The alligator and its allies

By Albert M. Reese

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Title: The alligator and its allies

Author: Albert M. Reese

Release date: December 30, 2023 [eBook #72548]

Language: English

Original publication: New York: G. P. Putnam's Sons, 1915

Credits: deaurider, Harry Lamé and the Online Distributed Proofreading Team at https://www.pgdp.net (This file was produced from images generously made available by The Internet Archive)

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Transcriber’s Notes

Texts printed in italics and bold face in the source document have
been transcribed between _underscores_ and =equal signs=. Small
capitals have been replaced with ALL CAPITALS.

More Transcriber’s Notes may be found at the end of this text.

[Illustration:

ALLIGATOR MISSISSIPPIENSIS. (After Ditmars.)

(Reproduced by Permission of Doubleday, Page & Company.)]

The
Alligator and Its Allies

By
Albert M. Reese, Ph.D.
Professor of Zoölogy in West Virginia University
Author of “An Introduction to Vertebrate Embryology”

_With 62 Figures and 28 Plates_

G. P. Putnam’s Sons
New York and London
The Knickerbocker Press
1915

The Knickerbocker Press, New York

PREFACE

The purpose of this volume is to bring together, in convenient form for
the use of students of zoölogy, some of the more important details of
the biology, anatomy, and development of the Crocodilia. For obvious
reasons the American Alligator is the species chiefly used.

In the first chapter the discussion of the alligator is largely the
result of the personal observations of the author; the facts in regard
to the less familiar forms are taken from Ditmars and others. The
description of the skeleton, with the exception of short quotations
from Reynolds, is the author’s.

The chapter on the muscular system is a translation from Bronn’s
_Thierreich_, and the author has not verified the descriptions of that
writer.

The description of the nervous system is partly the author’s and partly
taken from Bronn and others.

The chapters on the digestive, urogenital, respiratory, and vascular
systems are practically all from descriptions by the author.

The chapter on “The Development of the Alligator” is a reprint, with
slight alterations, of the paper of that title published for the author
by the Smithsonian Institution.

The bibliography, while not complete, will be found to contain most of
the important works dealing with this group of reptiles.

The author is grateful to Mr. Raymond L. Ditmars and to his publishers,
Messrs. Doubleday, Page & Co., and Messrs. Sturgis & Walton, for the
use of a number of plates; to the Macmillan Company and to the United
States Bureau of Fish and Fisheries for the same privilege; to the
National Museum for photographs of the skull of the gavial; and to
the Smithsonian Institution for the use of the plates from researches
published by them and included herein.

Proper acknowledgment is made, under each borrowed figure, to the
author from whom it is taken.

MORGANTOWN, W. VA.
_May 1, 1915._

CONTENTS

PAGE

CHAPTER I
THE BIOLOGY OF THE CROCODILIA 1

CHAPTER II
THE SKELETON 46

CHAPTER III
THE MUSCLES 90

CHAPTER IV
THE NERVOUS SYSTEM 131

CHAPTER V
THE DIGESTIVE SYSTEM 150

CHAPTER VI
THE UROGENITAL SYSTEM 192

CHAPTER VII
THE RESPIRATORY SYSTEM 197

CHAPTER VIII
THE VASCULAR SYSTEM 201

CHAPTER IX
THE DEVELOPMENT OF THE ALLIGATOR 226

BIBLIOGRAPHY 343

INDEX 349

ILLUSTRATIONS

PAGE

ALLIGATOR MISSISSIPPIENSIS. (In color.) _Frontispiece_

FIGURE

A. SKULL OF BELODON 5

1. MAP SHOWING THE PRESENT DISTRIBUTION OF CROCODILIA 6

2. HEADS OF AMERICAN ALLIGATOR AND CROCODILE _Facing_ 7

3. ALLIGATOR JOE IN THE EVERGLADES _Facing_ 10

4. ALLIGATOR HUNTER IN THE OKEFINOKEE _Facing_ 10

5. NEST OF C. POROSUS _Facing_ 21

6. JACKSON SLOUGH _Facing_ 21

7. A TYPICAL ALLIGATOR HOLE _Facing_ 23

8. ALLIGATOR NEST, MADE CHIEFLY OF GRASS _Facing_ 25

9. ALLIGATOR NEST, MADE CHIEFLY OF FLAGS _Facing_ 27

10. TWO SPECIES OF CAIMAN: BROAD-NOSED CAIMAN, SPECTACLED CAIMAN
_Facing_ 35

11. TWO AFRICAN CROCODILES: NILE CROCODILE, WEST AFRICAN CROCODILE
_Facing_ 39

12. SALT WATER CROCODILE _Facing_ 41

13. SKULL OF GAVIAL, VENTRAL VIEW _Facing_ 43

14. SKULL OF GAVIAL, LATERAL VIEW _Facing_ 43

15. ALLIGATOR SKINS _Facing_ 46

16. ENTIRE SKELETON OF CROCODILE 50

17. FIRST FOUR CERVICAL VERTEBRÆ OF CROCODILE 52

18. THORACIC AND SACRAL VERTEBRÆ OF CROCODILE 55

19. DORSAL VIEW OF SKULL OF ALLIGATOR _Facing_ 60

20. VENTRAL VIEW OF SKULL OF ALLIGATOR _Facing_ 63

20A. LONGITUDINAL SECTION OF TOOTH OF CROCODILE _Facing_ 66

21. LATERAL VIEW OF SKULL OF ALLIGATOR 69

22. POSTERIOR VIEW OF SKULL OF ALLIGATOR _Facing_ 70

23. SAGITTAL SECTION OF THE SKULL OF ALLIGATOR 71

24. DORSAL VIEW OF LOWER JAW OF ALLIGATOR _Facing_ 74

25. THE HYOID APPARATUS 77

26. THE STERNUM AND THE ASSOCIATED MEMBRANE BONES 79

27. THE PECTORAL GIRDLE AND ANTERIOR LIMB 82

28. THE PELVIS AND SACRUM 85

29. THE POSTERIOR LIMB 87

I. PLATE I. THE SHOULDER MUSCLES _Following_ 130

II. PLATE II. THE MUSCLES OF THE ANTERIOR REGION _Following_ 130

III. PLATE III. THE MUSCLES OF THE POSTERIOR REGION _Following_ 130

IV. PLATE IV. THE MUSCLES OF THE POSTERIOR REGION _Following_ 130

V. PLATE V. THE MUSCLES OF THE POSTERIOR REGION _Following_ 130

30. THE BRAIN OF THE ALLIGATOR _Facing_ 132

31. THE BRACHIAL PLEXUS OF C. ACUTUS 140

32. THE CRURAL PLEXUS OF A. MISSISSIPPIENSIS 142

33. INTERIOR OF THE MOUTH OF THE ALLIGATOR _Facing_ 151

34. THE DIGESTIVE SYSTEM OF THE ALLIGATOR 152

35. OUTLINE OF THE DIGESTIVE TRACT OF THE ALLIGATOR 158

36. COVERING OF THE ANTERIOR REGION OF THE TONGUE 160

37. COVERING OF THE POSTERIOR REGION OF THE TONGUE 161

38. GLAND FROM THE POSTERIOR REGION OF THE TONGUE 162

39. GLAND FROM THE POSTERIOR REGION OF THE TONGUE 164

40. COVERING OF THE ROOF OF THE MOUTH 166

41. TRANSSECTION OF THE ANTERIOR REGION OF THE ŒSOPHAGUS 169

42. TRANSSECTION OF THE POSTERIOR REGION OF THE ŒSOPHAGUS 170

43. EPITHELIUM OF ANTERIOR REGION OF ŒSOPHAGUS 172

44. EPITHELIUM OF ANTERIOR REGION OF ŒSOPHAGUS 173

45. TRANSSECTION OF WALL OF PYLORIC STOMACH 176

46. GLANDS OF FUNDUS OF STOMACH 177

47. TRANSSECTION OF WALL OF ANTERIOR REGION OF SMALL INTESTINE 181

48. TRANSSECTION OF WALL OF MIDDLE REGION OF SMALL INTESTINE 182

49. TRANSSECTION OF WALL OF POSTERIOR REGION OF SMALL INTESTINE 183

50. MUCOSA OF THE ANTERIOR REGION OF SMALL INTESTINE 184

51. TRANSSECTION OF THE WALL OF THE MIDDLE REGION OF THE SMALL
INTESTINE 185

52. TRANSSECTION OF THE WALL OF THE ANTERIOR REGION OF THE RECTUM 186

53. EPITHELIUM OF THE ANTERIOR REGION OF THE RECTUM 187

54. FEMALE UROGENITAL SYSTEM OF ALLIGATOR 193

55. MALE UROGENITAL SYSTEM OF ALLIGATOR _Facing_ 195

56. MALE ORGAN OF ALLIGATOR _Facing_ 195

57. RESPIRATORY ORGANS OF ALLIGATOR 198

58. HEART OF ALLIGATOR _Facing_ 202

59. VEINS OF THE POSTERIOR REGION OF ALLIGATOR 204

60. VEINS OF THE ANTERIOR REGION OF THE ALLIGATOR 209

61. ARTERIES OF THE POSTERIOR REGION OF THE ALLIGATOR 213

62. ARTERIES OF THE ANTERIOR REGION OF THE ALLIGATOR 215

VI.-XXVIII. PLATES VI TO XXVIII. A SERIES OF FIGURES TO ILLUSTRATE THE
DEVELOPMENT OF THE AMERICAN ALLIGATOR _Following_ 342

THE ALLIGATOR AND ITS ALLIES

CHAPTER I

THE BIOLOGY OF THE CROCODILIA

CLASSIFICATION

As in most groups of animals, there is considerable difference of
opinion as to the proper classification of the Crocodilia.

One of the older textbooks (Claus and Sedgwick) divides the order
Crocodilia into three sub-orders: the _Teleosauria_, _Steneosauria_,
and _Procœlia_, the last only being represented by living forms. The
Procœlia or Crocodilia proper are divided into three families,--the
_Crocodilidæ_, the _Alligatoridæ_ (including the caiman as well as the
alligator), and the _Gavialidæ_.

This division into families seems to be based mainly on the shape of
the head, or, at any rate, it throws those forms together that have
heads of the same outline.

It is this outline of the head that Ditmars (_Reptiles of the World_)
uses in classifying the Crocodilia, which, he says, are all included
in the single family--_Crocodilidæ_. The following list, taken from
his _Reptiles of the World_ (pp. 68-69), will give a clear idea of the
number, distribution, and maximum size of the members of the order
Crocodilia. More will be said of some of the members of this list later.

[1] These species are exceptions in their genus. The snout is blunt
like that of the genus _Alligator_.

[2] Alleged to grow to this size by competent observers.

Gadow in the _Cambridge Natural History_ (p. 450) agrees with Boulanger
in believing that the recent Crocodilia cannot be separated into
different families, yet he describes seven families of Crocodilia, two
of which, the _Gavialidæ_ and _Crocodilidæ_, include the living members
of the order; the former includes the gavials, of course, and the
latter the crocodiles, alligators, and caimans.

Though “doctors disagree” thus in regard to the scientific
classification of this small group of animals, this fact does not in
the least diminish the intense interest in the individual members of
the order.

ANCESTRY

Although the huge dragon-like dinosaurs or “terrible reptiles,” some
of which were probably more than one hundred feet long, became extinct
during the Mesozoic epoch, perhaps millions of years before man made
his appearance upon earth, we have one group of reptiles still living
in certain parts of the earth of which the Mesozoic lords of creation
need not feel ashamed. While most of the living Crocodilia are mere
pigmies in size, compared to the Atlantosaurus, there are a few
representatives of the living group, to be discussed later, that are
said to reach a length of thirty feet, which length makes pigmies, in
turn, of most of the other living reptiles.

Considering the extinct as well as the living Crocodilia, Gadow says it
is very difficult to separate them from the Dinosauria. In the Mesozoic
Crocodilia the fore limbs were much shorter and weaker than the hind
limbs, as was often the case with the dinosaurs; they were almost
entirely marine, but gave indications of descent from terrestrial forms.

Various facts point, thinks Gadow, “to some Theropodous Dinosaurian
stock of which the Crocodilia may well form an aquatic, further
developed branch” (_Cambridge Natural History_, p. 432).

[Illustration:

Skull of =Belodon=. A, from above; B, from below. _A_, orbit; _Bo_,
basi-occipital; _Ch_, internal nares; _D_, pre-orbital fossa;
_Exo._ exoccipital; _Fr._ frontal; _Ju._ jugal; _La._ lacrymal;
_Mx._ maxilla; _Na._ nasal; _Pa._ parietal; _Pl._ palatine; _Pmx._
pre-maxilla; _Por._ post-orbital; _Prf._ pre-frontal; _Pt._
pterygoid; _Qu._ quadrate; _S_, lateral temporal fossa; _S′_,
superior temporal fossa; _Sq._ squamosal; _Vo._ vomer. (From Zittel.)

FIG. A. A TRIASSIC ANCESTOR OF THE CROCODILIA.

From Parker & Haswell, _Textbook of Zoölogy_.]

The direct ancestors of the Crocodilia, Gadow says, are still unknown.

GEOGRAPHICAL DISTRIBUTION

As will be seen by examination of the table (p. 2) from Ditmars, and
of Figure 1, the recent Crocodilia are found in all of the great
continental areas except Europe; mainly in the tropical or subtropical
regions.

[Illustration: FIG. 1. MAP SHOWING PRESENT DISTRIBUTION OF CROCODILIA.

(After Gadow.)]

The alligator is found in the southwestern United States and in China.

The crocodile is the most numerous in species and is the most widely
distributed of the group. It is especially characteristic of Africa and
Madagascar, but is found also in Florida, Mexico, Central and South
America, the West Indies, South Asia, the East Indies, and Australia.

The gavial is found in India and some of the islands of the Orient,
especially Borneo and Sumatra.

The caiman occurs in southern Mexico, Central and South America.

[Illustration: FIG. 2. HEADS OF AMERICAN ALLIGATOR AND AMERICAN
CROCODILE; ALLIGATOR ON LEFT. (After Ditmars.)

(Reproduced by Permission of Sturgis & Walton Co.)]

The distribution of individual forms will be mentioned again when they
are discussed in detail.

ALLIGATOR MISSISSIPPIENSIS

Since this animal, generally known as the American or the Florida
alligator (formerly _A. lucius_), is the one upon which most of the
facts of this book are based, it will be discussed first.

At this point it may be well to answer the question that is sure to be
asked by someone early in any conversation upon the Crocodilia. The
writer, and doubtless every other zoölogist, has been asked countless
times, “What is the difference between an alligator and a crocodile?”
As a matter of fact there is, perhaps, no absolute distinction between
the two groups, but there are certain features that make it easy to
distinguish, say, between the American alligator and the American
crocodile.

The most striking difference is in the outline of the head; the
alligator has a broad, rounded snout, while that of the crocodile
is narrower and more pointed (Fig. 2). Again, in the crocodile the
fourth tooth from the front projects slightly outwards and fits into
a notch in the side of the upper jaw, while in the alligator (also in
the caiman) the corresponding tooth on each side fits into a socket
in the upper jaw and hence is hidden, except in some old animals with
very long teeth, in which it may pierce the upper jaw and show from
above. According to Ditmars, the crocodile has, as a rule, larger and
more exposed teeth than the alligator. Finally, as will be brought out
later, the crocodile is usually more quick and active, and also more
vicious, than the alligator.

Very young alligators are nearly black, with distinct, yellow cross
bands; as they grow older these markings become less distinct until in
maturity the animals are of a uniform gray or dirty black color.

_Habitat._ The American alligator is found in the rivers and swamps of
the Southern States, from the southern part of North Carolina to the
Rio Grande, though Florida is usually thought of as being the region
in which they particularly abound. Years ago, before the rifle of the
ubiquitous tourist and so-called sportsman had gotten in its deadly
work, the alligators were probably very abundant in the Southern
States; but they have been so ruthlessly destroyed by native hunters
for their skins, and by others for mere wanton sport, that one may
travel, perhaps, for days along the rivers of the South without seeing
a single ’gator.

The account quoted by Clarke from Bartram’s travels of more than one
hundred years ago, while probably exaggerated, gives an idea of the
abundance of the alligators at that time: “The rivers at this place
from shore to shore, and perhaps near a half mile above and below
me, appeared to be one solid bank of fish of various kinds, pushing
through the narrow pass of San Juans into the little lake on their
return down the river, and the alligators were in such incredible
numbers, and so close together from shore to shore, that it would have
been easy to have walked across their heads, had the animals been
harmless.” At the present time it is usually necessary to travel far
from the usual routes of the Northern tourists to find alligators in
any abundance.

At Palm Beach, Florida, lived, a few years ago, and probably still
lives, a well-known hunter and guide, “Alligator Joe.” Just what
nationality he may be is difficult to determine, but that he knows
that trackless waste, the Everglades, at least in the region of Palm
Beach, is evident. He has an “alligator farm” near the great hotels
of that famous winter resort, at which he keeps, or did a few years
ago, a large number of alligators of all sizes, as well as a number
of crocodiles. For a consideration (by no means a modest one) he
would take out a party of tourists for a day into the Everglades,
guaranteeing that he would find an alligator for them to shoot. It was
rumored by the natives that an accomplice was always sent ahead to free
the alligator at the psychological moment, after the hunters had been
paddled by a devious course to the selected spot, but whether this were
true or not the writer was not able to determine. It is true, however,
that he and the writer paddled in a rather graceful canoe, dug out
of a single cypress log, and waded through the Everglades for several
days, searching for alligator eggs, and that we found only one nest and
saw only one or two alligators (Fig. 3).

Doubtless in more remote parts of the Everglades the alligators are
much more numerous.

During another summer the writer, with a guide, penetrated the very
center of the State, to the region southeast of Lake Kissimmee,
forty miles from the nearest railroad; here the alligators, and in
consequence their nests, are fairly abundant, though the native hunters
are, even in this remote region, rapidly thinning their ranks.

A still greater number of alligators was found, the following summer,
in the Okefinokee Swamp in southern Georgia. In the center of this
great waste, ten miles or more from dry land, nearly one hundred
alligators, ranging from about four to eight feet in length, were
killed within a week by a small party of native hunters with whom the
writer was traveling (Fig. 4).

Whether this wholesale destruction by sportsman and native hunter will
eventually exterminate our giant reptile, as has been the case with the
buffalo and other game animals, it is impossible to say. Unless the
Everglades and the Okefinokee are largely drained it seems probable
that a few alligators will always remain in the most inaccessible
regions.

[Illustration: FIG. 3. ALLIGATOR JOE IN THE EVERGLADES.

(From a Photograph by the Author.)]

[Illustration: FIG. 4. ALLIGATOR HUNTER IN THE OKEFINOKEE.

(From a Photograph by the Author.)]

The collection of eggs for sale and for hatching purposes, as well as
their destruction for food by bears and other animals, will also tend
towards the annihilation of the species in the course of time. The
economic importance of the alligator will be discussed later.

While in the old days, as has been said, the alligator was common in
the larger rivers and lakes, and may even have ventured short distances
into salt water, he must now frequently be satisfied to hide his great
body in a “’gator hole” that is scarcely more than a puddle. These
“holes” (Fig. 7) are common in central Florida and are sometimes
scarcely large enough to allow the alligator to dive into them to seek
the underground cave in which he hides. It is on the edge of such a
hole that the nest is built, as will be described later.

Often from a small swamp or slough alligator “trails” lead off in
different directions. These trails are narrow, winding gullies such as
might be made by cattle in a damp pasture. If followed from the main
slough the trail will usually be found to end in a “hole,” in which an
alligator will probably be found (Fig. 7). In a great swamp like the
Everglades or the Okefinokee such holes would naturally not be found.

On one side of the hole is usually a smooth place where the vegetation
is worn away; it is here that the ’gator “pulls out” to sleep in the
sun; and wary must the hunter be to approach within sight of the
animal before being seen or heard by him. At the first alarm he slides
quietly or plunges quickly into the muddy water, and the hunter must
wait long if he expects to see the ’gator come to the surface.

The opening of the cave is always below the surface of the water,
but it is possible that there may be a subterranean chamber that is
not completely filled with water. How the animal is gotten from his
cave will be described later. According to some writers the alligator
retires to his cave to hibernate during the cooler winter months. This
is possibly true in the more northerly limits of his range. It is
well known that if kept in cool water the alligator will lie dormant
and refuse all food for months at a time. The writer has had young
alligators in captivity, under these conditions, that refused food from
late in the autumn until nearly the first of April.

The proprietor of one of the largest alligator farms in the country
says: “Our alligators stop eating the first week in October and do not
begin to eat until the latter part of April. We have experimented with
our stock to see if we could get them to eat in the winter, and found
that by keeping the water in the tanks at a certain temperature they
would eat, but we found out that the warm water would make their bowels
move, and that they would not eat enough to keep themselves up, as in
the summer, and as a result they would become very poor and thin, so
we do not force them to eat any more.” The effect upon the growth of an
animal of these two methods of feeding will be noted later when the age
and rate of growth are discussed. The same writer says, in answer to
a question about hibernation: “In their wild state they go into their
dens under water and remain dormant all winter.” Whether this statement
is the result of actual observation the writer is not able to say, but,
judging by some other statements from the same source, it is probably
from hearsay. The writer, having visited the alligator haunts only in
late spring and summer, has had no opportunity of studying the habits
of the animal in its natural habitat during the winter season. During
the heat of summer the animal does not seek the sun as he is said to do
during cooler weather, but spends more time on the bank at night and
during the cooler parts of the day.

That he sometimes wanders over dry land, perhaps going from hole to
hole, is evident from the tracks that are sometimes seen crossing a
dusty road or path. These trails are easily recognized by the clawed
footprints with a line, made by the dragging tail, between them.
Although most awkward on land, he can, if necessary, move very quickly.
It is, however, in the water that he shows to best advantage; he is an
active, powerful swimmer, his tail being used as a propeller as in the
fishes. When swimming actively the legs are held close against the
body in order that they may retard the animal’s motion as little as
possible. While swimming in a leisurely way the top of the head is at
the surface of the water, perhaps just the nostrils and eyes projecting
above the surface, so that the size of the animal can be estimated by
the distance between these projecting points. One afternoon the writer
and a guide, while paddling along an old canal that was dug years ago
into the Okefinokee Swamp, were preceded for perhaps half a mile by a
large alligator that swam just fast enough to keep out of our reach
until he came to the place where he wanted to turn off into the swamp.

Although so awkward on land, the alligator is said to be able to defend
himself very effectively with his tail, which he sweeps from side to
side with sufficient force, in the case of a large specimen, to knock
a man off his feet. Although the writer has seen captured and helped
to capture alive several alligators up to eight feet in length he has
never seen this vigorous use of the tail as a weapon of defense.

While the alligator, like most other wild animals, will doubtless
defend itself when cornered, it will always flee from man if possible,
and the writer has frequently waded and swam in ponds and lakes where
alligators lived without the least fear of attack. This might not have
been possible years ago when the animals were more numerous and had not
been intimidated by man and his weapons.

_Food._ The food of the adult alligator consists of fishes, birds,
mammals, and possibly smaller individuals of its own species. The young
eat small fish, frogs, insects, or worms.

If the animal be too large to swallow whole it is shaken and torn, the
shaking being so vigorous that, according to Ditmars, the entrails of
the prey may be thrown to a distance of twenty feet or more. Should
two alligators seize the same prey at the same time they whirl about
in opposite directions so violently that the prey is torn apart. This
action may be illustrated by giving two small captive alligators a
piece of tough meat; they hold on with bulldog tenacity, and each,
folding its legs close to its body, will use its tail like a propeller
until the animal whirls around with remarkable speed. The commotion
that two ten-foot alligators would cause when thus struggling can
easily be imagined. That a large alligator, if it tried, could easily
drag under the water and drown a man or possibly a much larger animal
is evident.

While the alligator has a valve-like fold of skin in its throat that
enables it to open its mouth and crush its prey under water, it is said
that it must raise its head above water in order to swallow its food.
A young alligator on land will usually throw back its head when trying
to swallow a large piece of meat, so that it may be simply this motion
that brings the head of the alligator above the surface of the water.

Ditmars thus describes the fate of a dog that approached too near a
very large alligator: “As a dog, weighing about fifty pounds, unwarily
approached the edge of this creature’s tank, it was suddenly grasped
and before completing its first yelp of terror was dragged beneath the
surface. A few minutes later the twelve-foot saurian appeared at the
top, holding the dead canine in its jaws. The dog was shifted about,
amid the sound of breaking bones, and swallowed head first, and entire,
after a few gulps.”

_Size and Growth._ Although, years ago, alligators of fifteen feet
length may have been common in favorable localities in the South, it
is probable that few if any such monsters now exist. A twelve-foot
alligator, owing to its great girth, is a huge animal and but few of
this size are to be found in captivity. The largest specimen the writer
has ever seen is the one in the Bronx Zoo, which is barely thirteen
feet in length. At hatching the alligator is about eight inches in
length.

Clarke (17) says: “The largest specimen I saw measured twelve feet in
length; and none of the many hunters and natives of Florida I have met
have seen any longer than thirteen feet. All the hunters agree that it
is only the males that acquire the great size; no one had ever seen a
female that measured over eight feet, and the majority are not over
seven. The male has a heavier, more powerful head, and during the
breeding season especially is more brilliantly colored.”

It is a very common belief, even among those who should be most
familiar with their habits, that the growth of the alligator is
remarkably slow, so that a large specimen may be described by the
exhibitor as more than a century old. The same dealer in alligators
quoted above says upon this subject: “You can figure about two inches a
year to their growth.” He also says: “We judge that an alligator about
twenty-five to thirty years old will breed.” Even scientific writers
of reputation have not been free from this error in their writings.
That the alligator may live to an extreme age, as seems to be true of
some of the tortoises, is quite possible, and it is probable that after
reaching a length of twelve or fifteen feet the growth is very slow.

In captivity, when kept in warm water and other favorable conditions,
the alligator will grow, according to measurements taken at the New
York Zoölogical Park, at the rate of about one foot a year, for about
the first ten years. Under unfavorable conditions the growth may be
exceedingly slow. Under favorable conditions in nature the rate of
growth may exceed that given above.

Instead of requiring twenty-five to thirty years to reach sexual
maturity, as quoted above, it is likely that the female may lay eggs
at five to ten years, though such a fact is difficult to determine of
animals in their native haunts.

_Voice._ The alligator, unlike most other members of its class, the
_Ophidia_, _Chelonia_, and _Lacertilia_, has a voice, which, in
an adult bull, may be heard for a mile or more. This bellowing is
difficult to describe; it is something between a moan and a roar, and
may be to attract the opposite sex or to serve as a challenge to other
large animals. It is usually ascribed to the male, but whether confined
to him or not the writer is unable to say.

In younger animals the voice is, of course, less deep and in very young
individuals it is a squeak or grunt, easily imitated by hunters for the
purpose of luring the animals from their hiding places.

_Breeding Habits._ Judging from the statements of native hunters the
laying season of the alligator might be thought to be at any time from
January to September. As a matter of fact the month of June is the time
when most, if not all, of the eggs are laid. S. F. Clarke gives June
9th and June 17th as the limits of the laying season in Florida, but I
found at least one nest in which eggs were laid as late as June 26th:
no eggs were found before the first date given by Clarke. It seemed
quite certain that the laying, during the season in question, had been
delayed by an extreme drought that had dried up the smaller swamps
and reduced the alligator holes to mere puddles. Nests were found in
considerable numbers as early as June 8th, but no eggs were laid in
any of them until the end of the dry period which occurred nearly two
weeks later. Almost immediately after the occurrence of the rains that
filled up the swamps eggs were deposited in all of the nests at about
the same time. From the fact that all of these completed nests had
stood for so long a time without eggs, and from the fact that all of
the eggs from these nests contained embryos in a well-advanced state of
development, it seemed evident that the egg-laying had been delayed by
the unusually dry weather. Eggs taken directly from the oviducts of an
alligator that was killed at this time also contained embryos that had
already passed through the earlier stages of development. Thus it was
that the earliest stages of development were not obtained during this
summer.

It is said that during the mating season, which precedes by some time,
of course, the laying season, the males are noisy and quarrelsome,
and that they exhibit sexual characteristics of color by which they
may be distinguished from the females. Never having been in the
alligator country at this season, the writer has made no personal
observations along these lines, but from the frequency with which
alligators with mutilated or missing members are found it is evident
that fierce encounters must sometimes take place, whatever the cause.
During June and July, at least, and probably during most of the year,
the alligators are very silent, an occasional bellow during the very
early morning hours being the only audible evidence that one has
that the big reptiles are in the neighborhood. Whatever may be the
sexual differences during the mating season, at ordinary times the two
sexes are so much alike that I have, on more than one occasion, seen
experienced hunters disagree as to the supposed sex of an alligator
that had just been killed.

Although I have never seen a nest actually during the process of
construction, it is easy to imagine, after the examination of a large
number of freshly made nests, what the process must be like.

The alligator, probably the female, as the male, after the mating
season, takes no interest whatever in the propagation of his species,
selects a slight elevation on or near the bank of the “hole” in which
she lives. This elevation is generally, though not always, a sunny
spot, and is frequently at the foot of a small tree or clump of bushes.
Where the alligator is living in a large swamp she may have to go a
considerable distance to find a suitable location for her nest; when
her hole is scarcely more than a deep, overgrown puddle, as is often
the case in the less swampy regions, she may find a good nesting place
within a few feet of her cave. That the female alligator stays in the
neighborhood of her nest after she has filled it with eggs seems pretty
certain, but that she defends it from the attacks of other animals is
extremely doubtful: certainly man is in very little danger when he
robs the nest of the alligator, and, according to the statement of
reliable hunters, bears are very persistent searchers for and eaters of
alligator eggs. Having selected (with how much care it is impossible
to say) the location for the nest, the alligator proceeds to collect,
probably biting it off with her teeth, a great mass of whatever
vegetation happens to be most abundant in that immediate vicinity. This
mass of flags or of marsh grass is piled into a conical or rounded heap
and is packed down by the builder repeatedly crawling over it.

[Illustration: FIG. 5. A NEST OF C. POROSUS: PALAWAN, P. I.

(From a Photograph by Rowley.)]

[Illustration: FIG. 6.--JACKSON SLOUGH; NEAR LAKE KISSIMMEE, FLORIDA.

In the vicinity of this pond several alligator nests were found,
either within a few yards of the edge, or on the banks of smaller
“holes” which were connected with the larger pond by narrow “trails.”
(From a Photograph by the Author.)]

There is a great deal of variation in the size and form of the
different nests, some being two meters or more in diameter and nearly a
meter in height, while others are much smaller in diameter and so low
as to seem scarcely more than an accidental pile of dead vegetation.
It is probable that the nests are under construction for some time,
perhaps to give time for the fresh vegetation of which they are
composed to ferment and soften, and also for the material to settle
into a more compact mass. The compactness of the alligator’s nest was
well illustrated one day when the writer used an apparently deserted
nest as a vantage ground from which to take a photograph: on opening
this nest it was found, after all, to contain eggs, and though some
of the eggs were cracked, none of them were badly crushed. This nest
although it was so low and flat that it was thought to be one that had
been used during some previous season, contained forty-eight eggs, a
greater number than was found in any other nest; while in other nests
that were twice as large as this one were found less than half as many
eggs, showing that there is no relation between the size of the nest
and the number of eggs. The average number of eggs per nest, in the
twelve nests that were noted, was thirty-one. One observer reported a
nest that contained sixty eggs, but this, if true, was a very unusual
case. Reports of still larger numbers of eggs in one nest probably
refer to crocodiles, which are said to lay one hundred or more eggs in
a nest. Although crocodiles may be found in certain parts of Florida,
the writer has had no opportunity of observing their nesting habits.

The eggs are laid in the nest without any apparent arrangement. After
the nest has been prepared, and has had time to settle properly, the
alligator scrapes off the top, and lays the eggs in a hole in the damp,
decaying vegetation; the top of the nest is again rounded off, and it
is impossible to tell, without examination, whether the nest contains
eggs or not.

As to whether the same nest is used for more than one season there is a
difference of opinion among alligator hunters, and the writer has had
no opportunity of making personal observations.

[Illustration: FIG. 7. A TYPICAL ’GATOR “HOLE.”

Only a few yards across, and surrounded by a dense growth of
vegetation. On the far side is seen an opening in the surrounding
grass and flags where the ground is worn smooth by the alligator in
crawling out of the hole. Under the bank, probably near the place
where the alligator “pulls out,” is the deep cave into which the
inhabitant of this hole quickly goes on the approach of danger. As
this cave may be fifteen or twenty feet deep it is not an easy matter
to get the animal out. When a female alligator inhabits such a hole,
a nest may often be found within three or four yards of the water,
though it is sometimes at a greater distance. Such a hole as this may
be connected by narrow, winding “trails” with larger ponds, as noted
under Fig. 6. (From a Photograph by the Author.)]

While it is usually stated that the eggs are incubated by the heat
of the sun, it is held by some observers that the necessary heat is
derived not from the sun but from the decomposition of the vegetable
matter of which the nest is composed. Possibly heat may be derived
from both of these sources, but it seems likely that the conditions
that are especially favorable to normal incubation are moisture and
an even, though not necessarily an elevated, temperature. Moisture is
certainly a necessary condition, as the porous shell allows such rapid
evaporation that the egg is soon killed if allowed to dry. The inside
of the nest is always damp, no matter how dry the outside may become
under the scorching sun, so that this condition is fully met. The eggs
of the Madagascar crocodile, according to Voeltzkow,[3] offer a marked
contrast to those of the alligator. Instead of being laid in damp nests
of decaying vegetation, they are laid in holes that are dug in the dry
sand, and are very sensitive to moisture, the early stages, especially,
being soon killed by the least dampness. A crocodile’s nest containing
eggs is shown in Figure 5. In this species of crocodile, probably
_C. porosus_, the nest resembles that of the Florida alligator. The
photograph was taken by Mr. Rowley on the edge of a small lake on the
Island of Palawan, P. I.

[3] Voeltzkow, A., “The Biology and Development of the Outer Form of
the Madagascar Crocodile,” _Abhandl. Senckberg. Gesell._, Bd. 26, Hft.
I.

The daily range of temperature in the Southern swamps is sometimes
remarkably great, so that if the eggs were not protected in some way
they would often pass through a range of temperature of possibly fifty
degrees or more; while in the center of a great mass of damp vegetation
they are probably kept at a fairly constant temperature. Unfortunately
no thermometer was taken to the swamps, so that no records of the
temperatures of alligator nests were obtained, but it was frequently
noticed that when, at night or very early in the morning, the hand was
thrust deep into the center of an alligator’s nest the vegetation felt
decidedly warm, while in the middle of the day, when the surrounding
air was, perhaps, fifty degrees (Fahrenheit) warmer than it was just
before sunrise, the inside of the same nest felt quite cool. It is
probable, then, that the conditions of temperature and moisture in the
center of the nest are quite uniform. One lot of eggs that had been
sent from Florida to Maryland continued to incubate in an apparently
normal way when packed in a box of damp sawdust, the temperature of
which was about 80 degrees Fahrenheit. Another lot of eggs continued to
incubate, until several young alligators were hatched, in the ordinary
incubator, at a temperature of about 95 degrees Fahrenheit.[4]

[4] Reese, A. M., “Artificial Incubation of Alligator Eggs,” _Amer.
Nat._, March, 1901, pp. 193-195.

The fact that eggs taken directly from the oviducts of the cold-blooded
alligator contain embryos of considerable size seems to indicate that
no such elevation of temperature as is necessary with avian eggs is
necessary with the eggs of the alligator.

[Illustration: FIG. 8. A TYPICAL ALLIGATOR’S NEST, MADE CHIEFLY OF
GRASS.

The guide is feeling for eggs without disturbing the outside of the
nest. Being made of the same material as the background, the nest
does not stand out very sharply, though in nature the contrast is
somewhat more marked, owing to the fact that the surrounding grass is
green while the grass of which the nest is built is dead and brown.
(From a Photograph by the Author.)]

The complete process of incubation probably extends through a period
of about eight weeks, but no accurate observations along this line
could be made. For some hours previous to hatching the young alligators
make a curious squeaking sound inside the shell, that may be heard
for a distance of several yards: this sound may be for the purpose of
attracting the attention of the female alligator, who will open the top
of the nest in time to allow the just hatched alligators to escape:
unless thus rescued, it would seem impossible for the little animals
to dig their way out from the center of the closely packed mass of
decaying vegetation.

At the time of hatching the alligator is, as already noted, about eight
inches in length, and it seems impossible that it should have been
contained in so small an egg.

The size of alligator eggs, as might be expected, is subject to
considerable variation. In measuring the eggs a pair of brass calipers
was used, and the long and short diameters of more than four hundred
eggs were obtained. A number of eggs of average size, when weighed in
mass on the scales of a country store, gave an average of 2.8 oz. per
egg.

There was more variation in the long diameter of eggs than in the short
diameter.

The longest egg of all those measured was 85 mm.; the shortest was 65
mm. The widest egg (greatest short diameter) was 50 mm.; the narrowest
egg (least short diameter) was 38 mm.

The average long diameter was 73.742 mm.; the average short diameter
was 42.588 mm.

The greatest variation in long diameter in any one nest of eggs was
15.5 mm.; the greatest variation in short diameter in the eggs of any
one nest was 11 mm.

The average variation in the long diameter of the eggs from the same
nest was 11.318 mm.; the average variation in the short diameter of the
eggs from the same nest was 5.136 mm.

It will be seen from the above that the average variation in the long
diameter of eggs from the same nest is between one sixth and one
seventh of the long diameter of the average egg; while the average
variation in the short diameter of the eggs from the same nest is less
than one eighth of the short diameter of the average egg.

S. F. Clarke[5] gives the limits of the long diameter as 50 mm. and 90
mm., and the maximum and minimum short diameters as 45 mm. and 28 mm.
No such extremes in size were noticed among the eight hundred or more
eggs that were examined.

[5] _Journal of Morphology_, vol. v.

[Illustration: FIG. 9. AN ALLIGATOR’S NEST, SOMEWHAT SMALLER THAN THE
ONE REPRESENTED IN FIG. 8, BUILT CHIEFLY OF FLAGS.

The nest has been opened to show the irregularly arranged mass of
eggs inside. The size and shape of the egg are shown by the one in
the guide’s hand. (From a Photograph by the Author.)]

_Economic Importance._[6] More than one hundred years ago attempts
were made to utilize the skin of the alligator, but it was not until
about 1855 that these attempts were successful and alligator leather
became somewhat fashionable and some thousands of hides were converted
into leather. The demand was short-lived, however, and was not again
felt until the demand for shoe-leather during the war between the
States revived the business. At the close of the war the business
again failed, but about 1869 the demand became greater than ever and
has continued unabated to the present time. The supply of skins from
our own States proving inadequate, large numbers of skins were soon
imported from Mexico and Central America. The skins from South America
are so heavy that they are of little value in making leather. Of the
States of the Union, Florida has been the chief producer, the most
important centers for hides being Cocoa, Melbourne, Fort Pierce, Miami,
and Kissimmee. Ten men at the first-named place took, in 1899-1900,
2500 skins; one man took 800 skins in one year; another man collected
42 skins in one night. At Fort Pierce twelve men took 4000 skins in
1889. In 1899, three firms at Kissimmee handled 33,600 hides. After
this time the total number of hides taken and the average per man
diminished greatly.

[6] The following figures are from an article by C. H. Stevenson in the
Report of the Bureau of Fisheries, 1902, pp. 283-352.

Besides being killed for their hides, the alligators have been
destroyed by the thousands merely for wanton sport, so that in 1902 it
was estimated that their numbers in Florida and Louisiana were less
than one fifth of what they were twenty years before that time, and
unless steps be taken to prevent it, the alligator hide, as an article
of commerce, may cease to exist in our Southern States.

It has been claimed that the destruction of the alligator has allowed
the cane rat and muskrat to increase to a serious extent, the former
doing great damage to crops, the latter often injuring the levees to a
dangerous extent. Legislation to forbid the killing of alligators of
less than five feet in length has been suggested and should be passed,
since animals of less size have almost no commercial value for leather.

In 1902, the annual output from the tanneries of the United States
approximated 280,000 skins, worth about $420,000. Of these about
fifty-six per cent. came from Mexico and Central America, twenty-two
per cent. from Florida, twenty per cent. from Louisiana, and the
remaining two per cent. from the other Gulf States. South American
hides are not handled by the United States markets.

In 1908, there were marketed from the South Atlantic and Gulf States
372,000 pounds of alligator hides, valued at $61,000.

According to the United States Bureau of Fisheries the hunter in 1891
averaged about 60 cents for the skin, while in 1902 the price averaged
about 90 cents, varying between 15 cents and $2.00, depending on the
size and condition of the skin. “Prime hides five feet long, with no
cuts, scale slips, or other defects, are worth about 95 cents each, in
trade, when the hunter sells them at the country stores, and about
$1.10 cash, at the tanneries. Those measuring seven feet are worth
$1.55, six feet, $1.12; four feet, 52 cents, and three feet, 25 cents.
Little demand exists for those under three feet in length” (Report
Commissioner of Fish and Fisheries, 1902, p. 345). Hides of seven feet
are in most demand, those over ten feet are not much used. The income
of the hunters is largely increased by the sale of otter, bear, deer,
and other skins.

The different varieties of skins are described by Stevenson (74) as
follows:

“There are several distinct varieties of alligator skins on the
markets, the most important being the Floridian, Louisianian, and
Mexican; each differs from the others in certain well-defined
characteristics, and owing to these differences each variety has its
special uses.

“The Florida skins are longer in the body--that is, from the fore
legs to the hind legs--than those from Louisiana and Mexico, and
consequently they are largely in demand by manufacturers of large
handbags. They also have a number of so-called ‘buttons’ or ‘corn
marks’ on the inside or under surface of an equal number of the scutes
resulting from imbedded horn-like tissues in the center of those
scales. These increase the difficulty in tanning the skins and detract
somewhat from the appearance of the finished article, and for this
reason the Florida skins are ordinarily the cheapest on the market.
The farther south the skins are secured in Florida the greater the
number of ‘corn marks,’ and those from the vicinity of Key West are
almost valueless for this reason.

“The Louisiana skins differ from those of Florida in the absence of the
‘corn marks’ above noted, and from both the Florida and Mexican skins
in being more pliable and in having the scales more artistically curved
and shaped. Consequently they are preferred for such small articles as
card-cases and pocketbooks, and usually sell at the highest prices.
Skins obtained in Mississippi and Texas are similar to those secured
in Louisiana, while those from Georgia and South Carolina are similar
to those from Florida, except that the ‘corn markings’ are not so
numerous. All the Florida and Louisiana skins show greater uniformity
of coloring, being of a bluish black on the upper surface and a
peculiar bluish white on the under side.

“In addition to an absence of the characteristics above noted the
Mexican and Central American skins are distinguished by having from
one to four small dots or markings like pin holes near the caudal
end of each scale. The length of the Mexican skins varies greatly in
proportion to the width, sometimes equaling that of the Florida skins.
Those from the east coast of Mexico are the best, being lighter in
color and with neat and attractively shaped scales. The west coast
skins are yellowish in color when in the green state, and the scales
are larger and not so artistically formed. The Florida and Louisiana
skins are almost invariably split down the back, or rather along each
side of the back, so as to preserve the under side in a solid piece,
but most of the Mexican skins are split down the middle of the abdomen,
keeping the back intact, making what is commonly known as ‘horn
alligator.’ (See Fig. 15.)

“The skin should be removed soon after death as, in warm climates,
putrefaction sets in very early and the value of the skin is
depreciated. After removal, the flesh side of the skin is thoroughly
rubbed with fine salt, and the skin is carefully rolled up with
the salted side inside and is ready for shipment, but must be kept
in a dry, cool place. Great care must be taken not to cut the hide
since small cuts that are not noticeable in the raw skin may be so
conspicuous in the dressed skin as to render it of much less value; a
large percentage of the hides received in the markets are thus damaged.

“Formerly only the ‘belly skin’ was removed, by two longitudinal
incisions just below the horny portion of the back; but it was later
found that the thick horny skin of the back could be tanned nearly as
well as the thinner belly skin, so that the entire skin is now usually
removed by a longitudinal incision along the mid-ventral line, with
lateral incisions along each leg to the foot (Fig. 15). The entire
skin is more commonly taken in Mexico and Central America than in our
States.

“Although the raw skins are sold according to length, the tanned hides
are sold by the width of the leather at the widest part. Standard hides
sell for $1.00 to $1.65 per twelve inches of width. Some skins tanned
and dyed in a superior manner sell for $2.00 or more for single skins
of 2¹⁄₂ feet in length. As a rule the Louisiana skins fetch the highest
prices, and those from Florida the lowest. Imitation alligator leather
is now prepared in large quantities, principally from sheepskins or
the buffing from cowhides. These are tanned according to the usual
process, and before the skins are finished they are embossed with the
characteristic alligator markings by passing them between two rollers.”
(Above-mentioned report, p. 346.)

Very little of the leather is now used in making shoes, the chief
demand being for handbags, music-rolls, etc.

In hunting alligators for their hides two methods are usually employed,
in our Southern States at least. The common method is “fire-hunting”
at night; the hunters here go, either singly or in pairs, usually in
boats, sometimes on foot, with shotgun and torch. The torch may be
fastened to the hunter’s hat, after the manner of the miner’s lamp.
One more progressive hunter that I knew had, as a torch, an acetylene
lamp, attached to his hat, with the tube for the gas extending down
his back to the generator in his pocket. This lamp threw a blinding
beam of light far across the swamp into the eyes of the unsuspecting
’gator, which usually remained fascinated until it could be approached
to within easy range. A shotgun at close range, of course, blows off
nearly the entire top of the animal’s head and kills it instantly; it
is then seized before it sinks out of reach and is either taken into
the boat or dragged upon the bank to be collected with others in the
early morning.

In daylight, with no glaring light to hypnotize it, the alligator is
difficult to approach within range and it usually disappears into its
cave before the hunter can get a shot at it. The daylight hunter, then,
should be supplied not, of course, with a light, but with a ten- or
fifteen-foot pole with a large iron hook at the end. If the alligator
be vigorously prodded with this mammoth fishhook he will usually
finally seize it with his mouth and can be pulled out of his hole
alive. It is then an easy matter to kill him with a bullet through the
base of the brain. I have seen an eight-foot alligator thus killed with
a little .22 calibre “cat” rifle. An eight-foot alligator will often be
all that two men can manage to drag out of his cave in this way; and,
in the torrid heat of the Southern swamp, this violent exercise is not
to the liking of the usually not very energetic hunter.

While the manufacture of leather gives the chief value to the
alligator there are other ways in which it has some economic
importance. Chief of these is probably the sale of alligator goods to
tourists. In 1891 there were in Jacksonville, Florida, twelve dealers
in live and stuffed alligators. In 1890, 8400 alligators were sold to
tourists, the price for the live animals varying from $10.00 to $35
per hundred. For individual animals of the smallest size (less than
twelve inches long) the price is usually from 50 cents to $1.00. For
a three-foot alligator the price is generally $3-$5.00; for sizes
over three feet $2.00 per foot may be charged, though for very large
specimens the price may be from $50 to $300 each.

Besides the live and stuffed animals the teeth are polished and sold
as souvenirs; about 450 pounds of teeth were sold in 1890, at a price
varying from $1.00 to $2.00 per pound. From 75 to 200 teeth will make a
pound.

In 1891 about forty people, in addition to the regular dealers, were
engaged, in the United States, in stuffing alligators, polishing teeth,
etc. The teeth are extracted by burying the head until decomposition
sets in.

The tiny alligators that are most commonly sold to tourists, to be
brought North, perhaps, and allowed to freeze or starve to death, may
either be caught by a wire noose at the end of a fishing rod, or they
may be hatched from eggs that are taken from the nests shortly before
they are ready to hatch. Such eggs may readily be hatched by simply
keeping them moist and at a fairly constant temperature, as has been
previously noted. Besides the above uses Ditmars says: “The eggs are
eaten in many portions of the South, and the search for eggs at the
proper season furnishes profitable employment for many persons, as each
nest contains a large number of eggs.”

[Illustration: BROAD-NOSED CAIMAN. (_Caiman latirostris._)

Distribution: Tropical South America.

Attains a length of about seven feet. (After Ditmars.)

(Reproduced by Permission of Sturgis & Walton Co.)]

[Illustration: FIG. 10. SPECTACLED CAIMAN. (_Caiman sclerops._)
Tropical America.

The length of an adult is about eight feet. (After Ditmars.)

(Reproduced by Permission of Sturgis & Walton Co.)]

Never having eaten an alligator egg I cannot speak from personal
experience of its flavor; but it has always seemed strange to me that
more use is not made of the flesh of the alligator. This flesh is often
said to have too strong a flavor to be palatable; I have eaten it when
it had no such rank taste but was decidedly agreeable, being, as might
perhaps be expected of so amphibious an animal, somewhat like both fish
and flesh, yet not exactly like either. Perhaps greater care should
be taken in skinning an animal that is to be used for food in order
that the flesh be not tainted with the musk. It may be a lack of care
in preparation that has given rise to the impression that alligator
meat is too strong to be pleasant. It is perhaps, also, the “idea”
of eating a reptile that makes the meat unpopular. A half-grown boy,
who was once in the swamps with me, had expressed a great aversion
to alligator meat, so the guide, one day, offered him a nicely fried
piece of alligator meat, saying it was fish; the meat was eaten with
evident relish and the diner was not told until after a second piece
had disappeared what he had been eating. It always seemed strange to
me that the poor people of the South should not more often vary the
monotony of fat pork and corn bread with alligator steaks. Whether the
meat could be smoked or salted so that it would keep in a hot climate I
do not know; I am not aware of any experiments along this line.

THE CHINESE ALLIGATOR

Beside the American form, _Alligator mississippiensis_, the only other
species of alligator is found in China, along the Yang-tse-Kiang
River; it is _Alligator sinensis_. It reaches a length of six feet and
externally resembles its American relative; it is greenish black above
speckled with yellow; grayish below.

THE CAIMAN

This is the nearest relative of the alligator and is found in Central
America and tropical South America. As seen by the table on page 2, it
is usually a small animal, though one species, the black caiman, is
said to reach a length of twenty feet (Fig. 10). The nasal bones do not
form a bony septum as in the alligator and the ventral armor consists
of overlapping bony scutes. The canine teeth of the lower jaw fit into
a pit in the upper jaw, as in the alligator.

They are said by some writers to be extremely abundant in the waters
of the upper Amazon, migrating to the flooded forests during the rainy
season and returning to the streams on the approach of the dry season.
According to Ditmars there are five species of caiman of which the
spectacled caiman, _C. sclerops_, and the black caiman, _C. niger_, are
the most striking. The former is so named because of the spectacled
appearance due to the swollen and wrinkled upper eyelids; it reaches a
length of eight feet and is said to be of a treacherous disposition.
The latter has a blunt snout like the alligator and is the largest of
the New World crocodilians.

THE AMERICAN CROCODILE

Of about a dozen existing species of crocodile, but one, the American
crocodile, _C. americanus_, is found in the United States, and it is
limited to the swamps and coast of southern Florida below Lake Worth;
its greater sensitiveness to cold is doubtless the cause of its not
being found so far north as the alligator. Its range extends south
through Mexico and Central America into South America. It was first
found in Florida by Dr. Hornaday in 1875. It sometimes reaches a length
of fourteen feet.

As has already been noted there is, besides certain structural
differences, a marked difference in the dispositions of the Florida
alligator and crocodile. While an alligator may snap its jaws, hiss,
and swing its tail from side to side, it is not difficult for a couple
of men with ropes and a pole to safely tie up a large specimen. The
struggles of a crocodile are of a more serious nature. Ditmars thus
describes an encounter with a captive Florida crocodile: “The writer
well remembers his first acquaintance with a big fellow from Florida.
Driven out of the crate the crocodile looked the picture of good
nature. Standing away from what he thought to be the reach of his
tail, the writer prodded the apparently sluggish brute with a stick to
start it for the tank. Several things happened in quick order. With a
crescentic twist of the body utterly beyond the power of an alligator,
the brute dashed its tail at the writer, landing him such a powerful
blow that he was lifted completely from the ground. As he left _terra
firma_, an almost involuntary inclination caused him to hurl his
body away from a pair of widely-gaping, tooth-studded jaws swinging
perilously near. Landing with a thud on one shoulder, though otherwise
unhurt, the writer threw himself over and over, rolling from the
dangerous brute that was actually pursuing him on the run, body raised
high from the ground. For an instant it seemed as if the crocodile
would win. As the writer sprang to his feet and glanced backward, he
beheld the brute throw itself flat on its belly, open the jaws widely,
then remain motionless as a statue. Such is the average crocodile--an
active, vicious and, above all, treacherous brute.” Ditmars says again,
in the same book: “When the keepers of the reptile house of the New
York Zoölogical Park clean out the big pool of the crocodilians, they
actually walk over the backs of some of the big ’gators, so tame are
these. They never become unduly familiar with the crocodiles, finding
it necessary to pen the latter behind heavily barred gates--and in the
process the men are often chased from the enclosure.”

[Illustration: NILE CROCODILE. (_Crocodilus niloticus._)

Distribution: Africa generally; Madagascar.

Grows to a length of sixteen feet. (After Ditmars.)

(Reproduced by Permission of Sturgis & Walton Co.)]

[Illustration: FIG. 11. WEST AFRICAN CROCODILE. (_Crocodilus
cataphractus._)

Distribution: West Africa.

Does not attain so large a size as the Nile Crocodile. (After
Ditmars.)

(Reproduced by Permission of Sturgis & Walton Co.)]

In contrast to this ferocious aggressiveness in captivity the American
crocodile is said to be very timid and retiring when in its native
habitat.

Young animals are greenish with black marking; as they become older
they are of an olive color, and old specimens are dull gray.

THE ORINOCO CROCODILE, _C. intermedius_

This is a species with a very narrow snout that is not quite so large
as the preceding. It is said to be abundant in the Orinoco River and
its tributaries.

Besides the two above mentioned there is a small species, _C.
rhombifera_, found only in Cuba and hence known as the Cuban crocodile.

THE AFRICAN OR NILE CROCODILE, _C. niloticus_

This well known and much feared species is found throughout the
continent of Africa, and is very common on the island of Madagascar
(Fig. 11). In the lower waters of the Nile it is now nearly
exterminated. It has always been a conspicuous animal in Egypt and was
one of the animals held sacred by the Egyptians and preserved by them
as mummies. It is discussed by Herodotus, and the “leviathan” mentioned
in the Book of Job is doubtless this crocodile. In fact the name is
said to be derived from the ancient Greek for lizard, just as the
word alligator is said to be derived from the Spanish for lizard--_el
lagarto_; the resemblance in form between these big saurians and their
smaller relatives is evident. The alligator, being confined to America
and a small part of Asia, was probably not known to the ancients.

An excellent account of the development and habits of the present
species is given by Voeltzkow (78), who says it is, perhaps, the most
common vertebrate in Madagascar. The largest specimen measured by this
observer was thirteen feet; Ditmars gives sixteen feet as the maximum
size. This _man-eating_ crocodile, according to Ditmars, destroys more
human lives than any other wild animal of the dark continent.

The story told by Herodotus of the bird, probably a species of plover,
which enters the gaping mouth of the crocodile to pick off the leeches
found there may be true, since there is such a bird that may be seen
perching on the backs of crocodiles, and as the Crocodilia frequently
lie with their mouths wide open it is quite possible that these birds
may pick off the worms that are often found within. It is also possible
that the alertness of these birds to danger may serve as a warning to
the crocodiles with which they associate.

[Illustration: FIG. 12. SALT-WATER CROCODILE. (_Crocodilus porosus._)

Distribution: India to North Australia. Occurs at sea.

Grows to a length of twenty feet. (After Ditmars.)

(Reproduced by Permission of Sturgis & Walton Co.)]

According to Voeltzkow these crocodiles dig caves of thirty-nine to
forty feet length in the banks of the streams they inhabit, into which
they retire on the approach of danger. The caves open under water
and slope upward towards the surface of the ground where a few small
air-holes are found. The natives locate the caves by means of the
air-holes and dig out the hidden animal, first stopping up the entrance.

In Madagascar the eggs are laid in August and September and hatch in
about twelve weeks; they are laid at night, usually shortly before
daybreak. From twenty to thirty eggs are laid in one nest, which is
merely a hole dug in the dry sand. As was said in connection with the
Florida alligator, the habits of the two animals are quite different
in this respect,--the moisture that is so important in the one case is
fatal to the embryo in the other. When the eggs are laid the nest is
filled in with sand so that there is nothing to indicate its position
except that the female crocodile is in the habit of lying on the spot
where her eggs lie buried.

Like the alligator the young crocodile makes a squeaking noise shortly
before hatching and the mother doubtless opens the nest, at this time,
to allow the young to escape. A fence that Voeltzkow built around a
nest was repeatedly broken down by the mother in attempting to get back
to her eggs.

The character of the crocodile’s egg is discussed, in comparison with
that of the Florida alligator, on page 23.

THE MARSH CROCODILE OR MUGGER, _C. palustris_

Found in India, Ceylon, Burmah, the Malay Peninsula, and many of the
islands in that region. It has a rather broad snout, and reaches a
length of twelve feet. It is a timid form and is harmless to man. It is
frequently venerated by the Hindoos and is kept in a semi-domesticated
condition in ponds where it is fed and becomes very tame.

In the dry season when the natural ponds are empty they sometimes
migrate overland in search of water, but generally they bury themselves
in the mud and lie dormant until the rains begin again.

THE SALT-WATER CROCODILE, _C. porosus_

This is one of the largest if not the largest of living reptiles (Fig.
12). It is said by Ditmars to reach a length of twenty feet and there
is a record of one specimen that was thirty-three feet in length. It
is said to be easily recognized by the prominent, longitudinal ridge
that extends in front of each eye, over the prefrontal bone, and by the
absence of the suboccipital scutes.

[Illustration: FIG. 13. SKULL OF GAVIAL.

(Dorsal View.)

(Photograph from U. S. National Museum.)]

[Illustration: FIG. 14. SKULL OF GAVIAL.

(Lateral View.)

(Photograph from U. S. National Museum.)]

It is typically an inhabitant of tidal waters and is sometimes found
swimming at sea, out of sight of land; it seldom goes inland to any
great distance from the sea. It is a _man-eating_ species and many
human lives are said to be destroyed by it in India and surrounding
countries. A British “blue book” states that in British India 244
deaths were caused by Crocodilia in the year 1910.

In captivity it is savage and untamable. Ditmars, in speaking of three
specimens that he had in captivity, says they were “positively the most
vicious reptiles” he had ever seen.

THE INDIAN GAVIAL, _Gavialis gangeticus_

This animal, which inhabits the Ganges and other rivers of northern
India, is, with the possible exception of the preceding species, the
largest of the Crocodilia; it is said to reach a length of thirty feet,
which is twice that of a very large Florida alligator. As previously
noted its snout is extremely long and narrow (Figs. 13 and 14), with a
large, fleshy hump at the tip, that projects above the muddy water in
which the animal lies concealed.

It is a timid animal and, in spite of its huge size, dashes quickly
into the water on the approach of man, to whom it is seldom or never
dangerous. Its Indian name, _gharial_, from which its generic name has
been corrupted, means fish-eater, since its food consists, it is said,
largely if not entirely of fish.

Considering its huge size and the character of its jaws and teeth as
shown in Figures 13 and 14, it is fortunate that it prefers fish to
human flesh.

Anderson (2) describes the eggs and young of the Indian gavial. He
found forty eggs in a nest of sand; they were in two layers, with a
foot of sand between them. The young were 15.8 inches long at hatching.
He says: “The young run with amazing rapidity the moment they are out
of the shell.... Some of them actually bit my fingers before I had time
to remove the shell from their bodies.” The following quotation from
Oldenburg (46), for which I am indebted to Dr. Hussakof, is perhaps
the earliest reference to the egg of the American alligator. It also
mentions the habit that is practiced by some of the recent Crocodilia
of swallowing stones to aid in digestion, as was apparently done by
some of the large extinct reptiles.

“The eggs of _Crocodiles_ and _Alligators_ are little bigger than a
Turkey’s. I thought to bring one to England, but it was lost. I never
broke any to see the Yolk and White; but the Shell is as firme and
like in shape to a Turkey’s, but not spotted. I inquired into the
Stone in the Stomach of a _Cayman_ or _Crocodile_, and I found by the
inquiry of a very observing gentleman there, that they were nothing but
several Stones, which that Creature swallows for digestion. He took
out of one a piece of a Rock as big as his head: out of others he had
taken sixteen or twenty lesser. None regards them much there, whatever
_Monardes_ relateth.”

CHAPTER II

THE SKELETON

A. THE EXOSKELETON

The exoskeleton is well developed in the Crocodilia, and forms a very
considerable protection to its bearer. It is both dermal and epidermal
in origin.

The _epidermal skeleton_ of the alligator consists of oblong horny
scales, arranged in transverse rows; the long axes of the scales are
parallel to that of the body. On the tail, except along the mid-dorsal
line, and on the ventral side of the trunk and head these scales are
very regular in outline and arrangement; on the sides of the head
and trunk and on the legs they are much smaller and less regularly
arranged, while along the mid-dorsal line of the tail, especially in
its posterior half, they are elevated into tall keels that give the
tail a large surface for swimming. The first three digits of both manus
and pes are armed with horny claws, which also belong to the epidermal
part of the exoskeleton.

[Illustration: FIG. 15. ALLIGATOR SKINS; UNDER-SURFACE AND HORN-BACK.]

The _dermal exoskeleton_ consists of bony scutes that underlie the
epidermal scales of the dorsal surface of the trunk and anterior part
of the tail. The overlying scales, except in very young animals, are
always rubbed off, so that the bony scales are exposed. The ventral
or inner surface of the scutes is flat, while the outer surface is
strongly keeled and in old animals is often rough and pitted. The
plates are nearly square in outline and are closely joined together in
most places.

The scutes are grouped in two fairly distinct areas known as the nuchal
and the dorsal shields. The former lies just back of the head, in the
region of the fore legs, and consists of four larger and a number
of smaller plates (Fig. 15). The latter, or dorsal shield, extends
over the back in fairly regular longitudinal rows and quite regular
transverse rows. At the widest part of the trunk there are six or eight
of these scutes in one transverse row. They become smaller towards the
tail.

The _teeth_ are exoskeletal structures, partly of ectodermal, partly
of dermal origin. They are conical in shape, without roots, and are
replaced when lost. They will be described in connection with the skull.

Musk glands, said by Gadow to be present in all Crocodilia, are found
in both sexes and are derivations of the skin. One pair, each of which
may be as large as a walnut, is found on the lower side of the head,
one on the inside of each half of the mandible. The other pair is
inside of the lips of the cloaca.

_The Histology of the Integument._ To understand the structure of the
integument of the Crocodilia it is well to begin with the embryo. A
cross section of the epidermis of such an embryo will show the rete
Malpighii as a single layer of short, cylindrical cells; over these are
found more or less flattened, disk-shaped cells formed by transverse
division of the underlying cells of the rete. On the outside lies
the epitrichial layer which consists of a mosaic of polygonal cells,
near the middle of each of which lies an oval nucleus. Between the
epitrichial cells are small oval holes, not unlike the stomata in the
epidermis of plant tissues. Bronn thinks these are not artifacts, but
he does not suggest any explanation of their occurrence.

In the epidermis of young and half-grown animals the rete Malpighii
is seen still as above noted. On these cylindrical cells are found
flattened cells that gradually become very flat and lose their nuclei
as they pass over into the horny layer.

The stratum corneum consists of strongly flattened cells in which the
nuclei can no longer be clearly seen, though their location can usually
be determined by the groups of pigment granules. On the cells of the
more superficial layers of the stratum corneum are seen straight, dark
lines, perhaps ridges caused by pressure of the over- or underlying
polygonal cells. The individual cells of the horny layer are usually
easily isolated in the belly and neck regions where they never become
very thick; but in the back the cells in this layer are very numerous
and fuse with each other to form the bony plates; here the rete is the
only clearly differentiated layer. Whether prickle cells are present in
the epidermis of the crocodile Bronn is not certain, though he thinks
they probably are.

Rathke pointed out that on the surface of certain folds of the
integument, especially in the region of the jaws, are found in all
Crocodilia certain small, scattered, wart-like elevations, around
each of which is customarily a narrow, shallow, circular groove; they
usually have a dark brown but sometimes a gray or even white color.
Microscopic examination shows these warts to be of epidermal origin,
consisting of bright, round cells that are closely united, without
visible intercellular substance. Treatment with potassium hydroxid and
then with water will show sometimes, though not always, fine granular
nuclei in the cells.

In probably all members of the genus Crocodilus at least is found, on
the thick swelling on the right and on the left side of the neck and
trunk, a small, flat pit which has the appearance of the opening of
an integumental gland. The pits are present also in the scales of the
throat, under the side of the neck, sides of the body, lateral and
ventral surfaces of the anterior half of the tail, and the legs. They
are near the hinder border of the scales. Only occasionally are two
pits found in one scale. These pits are found in the gavials but are
absent in some, probably all, alligators. A small knob projects from
the center of some of the pits. These pits are not openings of glands
but have about the same structure as the pits seen in the head.

The integumental bones in the Crocodilia originate in the connective
tissue of the cutis. Investigations in young animals show that these
bones usually take their origin in the under and middle layers of the
cutis and generally work towards the periphery.

B. THE ENDOSKELETON

_I. The Vertebral Column._

[Illustration:

FIG. 16.--Skeleton of Crocodile. _D_, dorsal region; _L_, lumbar
region; _Sa_, sacral region; _Ri_, ribs; _Sc_, scapula; _H_, humerus:
_R_, radius; _U_, ulna; _Sta_, sternum abdominale: _Fe_, femur; _T_,
tibia; _J_, ischium; _C_, caudal vertebræ. (From Claus & Sedgwick.)]

The vertebral column consists of about sixty-five vertebræ, which
may be separated into the usual regions; there are nine cervical, ten
dorsal, five lumbar, two sacral, and about thirty-nine caudal. It is
likely that the number of caudals may be subject to frequent variation;
one complete skeleton had sixty-five vertebræ in all, another had
sixty-eight. A complete skeleton of the crocodile (species not known)
had sixty vertebræ. A thirteen-foot skeleton at Western Reserve
University had only sixty-one vertebræ, but some of the caudals were
evidently missing. Two skeletons of _C. porosus_ in the museum at
Singapore had sixty and sixty-three vertebræ respectively. A skeleton
of _Tomistoma schlegeli_ in the same museum had sixty vertebræ.

_The Cervical Vertebræ._ Since all of the cervical vertebræ bear ribs,
we shall assume the distinction between them and the dorsal vertebræ
to be that the ribs of the latter meet the sternum, while those of the
former do not reach to the sternum. Assuming this distinction, there
are, as was said above, nine cervical vertebræ.

[Illustration: FIG. 17. FIRST FOUR CERVICAL VERTEBRÆ OF A CROCODILE

(_C. vulgaris_). (From Reynolds, partly after Von Zittel.)

1. pro-atlas.
2. lateral portion of atlas.
3. odontoid process.
4. ventral portion of atlas.
5. neural spine of axis.
6. postzygapophysis of fourth vertebra.
7. tubercular portion of fourth cervical rib.
8. first cervical rib.
9. second cervical rib.
10. convex posterior surface of centrum of fourth vertebra.]

With the exception of the first two, to be discussed later, these are
all essentially alike and the fourth will be described as a type (Fig.
17). Its centrum is cylindrical or somewhat hourglass shaped, concave
anteriorly and convex posteriorly; it is not completely fused with the
neural arch but is united with it by sutures. From the anterior end
of the ventral surface of the centrum projects downward and forward
a small plow-shaped process, the hypapophysis. On each side of the
centrum, near its anterior end, is a facet with which the lower branch
(capitulum) of the rib articulates. The neural arch is strongly
developed and is extended dorsally into a prominent neural spine and
on each side as a short, blunt, transverse process with which the
tubercle or upper branch of the rib articulates. Posteriorly the arch
is notched on each side to form the openings for the exit of the spinal
nerves. Projecting dorsally and anteriorly from the arch are two short
processes which bear the medially and dorsally facing prezygapophyses
(Fig. 17). Just caudad to these processes are somewhat shorter
processes that bear the laterally and ventrally facing postzygapophyses
(Fig. 17, 6).

The _atlas_, as in other vertebrates, is highly specialized. It
consists (Fig. 17), even in the adult animal, six feet or more in
length, of four distinct portions, a ventral (4), a dorsal (1), and two
lateral (2) parts. The ventral portion is relatively more massive than
in most animals; its anterior surface is concave and forms the main
part of the articular surface for the occipital condyle of the skull.
Its postero-dorsal surface articulates with the odontoid process of the
axis. On its postero-lateral surfaces are the facets for articulation
with the first ribs, which, unlike the other cervical ribs, have but
one articular surface. Articulating dorsally with this ventral element
of the atlas are the two rather heavy lateral elements which form the
neural arch. Anteriorly they form the lateral parts of the articular
surface for the condyle and dorsally they unite for a short distance
with each other. Projecting ventrally from the posterior part of their
dorsal portion are the small postzygapophyses. Ventrally and laterally
they articulate with the odontoid process (Fig. 17, 3). Projecting
dorsad and cephalad from the dorsal surface of these lateral elements
is the dorsal element of the atlas (Fig. 17, 1), the pro-atlas, which
may not be properly a part of the vertebral column at all, since it is
said to be merely a membrane bone. Gadow says it is the detached neural
spine of the atlas. It is thin and triangular in shape, resembling in
contour a large, mammalian epiglottis. It forms an arch over the space
between the skull and the front of the atlas proper. Reynolds calls it
the pro-atlas.

_The Axis._ The centrum differs from those following it (described
above) mainly in its close articulation (not fusion) with the large
odontoid process; this process not only projects into the atlas,
as is usually the case, but articulates with its postero-lateral
border on each side, and is distinctly visible in a lateral view of
the neck (Fig. 17, 3). Like the rest of the cervical vertebræ the
posterior surface of the centrum is convex. The neural arch of the
atlas differs from those following mainly in having a much wider (in
an antero-posterior direction) neural spine. The lateral processes and
those bearing the prezygapophyses are also less strongly developed than
on the following vertebræ.

[Illustration: FIG. 18. ANTERIOR VIEW OF A, A LATE THORACIC AND B, THE
FIRST SACRAL VERTEBRA OF A YOUNG CROCODILE

(_C. palustris_). × ¹⁄₃. (After Reynolds.)

1. neural spine.
2. process bearing prezygapophysis.
3. facet for articulation with the capitulum of the rib.
4. sacral rib.
5. surface which is united with the ilium.
6. concave anterior face of centrum.]

_The Thoracic Vertebræ._ The first thoracic vertebra differs scarcely
at all from the ninth cervical; and the tenth thoracic differs from the
first lumbar only in bearing a short rib. Only the first three thoracic
centra bear the hypapophyses noted in connection with the cervical
vertebræ. The ribs of the first two thoracic vertebræ articulate with
them by two processes, as in the typical cervical vertebræ; the other
ribs articulate only with the transverse process. The fourth thoracic
may be described as a type of this region (Fig. 18, A). Its centrum
is rather longer than in the first two thoracic and in the cervical
vertebræ and has no process for articulation with the head of the rib,
otherwise it is essentially the same. Like all of the vertebræ behind
it and unlike those in front it is apparently completely fused with
its neural arch. The neural arch is very broad (in an antero-posterior
direction) and is extended dorsally as a wide neural spine (1). The
neural spines of the following thoracic and the first two or three
lumbar vertebræ are increasingly broad and truncated. The transverse
processes are very broad, long and thin, and in the third to eighth
vertebræ they have two articular surfaces, an anterior and more medial
one for articulation with the head of the rib (3) and a posterior and
more distal one for articulation with the tubercle of the rib. These
two surfaces approach each other as the vertebræ are followed caudad
until, in the last two thoracic vertebræ, they form practically one
surface. The processes of the pre- and postzygapophyses spring from
the arch at the base of the transverse process; the former surface is
directed dorsally and medially, the latter ventrally and laterally. The
intervertebral foramina are smaller and more nearly circular than in
the cervical region, and are more closely surrounded by bone.

_The Lumbar Vertebræ._ The five lumbar vertebræ are essentially like
the thoracic except that the transverse processes, which, of course,
bear no ribs, are both shorter and narrower. The postero-lateral border
of the centrum of the last of these five vertebræ has a small surface
for articulation with the antero-medial border of the transverse
process of the first sacral vertebra.

_The Sacral Vertebræ_ (Fig. 18, B). These are two in number. The
centrum of the first is concave in front and flat behind, instead of
being convex behind, and the second is flat (instead of concave) in
front, and convex behind. The neural spine and zygapophyses are as in
the lumbar region. Projecting laterally from each sacral vertebra,
forming a close, sutural joint with both centrum and neural arch, is a
heavy bone shaped like a truncated pyramid (4); the base of the pyramid
is ankylosed with the ilium. These bones seem to be much thickened
transverse processes, but since they are not completely fused with
their respective vertebræ and are said to ossify separately they should
probably be called sacral ribs. The two sacral vertebræ do not seem to
be any more closely united than are any other two vertebræ.

_The Caudal Vertebræ_ (Fig. 16, _C_). These are characterized by the
entire absence of ribs, and by the presence on all but the first
and the last four or five of V-shaped chevron bones. The first
ten or twelve of these chevron bones articulate chiefly with the
postero-ventral ends of the centra, but they also articulate with the
antero-ventral ends of the vertebra behind themselves; and as they are
followed caudad they seem to lie directly below the intervertebral
regions and to articulate equally with the vertebræ before and behind.
The chevron bones gradually diminish in size from before back. The
neural processes of the first four or five caudals are broad, like
those of the more anterior regions, but caudad to this point they
become narrower and more pointed, though they retain the same height
until about the last ten or twelve vertebræ. Towards the tip of the
tail the dorsal spines diminish in height and finally disappear. The
transverse processes of the first five or six of the caudals are long
and narrow. They gradually diminish in length until the eighteenth
caudal, back of which they are no longer to be seen. The zygapophyses
are mostly about the same as in the more anterior vertebræ, but towards
the posterior end of the tail the postzygapophyses come to lie between
rather than above the prezygapophyses. The neural canal diminishes,
of course, in size towards the tip of the tail until it is no longer
present, the last five or six vertebræ consisting only of the centra.

_II. The Skull._

The skull of the alligator is very massive and has several
peculiarities. 1. The bones of the dorsal surface are rough and
pitted, especially in old animals. 2. The jaws are enormously large in
proportion to the brain cavity, and are armed with many large teeth. 3.
The mandibular articulation is some distance caudad to the occipital
condyle. 4. The interorbital septum is mainly cartilaginous. 5. There
is a complicated system of Eustachian passages connecting with the back
of the mouth by a single opening. 6. The posterior nares are placed
very far back and the palate is correspondingly long.

The skull as a whole may be divided into three regions: the cranium,
the lower jaw, and the hyoid; these will be described in the order
given.

_The Cranium._ As a matter of convenience the bones will be described
as seen from the different aspects--dorsal, ventral, lateral,
posterior, and in sagittal section--without particular regard to their
grouping into segments or regions.

_The Dorsal Aspect_ (Fig. 19). At the extreme posterior end of the
median line lies the _parietal_ (23), double in the embryo but a
single bone in the adult. It forms a part of the roof of the cranial
cavity and articulates anteriorly with the frontal, laterally
with the postfrontals, squamosals, and, according to Reynolds,
with alisphenoids, pro-otics and epiotics, and ventrally with the
supraoccipital. It forms the median boundary of each of the two
_supratemporal fossæ_ (sf).

On each side of the parietal and forming the posterior comers of the
rectangular postero-dorsal region of the skull are the _squamosals_
(7). Each squamosal articulates medially with the parietal, anteriorly
with the postfrontal, and ventrally with the quadrate and exoccipital.
It forms part of the posterior and lateral boundaries of the
supratemporal fossa and a part of the roof of the external auditory
meatus.

Articulating with the anterior border of the squamosals and forming
the anterior corners of the rectangular region mentioned above are
the _postfrontals_ (6). The postfrontal articulates medially with the
parietal and frontal, and ventrally with the alisphenoid and a small
part of the quadrate. It sends, in a ventro-lateral direction, a thick
process that unites with a similar process from the jugal to form
the _postorbital bar_ (pb) which lies between the orbit (o) and the
temporal fossa (tf). The postfrontal forms the antero-lateral boundary
of the supratemporal fossa.

Articulating posteriorly and laterally with the parietal and the
postfrontals, and forming the highest point of the skull, is the single
_frontal_ bone (24), which, like the parietal, is paired in the embryo.
It is a heavy bone whose dorsal surface is flattened posteriorly,
deeply concave in the middle region, and drawn out into a long
projection anteriorly. It forms part of the roof of the cranial cavity
and articulates ventro-laterally with the alisphenoid and anteriorly
with the prefrontals and nasals. It forms a part of the median boundary
of the orbit.

The _prefrontal_ (4) is an elongated bone in the latero-median border
of the orbit. Medially and anteriorly it articulates with the frontal
and nasal, laterally with the maxillary and lachrymal, and ventrally,
by a heavy process, with the pterygoid.

[Illustration: FIG. 19. DORSAL VIEW OF THE SKULL OF THE ALLIGATOR (_A.
Mississippiensis_).

1. premaxilla.
2. maxilla.
3. lachrymal.
4. prefrontal.
5. jugal.
6. postfrontal.
7. squamosal.
8. quadrate.
12. quadratojugal.
23. parietal.
24. frontal.
25. nasal. _an_, anterior nares; _o_, orbit; _pb_, postorbital bar;
_sf_, supratemporal fossa; _tf_, lateral temporal fossa.]

The _nasal_ (25) is a long narrow bone forming the greater part of
the roof of the nasal passage. Along the median line of the skull it
articulates with its fellow; posteriorly with the frontal; laterally
with the prefrontal and maxillary; and anteriorly with the premaxilla.
In the crocodile, caiman, and gavial it also articulates with the
lachrymal. In the alligator the anterior ends of the two nasals form a
narrow rod of bone that extends across the anterior nares, and, meeting
a projection from the premaxillaries, divides the opening into right
and left halves. In the crocodile the nasals project only a very little
way into the nares; in the caiman (according to Reynolds) they do not
extend into the nares at all, and in the gavial, whose much elongated
snout is mainly due to the great length of the maxillaries, the nasals
do not extend more than a third of the distance from the prefrontals to
the anterior nares.

The _maxilla_ (2) is a large bone that forms a large part of the upper
jaw and that holds most of the teeth of that jaw. On the ventral
side, as will be described later, it articulates with its fellow in
the middle line, with the premaxilla, with the palatine, and with
the transpalatine. Dorsally it articulates with the premaxilla in
front; with the nasal and prefrontal on the medial side; and with the
lachrymal and jugal behind.

The _premaxilla_ (1) forms, with its fellow, the extreme tip of the
upper jaw. Each bone forms the anterior and lateral borders of its half
of the anterior nares. It articulates medially with its fellow and
posteriorly with the nasal and maxilla. Ventrally, as will be noted
later, it bears five teeth and articulates with its fellow medially and
with the maxilla posteriorly. Between the premaxillæ on the ventral
side is the large anterior palatine foramen.

The _lachrymal_ (3) is a fairly large bone that forms the anterior
border of the orbit. It is bounded laterally by the jugal, anteriorly
by the maxilla, and medially by the prefrontals. Its postero-medial
border is pierced by a large lachrymal foramen that extends lengthwise
through the bone and opens, at its anterior end, into the nasal chamber.

The _supraorbital_, missing in the skull figured, is a small bone lying
in the eyelid close to the junction of the frontal and prefrontal.
Being unattached it is usually absent from prepared skulls.

The _jugal_ or _malar_ (5) is an elongated bone that forms a part
of the lateral border of the head, on the one hand, and most
of the lateral border of the orbit on the other. Anteriorly it
articulates with the maxilla; medially with the lachrymal and
prefrontal; posteriorly with the quadratojugal, and ventrally with
the transpalatine. With the transpalatine it sends, in a dorso-medial
direction, a process that meets the process, described above in
connection with the postfrontal, to form the postorbital bar.

The _quadratojugal_ (12) is a small bone, wedged in between the jugal
in front and the quadrate behind.

[Illustration: FIG. 20. VENTRAL VIEW OF THE SKULL OF THE ALLIGATOR (_A.
Mississippiensis_).

2. maxilla.
5. jugal.
8. quadrate.
9. palatine.
10. pterygoid.
11. transpalatine.
12. quadratojugal.
14. basioccipital. _a_, anterior palatine vacuity; _eu_, opening of
the median Eustachian canal; _pn_, posterior nares; _pv_,
posterior palatine vacuity.]

The _quadrate_ (8) is more irregular and has more complicated
articulations than almost any bone in the skull. Its posterior end,
which forms the articular surface for the lower jaw, is elongated
laterally and slightly concave. Anteriorly the quadrate articulates
with the quadratojugal; medially with the basisphenoid and exoccipital;
dorsally with the exoccipital, squamosal, postfrontal, and, possibly,
with the pro-otic; ventrally with the pterygoid, alisphenoid, and
probably with some of the otic bones. Its dorsal side forms most of
the floor of the external auditory meatus which will be described
later. While the basioccipital may be seen from the dorsal side, it is
not really one of the dorsal bones of the skull and will be described
later; the same is true of the pterygoids and palatines which may be
seen through the empty orbits.

_The Ventral Aspect_ (Fig. 20). The larger part of this side of the
skull is made up of four pairs of bones: the premaxillæ, the maxillæ,
the palatines, and the pterygoids, lying, from anterior to posterior,
in the order named.

The _premaxilla_ (1), as described in the dorsal view of the skull, is
a triangular bone which, with its fellow, forms the anterior end of
the snout. Each premaxilla bears five teeth, not only in the alligator
but in the crocodile, the caiman, and in the gavial. Of these teeth
the fourth from the front is the largest; the first two are small,
and the third and fifth are of intermediate size. This arrangement as
to size is also true, apparently, in the other groups of Crocodilia.
The ventral surface of the premaxilla, which is more or less flat and
horizontal, is pierced by a number of small foramina, in a row parallel
to the curved outer margin of the bone. Between these foramina and the
base of the teeth are four rounded depressions to receive the points of
the first four teeth in the lower jaw; of these depressions the first
and fourth are the deepest. The first pit often becomes so deep as to
perforate the bone; this is true also with the crocodile and, according
to Reynolds, with the caiman, but is not true of the gavial, whose
interlocking teeth project outside of the jaws. It will be remembered
that one of the chief distinctions, given early in this work, between
the crocodile and the alligator is that in the former the fourth tooth
in the lower jaw fits into a notch and not into a pit in the upper jaw.

The _maxilla_ (2), which with its fellow forms most of the hard palate,
has also been mentioned in connection with the dorsal aspect. Each
maxilla is notched, posteriorly, to form the anterior border of the
posterior palatine vacuity, and together they are notched to receive
the rectangular anterior ends of the palatines. The postero-lateral
extremity of the maxilla articulates with the transpalatine. Along the
outer border of the bone are the teeth, of which there are fifteen or
sixteen in the alligator, about the same number (perhaps one or two
less) in the caiman and crocodile, and about twenty-four in the gavial.
The first or anterior eight or ten teeth have individual sockets, the
rest are placed in a groove. In the crocodile none of the teeth have
individual sockets, and in the gavial they all have sockets. The
premaxillary and more anterior of the maxillary teeth are slightly
recurved and are sharper than the posterior maxillaries which besides
being blunt have a constriction above the surface of the socket.

The crocodilian tooth consists of three layers (Fig. 20 A).

The enamel (e) forms a fairly thick layer over the crown of the tooth;
it exhibits a very clear striated structure, the striations being
apparently due to stratification.

Some of the tubules of the dentine (d) continue into the enamel, where
they may be distinguished by their remarkable fineness and their
straight course.

The cement (c) covers the root of the tooth that projects into the
alveolus of the jawbone; it is much more strongly developed than in
the lizards and contains a very large number of bone corpuscles which
are distinguished from the bone corpuscles proper by their greater
circumference.

The fairly large pulp cavity (p) has, like the tooth itself, a conical
form.

Parallel to the teeth is a row of small foramina, a continuation of
those noted in the premaxilla; some or all of these foramina open
into a longitudinal sinus along the alveolar border of the maxilla;
this sinus opens posteriorly by one or more large apertures into the
posterior palatine vacuity.

The _palatines_ (9) form a broad bar of bone from the pterygoids
behind to the maxillæ in front. They are united with each other by a
straight median suture and form a considerable part of the floor as
well as a part of the side walls and roof of the nasal passage. They
form most of the median boundaries of the posterior palatine vacuities
(pv). Dorsally they articulate with the pterygoids, prefrontals, and
vomers.

The _pterygoids_ (10) are the very irregular bones that project ventrad
and caudad from beneath the orbits. Their suture is continuous, caudad,
with that between the palatines and at the posterior end of this
suture is the posterior opening of the nasal chamber, the posterior
nares (pn). This opening is divided by a vertical, longitudinal, bony
septum, and the part of the chamber into which it immediately opens,
which lies in the pterygoids, is divided by a number of transverse,
vertical septa. Posterior and dorsal to the posterior nares the
pterygoids are fused. Anteriorly the pterygoids articulate with the
palatines; dorsally with the quadrates, basisphenoid, alisphenoids, and
prefrontals, and dorso-laterally with the transpalatines. The lateral
vertical border of the pterygoid is roughened and is, according to
Reynolds, covered, during life, with a pad of cartilage against which
the medial side of the mandible plays.

The _transpalatine_ (11) is a T-shaped bone articulating ventrally
with the pterygoid and dorsally with the maxilla, the jugal, and the
postfrontal.

[Illustration: FIG. 20A. LONGITUDINAL SECTION OF THE JAW AND TOOTH OF A
CROCODILE.

(After Bronn.)

_c_, cement; _d_, dentine; _e_, enamel; _p_, pulp, of functional
tooth; _c′_, cement; _d′_, dentine; _e′_, enamel, of rudimentary
tooth; _e″_, epidermis; _k_, bone of jaw.]

The _basioccipital_ (14) is seen projecting caudad as the single
occipital condyle; it will be described in connection with the
posterior aspect of the skull.

The _jugal_ (5), _quadratojugal_ (12), and _quadrate_ (8) may all be
seen from this view. The first two have been sufficiently described in
connection with the dorsal aspect; the last will be further described
in connection with the lateral aspect.

Just caudad to the posterior nares is a small opening, the unpaired
Eustachian canal (eu).

_The Lateral Aspect_ (Fig. 21). As will be seen by the figure,
practically all of the bones visible in this view have already been
described, except those of the mandible, which will be described
separately. At the base of the skull are, however, two bones, the basi-
and alisphenoid, that have not been described and that show as well
in this as in any other view. The _basisphenoid_ (just below v and
hidden by the pterygoid) was mentioned in connection with the quadrate,
with whose posterior margin it articulates. It is an unpaired bone
of very irregular shape. Anteriorly it is flattened out to form the
rostrum, a rectangular process that forms the posterior part of the
interorbital septum; in fact it is the only part of the septum present
in a prepared skull, since the rest is cartilaginous. Dorso-laterally
the basisphenoid articulates with the alisphenoid; posteriorly with
the basioccipital; ventrally with the pterygoid; and posteriorly with
the exoccipital and basioccipital. On the dorsal surface of the
basisphenoid is the pituitary fossa, not seen, of course, in this view
of the skull.

The _alisphenoids_ (crossed by the dotted line from V) are a pair of
very irregular bones that form most of the antero-lateral walls of
the brain case. They articulate dorsally with the parietal, frontal,
and postfrontal; ventrally with the basisphenoid and pterygoid; and
posteriorly with the quadrate and some of the otic bones not visible in
this view. Between it and the quadrate, plainly visible in this view,
is a large opening, the _foramen ovale_ (V), through which, according
to Reynolds, the trigeminal nerve passes. In the middle line, directly
under the frontal bone, is an opening between the anterior wings of
the two alisphenoids, for the exit of the optic nerves. Ventrad and
caudad to this opening, and sometimes continuous with it, is another
large foramen, just dorsad to the rostrum, for the exit, according to
Reynolds, of the oculomotor and abducens nerves. Projecting caudad
is seen the rounded condylar part of the _basi-occipital_ (14) to
be described later, and dorso-cephalad to this is a part of the
exoccipital (13) in which four foramina may be seen; of the dorsal
three the one nearest the condyle and foramen magnum is for the exit
(Reynolds) of the hypoglossal nerve (XII); slightly dorso-cephalad to
this is one for the vagus nerve (X); between these two is a very small
one for a vein; the largest and ventrally located foramen is for the
entrance of the internal carotid (15). Another large foramen in the
exoccipital bone will be seen and described in connection with the
posterior view of the skull. Dorsal to the quadrate and largely bounded
by it is the wide _external auditory meatus_ (16), which leads into the
tympanic cavity. This cavity is complicated by a number of canals that
lead from it in various directions. Overhanging the cavity and meatus
is the squamosal bone, described in connection with the dorsal aspect
of the skull.

[Illustration: FIG. 21. LATERAL VIEW OF THE SKULL OF AN ALLIGATOR
(_Caiman latirostris_). ×¹⁄₃. (Brit. Mus.)

(After Reynolds.)

1. premaxilla.
2. maxilla.
3. lachrymal.
4. prefrontal.
5. jugal.
6. postfrontal.
7. squamosal.
8. quadrate.
9. palatine.
10. pterygoid.
11. transpalatine.
12. quadratojugal.
13. exoccipital.
14. basi-occipital.
15. foramen by which carotid artery enters skull.
16. external auditory meatus.
17. frontal.
18. supra-angular.
19. articular.
20. dentary.
21. coronoid.
22. angular.
III, VI, opening for exit of oculomotor and abducens nerves.
V, foramen ovale.
X, pneumogastric foramen.
XII, hypoglossal foramen.]

_The Posterior Aspect_ (Fig. 22). Most of the bones seen in this
view have already been described. The _pterygoids_ (10) form the two
prominent, ventro-lateral projections, while dorsal to these is the
large process formed by the quadrate (8) and quadratojugal (12). The
dorsal margin is formed by the edges of the parietal (23) and the
squamosals (7). Immediately below the parietal is the supraoccipital
(26); it is a small, triangular bone, articulating above with the
parietal and squamosals, below with the exoccipitals, and anteriorly
with the epiotic. It takes no part in the formation of the foramen
magnum.

The _exoccipitals_ (13) form the entire boundary of the foramen
magnum except the narrow ventral portion formed by the basioccipital.
Each exoccipital is a wing-shaped bone, articulating dorsally with
the squamosal and supraoccipital, ventrally with the quadrate,
basioccipital, and basisphenoid, and anteriorly with the opisthotic. It
is pierced by five foramina, four of which were described in connection
with the lateral view. Some distance laterad and somewhat dorsad to
the pair already described is the fifth and largest foramen (VII); it
really lies between the exoccipital and quadrate, but the former bone
forms almost its entire boundary; through it, according to Reynolds,
pass the seventh nerve and certain blood-vessels.

[Illustration: FIG. 22. POSTERIOR VIEW OF THE SKULL OF A.
MISSISSIPPIENSIS.

7. squamosal.
8. quadrate.
10. pterygoid.
11. transpalatine.
12. quadratojugal.
13. exoccipital.
14. basioccipital.
15. foramen for carotid artery.
23. parietal.
26. supraoccipital.
VII. foramen for 7th nerve and certain blood-vessels. _bs_, position
of basisphenoid.]

[Illustration: FIG. 23. LONGITUDINAL SECTION THROUGH THE SKULL OF AN
ALLIGATOR (_Caiman latirostris_). ×¹⁄₃. (Brit. Mus.) (After Reynolds.)

1. premaxilla.
2. nasal.
3. frontal.
4. parietal.
5. supra-occipital.
6. epiotic.
7. prootic.
Immediately in front of the figure 7 is the prominent foramen for
the trigeminal nerve.
8. opisthotic.
9. basioccipital.
10. quadrate.
11. pterygoid.
12. basisphenoid.
13. alisphenoid.
14. prefrontal.
15. vomer.
16. maxilla.
17. palatine.
18. dentary.
19. splenial.
20. angular.
21. supra-angular.
22. articular.
23. coronoid.
24. exoccipital.
25. squamosal.
26. jugal.
27. external mandibular foramen.
28. internal mandibular foramen.
VIII. internal auditory meatus.
XII. hypoglossal foramen.]

[Illustration: FIG. 24. DORSAL VIEW OF LOWER JAW OF ALLIGATOR (_A.
Mississippiensis_).

18. dentary.
19. splenial.
21. supra-angular.
22. articular.
23. coronoid.]

The _basioccipital_ (14) which, as has been said, forms a small part
of the ventral wall of the foramen magnum, consists of a heavy dorsal
portion, the ventrally curved condyle, and of a broader, irregular
ventral portion, between which and the basisphenoid is the single
opening of the Eustachian canals (eu). Dorsally and laterally the
basioccipital articulates with the exoccipitals; ventrally, laterally,
and anteriorly with the basisphenoid which was described in the lateral
view.

_The Sagittal Section_ (Fig. 23). The only bones shown in this figure
(besides those of the mandible, to be described later) that have
not already been described are the vomers and those of the auditory
capsules.

The _vomers_ (15) are delicate bones articulating with the maxillæ, the
palatines, the pterygoids, and with each other. They form a part of the
septum and roof of the nasal passage.

The _mesethmoid_ is not ossified.

Reynolds describes the bones of the auditory capsules as follows:

“Three bones, the _epiotic_, _opisthotic_, and _pro-otic_, together
form the auditory or _periotic_ capsule of each side. They are wedged
in between the lateral portions of the occipital and parietal segments
and complete the cranial wall in this region. Their relations to the
surrounding structures are very complicated, and many points can be
made out only in sections of the skull passing right through the
periotic capsule. The relative position of the three bones is, however,
well seen in a median longitudinal section. The _opisthotic_ early
becomes united with the exoccipital, while the _epiotic_ similarly
becomes united with the supraoccipital, the pro-otic (Fig. 23, 7)--seen
in longitudinal section to be pierced by the prominent _trigeminal
foramen_--alone remaining distinct throughout life. The three bones
together surround the essential organ of hearing which communicates
laterally with the deep tympanic cavity by the _fenestra ovalis_.

“The _tympanic cavity_, leading to the exterior by the _external
auditory meatus_ (Fig. 21, 16), is well seen in a side view of
the skull; it is bounded on its inner side by the periotic bones,
posteriorly in part by the exoccipital, and elsewhere mainly by the
quadrate. A large number of canals and passages open into it. On its
inner side opening ventro-anteriorly is the _fenestra ovalis_, opening
ventro-posteriorly the _internal auditory meatus_ (Fig. 23, VIII),
while dorsally there is a wide opening which forms a communication
through the roof of the brain case with the tympanic cavity of the
other side. On its posterior wall is the prominent foramen through
which the facial nerve passes on its way to its final exit from the
skull through the exoccipital; this foramen is bounded by the quadrate,
squamosal, and exoccipital. The opening of the _fenestra ovalis_ is in
the fresh skull occupied by the expanded end of the auditory ossicle,
the _columella_, whose outer end articulates by a concave facet with a
trifid _extracolumellar_ cartilage which reaches the tympanic membrane.
The lower process of this extracolumella passes into a cartilaginous
rod which lies in a canal in the quadrate and is during life continuous
with Meckel’s cartilage within the articular bone of the mandible.

“The columella and extracolumella are together homologous with the
chain of mammalian auditory ossicles.”

_The Lower Jaw_ (Figs. 21, 23, and 24). The mandible consists of two
similar rami, rather closely united at the anterior-median symphysis
with each other. Each ramus consists of six bones.

The _dentary_ (Figs. 23 and 24, 18; Fig. 21, 20) is a long bone that
unites at the symphysis with its fellow to form the point of the
jaw. It bears, along its dorsal edge, about twenty teeth; all but
the posterior four or five of these teeth are in individual sockets;
this may vary somewhat with age. The outer surface of the dentary,
especially towards the symphysis, is covered with numerous, small,
deep pits, while along its inner side, parallel to the row of teeth,
is a row of somewhat larger pits like those noted in the maxilla and
premaxilla. Articulating with the mesial side of the dentary along the
greater part of its length is a flat bone, the _splenial_ (Figs. 23 and
24, 19); between these two bones is a long cavity that makes the ramus
hollow almost to the symphysis. A large foramen, not shown in any of
the figures, leads through the splenial into this cavity.

Articulating with the caudal end of the splenial and forming the
anterior border, as seen from the mesial side, of the large _external
mandibular foramen_ (Fig. 23, 27) is a small bone, the _coronoid_
(Figs. 23 and 24, 23, Fig. 21, 21); it articulates with the splenial
anteriorly, with the supra-angular dorso-caudally, and with the angular
ventrally.

The _supra-angular_ (Figs. 23 and 24, 21, Fig. 21, 18) is an elongated
bone that forms the dorsal border of the external mandibular foramen;
it also forms the lateral edge of the articular surface for the
quadrate. It articulates anteriorly with the splenial, the dentary, and
the coronoid; and posteriorly with the angular and articular.

The _articular_ (Figs. 23 and 24, 22, Fig. 21, 19), which is scarcely
visible in a lateral view, forms most of the surface for articulation
with the quadrate, and sends back the large process so characteristic
of the crocodilian skull. On the dorsal side of this process is a
concavity that looks like another articular surface. Laterally the
articular articulates with the supra-angular; ventrally and posteriorly
with the angular.

The _angular_ (Fig. 23, 20, Fig. 21, 22) forms the ventro-posterior
border of the jaw and of the external mandibular foramen. Its narrow,
posterior end forms a part of the prominent process mentioned in
connection with the articular. Between it and the posterior edge of
the splenial is the _internal mandibular foramen_, which is much
smaller than the external (Fig. 23, 28). Anteriorly the angular
articulates with the dentary, coronoid, and splenial; dorsally with the
supra-angular and the articular.

_The Hyoid_ (Fig. 25). The hyoid being mainly of cartilage is usually
not seen in prepared skeletons. It is thus described by Reynolds:

“The hyoid of the Crocodile consists of a wide flattened plate of
cartilage, the _basilingual plate_ or _body of the hyoid_, and a pair
of _cornua_.

“The _basilingual plate_ (Fig. 25, 1) is rounded anteriorly and marked
by a deep notch posteriorly. The _cornua_ (Fig. 25, 3), which are
attached at a pair of notches near the middle of the outer border of
the basilingual plate, are partly ossified, but their expanded ends are
formed of cartilage. They pass at first backwards and then upwards and
inwards. They are homologous with part of the first branchial arches of
Selachians.”

[Illustration: FIG. 25. HYOIDS OF AN ALLIGATOR (_Caiman latirostris_)
(TO THE LEFT) AND OF A GREEN TURTLE (_Chelone midas_) (TO THE RIGHT).
×⁵⁄₈. (Brit. Mus.) (After Reynolds.)

The cartilaginous portions are dotted.

1. basilingual plate or body of the hyoid.
2. hyoid arch.
3. first branchial arch (anterior cornu).
4. second branchial arch (posterior cornu).]

_III. The Ribs and Sternum._

_The Cervical Ribs._ As noted above, all of the cervical vertebræ
possess ribs. The first rib, attached to the atlas, consists of a
single, long blade projecting backward at an acute angle (Fig. 17, 8)
as far as the middle of the fourth vertebra. As described above it
articulates with the atlas at but one place. All of the other cervical
ribs have two articular surfaces, a tuberculum and a capitulum, with a
well-marked vertebrarterial canal between them. The ventral surface or
capitulum articulates with a short process on the centrum; the dorsal
surface or tuberculum (7) articulates with the transverse process. The
third to seventh ribs are somewhat T-shaped, the stem of the T being
the tubercle and head, while the cross arm of the T extends parallel to
the axis of the neck (Fig. 17, 7). In the eighth rib the posterior arm
of the T is elongated and projects out at a wide angle from the body;
and in the ninth or last cervical rib this arm extends laterally as far
as the vertebral portion of the thoracic ribs and has a cartilaginous
tip.

_The Thoracic Ribs_ (Figs. 16 and 26). These are ten in number, the
first eight pairs being connected with the sternum. The fourth may be
taken as typical. It consists of a bony vertebral portion and partially
ossified intermediate and sternal portions. The vertebral portion
articulates with its corresponding transverse process by two surfaces,
as described in connection with the thoracic vertebræ. In the first
and second ribs only the tuberculum articulates with the transverse
process, the head having a separate articular surface on the side of
the centrum, as in the typical cervical rib. In the last thoracic rib
the head and tubercle are not distinguishable from each other. Near
the distal end of all the vertebral portions except the first and the
last two ribs is a caudally projecting, partially ossified, uncinate
process. The intermediate portion is present in all but the tenth rib,
and wherever present, except in the ninth rib, it articulates distally
with the sternal portion. The sternal portions extend medio-cephalad in
a direction at right angles to the intermediate portion; the first two
articulate with the sternum, the next six with the xiphisternal horns,
and the ninth and tenth are missing.

[Illustration: FIG. 26. STERNUM AND ASSOCIATED MEMBRANE BONES OF A
CROCODILE (_C. palustris_). ×¹⁄₃. (Brit. Mus.) (After Reynolds.)

The last pair of abdominal ribs which are united with the epipubes by
a plate of cartilage have been omitted.

1. interclavicle.
2. sternum.
3. sternal rib.
4. abdominal splint rib.
5. xiphisternal horn.]

_The Abdominal Ribs_ (Fig. 26, 4). While these ribs are membrane
bones and are not homologous with the other ribs, they may as well be
mentioned at this time. They consist of about seven V-shaped sets of
slender bones, the point of each V being directed cephalad. Each V is
made up of from two or five slender bones, the number and arrangement
being subject to considerable variation. The last V of the series
(not well shown in the figure) is considerably larger than the rest
and is made up of four curved bones that extend around the anterior
ends of the pubic bones and are united to them by a broad tough
membrane. The first or most anterior V is united by a narrow membrane
(not shown in the figure) with the membrane that extends between the
xiphisternal horns. All of the V’s are more or less connected with each
other by fibrous membranes. Since these ribs lie superficial to the
recti muscles of the ventral body wall they are sometimes missing in
carelessly prepared skeletons.

_The Sternum_ (Fig. 26). The sternum consists of the cartilaginous
sternum proper (2), the xiphisternal horns (5), and the bony episternum
or interclavicle (1). The latter is an elongated, flattened bone of
somewhat spatulate outline, lying in the midventral line; it projects
forwards to about the sixth cervical vertebra, while the anterior edge
of the sternum is below the eighth cervical. Lying dorsal and lateral
to the episternum is the flat, almost membranous sternum, to the
posterior border of which the first two thoracic ribs are attached.
The xiphisternum consists of two long, slender rods of cartilage; the
anterior ends of these rods are in contact with each other and with the
posterior border of the sternum; from this point they gradually diverge
from each other as they extend caudad. A membrane extends between the
horns as far back as the attachment of the last thoracic ribs.

_IV. The Appendicular Skeleton._

_The Pectoral Girdle_ and _Anterior Limb_. The pectoral girdle (Fig.
27) is of a very simple type, consisting, unless the episternum
(interclavicle) be counted, of but two bones, the _scapula_ (_s_) and
_coracoid_ (_c_). The former consists of an upper, flat, paddle-shaped
portion and a thicker lower portion which articulates anteriorly with
the coracoid, and posteriorly forms about half of the notch-like
glenoid cavity. The dorsal edge of the flattened portion is continued
as a small, cartilaginous suprascapula. The coracoid is a flattened
bone, wide at either end and narrow in the middle, so that in a dorsal
view it is shaped like an hourglass. It is decidedly curved, with the
convex side down. Its outer edge articulates with the scapula and is
thickened to form the anterior border of the glenoid cavity. Its median
end is attached to the sternum. Near its scapular articulation there
is a well-marked foramen that passes entirely through the bone. The
_episternum_ (_e_) or interclavicle was described in connection with
the sternum and ribs. There is no clavicle nor other coracoid elements.

[Illustration: FIG. 27. PECTORAL GIRDLE AND ANTERIOR LIMB.

_c_, coracoid; _ce_, centrale; _cl_, claw; _e_, episternum; _h_,
humerus; _m_, metacarpals; _p_, pisiform; _r_, radius; _r′_, radiale;
_s_, scapula; _u_, ulna; _u′_, ulnare.]

The _anterior limb_ consists of the usual parts,--the upper arm,
forearm, and manus. The _humerus_ (Fig. 27, _h_) is rather thick in
proportion to its length; it has an elongated articular surface at
its proximal end for articulation with the glenoid cavity, and a
larger, somewhat bilobed surface for articulation with the radius
and ulna. On its ventral side, near the proximal end, is a very
prominent protuberance, the deltoid ridge. The _ulna_ (_u_) is slightly
heavier and longer than the radius and forms the greater part of the
elbow joint and about half of the wrist joint. Its proximal end is
considerably larger than the distal, but has no olecranon process. Its
distal end articulates with the ulnare and pisiform. The ulna as a
whole is slightly curved, while the radius is quite straight.

The _radius_ (_r_) consists of a cylindrical shaft with enlargements
of about equal size at the ends. The proximal end articulates with the
side of the ulna and with the humerus; the distal end with the radiale.

The carpus consists of a proximal row of three distinct bones and
a distal row of smaller and less fully ossified elements. Of the
proximal row the _radiale_ (_r′_) is much the largest bone. It is
hourglass shaped, with the proximal end somewhat larger than the
distal. Proximally it articulates mainly with the radius but also
slightly with the ulna and ulnare. Distally it articulates with the
centrale. The _ulnare_ (_u′_), the second bone in size in the wrist,
has about the same shape as the radiale but is much smaller. Proximally
it articulates with the pisiform, radiale, and, apparently, with the
ulna; distally it is in contact with the fused carpalia elements. The
_pisiform_ (_p_) is a small, irregular bone, articulating with the
ulna and the ulnare; it is apparently connected by a long ligament with
the fifth metacarpal but does not actually articulate with it. The
_centrale_ (_ce_) is a flattened, partially ossified element between
the radiale and the first and second metacarpals. The distal carpal
bones are represented by two irregular, partially ossified elements
between the ulnare and the third, fourth, and fifth metacarpals.

The manus proper consists of five digits. The _metacarpals_ (_m_)
are of about the same shape, but vary in length and thickness; each
consists of a cylindrical shaft with a slight enlargement at each end.
The first digit or pollex has two phalanges, the second has three,
the third has four, the fourth has four, and the fifth has three. The
terminal phalanx of each of the first four digits is pointed, has a
pair of lateral grooves, and is encased in a large, horny _claw_ (_cl_).

_The Pelvic Girdle_ and _Posterior Limb_. The pelvic girdle is
described differently by Wiedersheim and Reynolds; the bone called by
the former the pubis, the latter calls the epipubis. The bone called by
Wiedersheim the pubis takes no part in the formation of the acetabulum;
the pubis of Reynolds helps form the acetabulum but is a very small,
unossified structure. Gadow also calls the lower bone the epipubis. I
shall follow Reynolds’s interpretation.

The _ilium_ (Fig. 28, 1) is a heavy bone with a dorso-laterally
projecting crest; medially it is firmly united to the sacral ribs (Fig.
18, 5) while its outer side forms the upper and greater part of the
acetabulum. Its outer and lower border has two surfaces, the larger
and more posterior articulating with the ischium, the other with the
cartilaginous pubis.

[Illustration: FIG. 28. PELVIS AND SACRUM OF AN ALLIGATOR (_Caiman
latirostris_). ×¹⁄₂. (Brit. Mus.) (After Reynolds.)

1. ilium.
2. ischium.
3. true pubis.
4. epipubis (so-called pubis).
5. acetabular foramen.
6. neural spines of sacral vertebrae.
7. symphysis ischii.
8. process bearing prezygapophysis.]

The _ischium_ (2) is a slightly arched bone, its ventral end a
flattened blade articulating with its fellow, its dorsal end enlarged
and thickened to articulate with the ilium, pubis, and epipubis. This
dorsal end, which forms the ventral side of the acetabulum, is divided
into two distinct articular surfaces by a deep, rounded notch; the
posterior and larger surface articulates with the ilium, the anterior
surface about equally with the pubis and epipubis.

The _pubis_ (3), which is much the smallest element of the pelvis, is a
small mass of cartilage lying between the ilium above and the ischium
below. It forms a small part of the anterior wall of the acetabulum.

The _epipubis_ (4) is a slightly arched bone, somewhat enlarged at its
proximal end where it unites with the ischium, and flattened out into
a fan-shaped extremity, where it is united with its fellow and with
the last pair of abdominal ribs by the broad, thin sheet of cartilage
or fibrous tissue noted in connection with the abdominal ribs. As
mentioned above, it is called by Wiedersheim and others the pubis. Near
the center of the acetabulum there is a small foramen.

The _posterior limb_ (Fig. 29) consists of the usual divisions--thigh,
shin, and foot. The _femur_ (f) is a bone of the same general outline
as the humerus, though slightly longer and heavier. The head, for
articulation with the acetabulum, is rather hemi-elliptical than
hemispherical in shape, the long axis of the ellipse being vertical.
The distal enlargement is of at least as great, if not greater, bulk
than the proximal and shows some indication of a division into two
articular surfaces. The ventral side of the femur near the proximal end
shows a fairly distinct trochantal ridge.

The shin or crus is made up of two well-developed bones, the
_tibia_ (t) and _fibula_ (fb), the former being somewhat longer and
considerably thicker than the latter.

[Illustration: FIG. 29. POSTERIOR LIMB.

_ca_, calcaneum or fibulare; _cl_, claw; _f_, femur; _fb_, fibula;
_t_, tibia; _t³_, _t⁴⁻⁵_, tarsalia; _tb_, _tb′_, tibiale-centrale;
_IV_, _V_, 4th and 5th metatarsals.]

The tibia consists of a cylindrical shaft with enlargements of about
equal size at the ends. The proximal end forms most of the knee joint,
the distal end articulates with a tarsal element said by Reynolds to
represent the fused astragalus and centrale, by Wiedersheim called the
astragalus, and said to represent the united tibiale, intermedium, and
centrale (tb, tb′). The fibula articulates by a small enlargement at
its proximal end with the femur, and by an enlargement of about equal
size, at its distal end, with the fibulare or calcaneum (ca), and with
a small facet on the above-mentioned tibiale-centrale element.

The tarsus is much modified and consists of four elements, in two rows;
those of the proximal row are much larger than the two distal elements.
Articulating with both tibia and fibula, as mentioned above, and with
the first metatarsal and one of the distal tarsalia, is the large and
irregular _tibiale-centrale_ element of Reynolds (tb, tb′). In the
tarsus here shown it consists of two elements. Post-axial in position
is the _calcaneum_ or _fibulare_ (ca), articulating with the preceding
tarsal element, with the fibula, with the rudimentary fifth metatarsal,
and with the distal tarsal element said by Reynolds to represent the
fourth and fifth tarsalia. The calcaneum is extended caudad into a
prominent knob quite like the heel of the higher mammals.

The two distal tarsal bones are small; one is said by Reynolds to
represent the first three _tarsalia_ (t³), the other (t⁴⁻⁵) the fourth
and fifth. Wiedersheim says one of these bones represents the first
three tarsalia, the other the fourth. In the tarsus here shown these
two elements are fused.

The foot has five digits, though the fifth is small and consists merely
of a small, distally pointed metatarsal bone. According to Wiedersheim
this fifth metatarsal is fused with the fifth tarsalia. The metatarsals
of the first four digits are long and progressively more slender from
the first to the fourth; each is distinctly enlarged at the ends. The
first digit or hallux has two phalanges, the second has three, the
third has four, and the fourth has four. According to Reynolds, the
fourth toe has five phalanges; the figure here shown, which was drawn
from nature, has only four on the fourth toe; the latter is the number
given by Bronn for the crocodiles. The terminal phalanges of the first
three digits are large and pointed, with the same lateral grooves noted
in connection with the fore foot; each is sheathed in a horny claw. The
four fully developed digits of the pes are nearly twice as long as the
corresponding digits of the manus, but they are not proportionately
thicker.

CHAPTER III

THE MUSCLES

The description of the muscles here given is taken from Bronn (11),
who, in turn, largely follows Gadow. The animal described is the
crocodile, but while Bronn does not indicate the species, it is
probable that the differences between the various members of the
Crocodilia would be slight. The figures of the muscular system are
mainly from the Florida alligator.

In his description Bronn gives for each muscle the various synonyms
(often more than half a dozen) that are employed by different writers;
in this work Bronn’s nomenclature is given first and the synonyms
follow in parentheses.

THE CHEWING MUSCLES

_Temporalo-maxillaris_ (Temporalis) (Masseter, Temporal, Aeussere ober
Heber or Schlafmuskel). Arises in the temporal fossa, passes under the
zygoma, and inserts itself on the inner and outer sides of the lower
jaw.

_Pterygo-maxillaris_ (Pterygoideus) (Pterygoidien, Aeusser
Flügelmuskel, Pterygoideus externus, Pterygoideus internus). A large
muscle which consists of two portions: the outer, weaker portion
springs from the pterygoid process, the inner stronger part from the
pterygoid fossa and pterygoid process; they run together around the
angle of the lower jaw, where they form a large, bulging fold. They
are the chief muscles of this part of the body since the masseter is
lacking and the temporalis is weakly developed.

_Occipito-maxillaris_ (Digastricus maxillæ) (Niederzieher des
Unterkiefers, Abaisseur ou l’analogue du digastrique, Senker des
Unterkiefers, Aristotelis apertor oris, Digastricus, Aperator oris).
Arises from the hinder border of the lateral occipital and is inserted
at the hinder end of the lower jaw. Its course is from front to back.
If the skull be stationary this muscle drops the lower jaw; if the jaw
be fixed it raises the skull.

MUSCLES OF THE VENTRAL SURFACE OF THE NECK

_Intermaxillaris_ and _Sphincter Colli_ (Intermaxillaire,
Mylo-hyoideus, Zwischenkiefermuskel, Latissimus colli). This muscle
consists chiefly of transversely running fibers, and has in its
middle third a small, median, longitudinal raphe or aponeurosis. In
the posterior part of the neck it is very thin, but increases in
thickness more and more as it passes cephalad. A short anterior and a
long posterior portion may be distinguished. The former extends from
the inner side of the right to that of the left half of the lower jaw,
without a median aponeurosis. The hinder half of this muscle is united
by a pair of aponeuroses to the lower jaw, on one hand (the smaller
part), and to a fascia, on the other hand (the far larger part),
that separates several of the neck muscles. The smaller part begins
immediately behind the pterygoid on the inner side of the halves of the
lower jaw but ends on the outer side of the two halves of the jaw.

_Latus Colli_ (Latissimus colli accessorius). Lies underneath the
preceding. Its muscle bundles lie between the collo-capitis muscle and
the bodies of the first three cervical vertebræ, and form a broad band
that extends from the hyoid bone to the backwardly directed cervical
ribs of the first and second pairs.

_Coraco-ceratoideus_ (Omo-hyoideus, Coraco-hyoideus). A long, narrow,
and moderately thick muscle which takes its origin from the upper
border of the coracoid, where the latter touches the scapula. It
extends forward near the œsophagus and attaches itself to about the
middle of the backwardly turned border of the horn of the hyoid of that
side.

_Episterno-ceratoideus_ (Niederzieher des Zungenbeins, or
Brustbeinzungenbeinmuskel, Sterno-hyoideus). A flat and fairly broad
muscle which springs from the ventral surface of the episternum;
behind, it is separated by a slight space from the corresponding muscle
of the other side, with which it nearly covers the cervical part of the
trachea. Towards its anterior end it divides into two heads; one of
these inserts itself on the outer border and outer surface of the cornu
of the hyoid; the other head, lying laterad to the former, is suddenly
reduced to a short tendon by which it is attached to the following
muscle.

_Maxillo-coracoideus_ (Mylo-hyoideus anterior, Sterno-maxillare). This
muscle arises from the upper border and inner surface of the caudal
third of the lower jaw. In its further course it becomes tendinous
and projects by a short tendon outwards from the hyoid cornu to unite
with the head of the preceding muscle, as noted above; it then becomes
fleshy again and is inserted on the medial part of the upper border of
the coracoid.

_Maxillo-hyoideus_ (Genio-ceratoidien, Hyomaxillaris, Hyoglossus,
Hyomandibularis, Mylo-hyoideus posterior). This muscle arises, very
thin, from the mandibular symphysis, goes thence immediately backward
and inward to insert itself, by its broad end, on the whole anterior
end of the horn of the hyoid and on the hyoid itself.

_Cerato-hyoideus._ Arises from the horn of the hyoid and inserts itself
on the body of the hyoid.

_Costo-coracoideus._ This muscle arises from the distal ends of the
first and second ribs and is inserted on the ventral surface of the
coracoid at the boundary of the scapula.

_Costo-scapularis_ (Collo-scapularis superficialis, Levator scapulæ
superficialis). See shoulder muscles.

_Costo-vertebralis Medialis_ (Scaleni). Fairly large, flat, and
long-drawn-out three-cornered muscle. Attached by its base to the most
anterior sternal rib, by its upper border to the fifth cervical rib,
and by its point to the end of the second cervical rib.

_Costo-vertebralis Lateralis_ (Longus colli). Originates thin and sharp
on the body of the fifth thoracic vertebra, increases in thickness
slowly but decidedly cephalad, then again becomes thinner and inserts
itself on the inner side of the ribs of the most anterior two cervical
vertebræ.

_Collo-capitis_ (Rectus capitis anterior). Arises, as a rule, from the
cervical centra, at times from the second thoracic vertebra (Gavialis).
It extends forward and is inserted on the basi-occipital and the hinder
border of the pterygoid. For a greater part of their length the two
muscles lie close together, but forward they separate somewhat from
each other.

DORSAL NECK MUSCLES

_Occipito-cervicalis Medialis_ (Complexus cervicis, Biventer cervicis,
Zweibäuchiger Strecker or Zweibäuchiger Nackenmuskel, Splenius
capitis). It springs, by separate points, from the dorsal processes of
the four anterior body vertebræ and the six posterior neck vertebræ;
it is convex on its dorsal, weakly concave on its ventral surface; it
leads cephalad as a short, strong tendon by which it is attached to
the angle between the upper hinder border of the skull, _i.e._ to the
superior and lateral occipital region.

_Squamoso-cervicalis Medialis_ (Kopfbäuchmuskel [Splenius] or
durchflochtener Muskel [Complexus], Trachelo-mastoideus, Complexus).
This muscle lies laterad and ventrad to the preceding and is at times
partly covered by it in its posterior half. It arises from separate
heads from the spinal processes of the two anterior and six posterior
cervical vertebræ; beginning caudad, thin and sharp, it gradually
becomes thicker as it passes cephalad until it becomes partially
tendinous and inserts itself on the hinder border of the squamosal,
laterad to the occipito-cervicalis medialis muscle.

_Epistropheo-vertebralis_ (Splenius colli). This muscle springs from
the spinous processes of the most anterior three body vertebræ and the
last cervical vertebra; it receives fibers from the articular processes
and intermediate parts of the six posterior cervical vertebræ and is
inserted on the second cervical vertebra.

_Collo-squamosus_ (Splenius capitis, Nackenwarzenmuskel,
Trachelo-mastoideus). Springs from the upper transverse processes of
the last three neck vertebræ, and, becoming tendinous, is inserted on
the hinder border of the squamosal.

_Collo-occipitis._ Arises from the transverse processes of the
posterior five cervical vertebræ, extends directly forwards on the ribs
of the vertebræ, and is inserted under the articular surface of the
lateral occipital.

_Occipito-epistropheus_ (short, straight, hinder head-muscle, or
extensor). This muscle springs from the lateral surface of the body of
the second neck vertebra and inserts itself on the basi-exoccipital,
under the preceding muscle.

_Cervicalis Adscendens._ Arises in great part from the angles under
the most anterior ribs; a smaller part appears farther above where it
is covered by the rhomboideus muscle. It is inserted on the upper side
of the five posterior cervical ribs and on the distal ends of the long
second cervical rib.

THE MUSCLES OF THE SCAPULA

_Capiti-sternalis_ (Sterno-mastoideus). This is a fairly large muscle,
on the side of the neck, that extends from the skull to the breast
and from the middle of the neck is divided into two portions: (a) an
anterior part or atlanti-mastoideus (Plate I., Figs. 1 and 2, cst¹)
(upper end of the “head nodder,” sterno-mastoideus, anterior part
of sterno-mastoideus, anterior part of atlanti-mastoideus); (b) a
posterior part or sterno-atlanticus (Plate I., Figs. 1 and 2, cst²)
(sterno-mastoideus, inner belly of the “head-nodder,” posterior part of
the sterno-atlanticus). The former part is a rather short but not weak
muscle that arises from the squamosum and inserts itself on the rib of
the atlas (alligator) or of the atlas and epistropheus (crocodile).

The latter part is fairly strong and exceeds the anterior part in
length; it springs from the rib of the first cervical vertebra,
opposite the insertion of the anterior part, and inserts itself on the
anterior border of the outer surface near the episternum. At times
superficial fibers pass into the pectoral fascia.

_Dorso-scapularis_ (Cucullaris) (Plate I., Figs. 1 and 2, Cu)
(Trapezius). A broad but thin muscle that begins as an aponeurosis from
the dorsal fascia in the middle line of the hinder part of the neck
and beginning of the back; with converging fibers it passes within
to insert itself partly on the spine of the scapula and partly by
superficial fibers in the fascia that cover the deltoides scapularis
inferior muscle.

_Collo-scapularis Superficialis_ (Plate I., Fig. 1, cssp) (Levator
scapulæ superficialis, Levator scapulæ, Heber des Schulterblatts,
Acromio-trachélien, Teil des Serratus magnus, Levator anguli scapulæ).
A considerable muscle on the side of the neck. It arises from the tips
of the ribs of the first and second cervical vertebræ (where it is
fused with the sterno-atlanticus muscle), and also from the transverse
process of the third and fourth cervical vertebræ; it goes with
diverging fibers to the entire anterior border of the scapula.

_Thoraci-scapularis Superficialis_ (Serratus superficialis, Pectoralis
minor, Hinterer Theil des inneren grösseren Rückwärtsziehers, Pars
posterior m. serrati antici majoris, Theil des Grand dentelé, Serrati
posteriores, Latissimus dorsi scapulo-costalis). A strong muscle of
three prongs that go directly, by superficial fibers, over into the
oblique abdominal muscle and meet the ribs. The first and smallest
prong arises from the under end of the rib of the ninth vertebra (last
cervical); the second and medium-sized prong comes from the uncinate
process of the tenth rib (first thoracic) and from beneath the uncinate
process of the second thoracic rib; the third and strongest prong takes
its origin from the uncinate processes of the second and third thoracic
ribs. All three prongs unite to form a broad, homogeneous muscle which
passes forward and above to the hinder border of the scapula, upon
whose entire surface, except at the lower end, it is inserted.

_Collo-thoraci-suprascapularis Profundus_ (Plate I., Fig. 3, cthspr)
(Levator scapulæ et serratus profundus, Serrati anteriores, Serratus
anticus major, Vorderer Theil des inneren grösseren Rückwärtsziehers
or vorderen grossen gezahnten Muskels, Pars anterior m. serrati antici
majoris, Theil des Grand dentelé, Theil des Serratus magnus). This
muscle arises in varying extent from the transverse process of the
fifth cervical vertebra to the first (crocodile) or second (alligator)
ribs. It is inserted on the inner surface of the suprascapula, except
on its forward part, and is made up of two layers--a superficial and a
deep one. The former layer (Fig. 3, cthspr¹) is weakly developed and
is composed of two or three thin, distinct bundles, that extend from
the ribs of the eighth, ninth, and eleventh vertebræ (alligator) or
from the transverse process of the seventh vertebra and the rib of the
tenth. The deeper layer is considerably developed; its bundles come, in
the alligator, from the fifth to tenth vertebræ; in the crocodile from
the fifth to ninth.

_Rhomboideus_ (Plate I., Fig. 3, rh) (Rautenmuskel, Angulaire de
l’omoplate). This is a very small, independent muscle that springs, by
two or three distinct bundles, from the fascia covering the longissimus
dorsi muscle, in the region of the eighth and ninth vertebræ; after
a short course it inserts itself on the antero-dorsal angle of the
suprascapula.

_Costo-coracoideus_ (Plate I., Fig. 3, cc) (Subclavius et Triangularis
sterni and Levator secundæ superioris costæ, Petit dentelé, Pectoralis
minor, Pectoralis). This is a broad muscle of considerable size on the
ventral side of the breast; it consists of a lateral and of a medial
portion, the former springing from the last cervical rib, the latter
from the anterior border of the first sternocostal ridge. The two parts
unite and are inserted on the whole posterior border of the coracoid.

_Pectoralis minor_ (Pectoralis, Costo-coracoideus). A broad,
considerable muscle on the under side of the breast, which is made up
of two parts, of which the lateral springs from the anterior border of
the last (ninth) cervical rib, and the medial from the anterior border
of the first sternocostal ridge. Both parts unite into a homogeneous
layer which is inserted broadly on the whole hinder border of the
coracoid.

_Pectoralis_ (Plate I., Figs. 1 and 2, p) (Pectoralis major, Grosser
Brustmuskel). A broad muscle on the under side of the breast, bounded
behind by the rectus abdominis and obliquus abdominis externus muscles,
with which it is united. It arises from the whole episternum, from
the whole sternum, except from the median line of its posterior part,
from the sternal ends of the first six thoracic ribs, from all six
sternocostal ridges, and, with a small prong, from the eighth rib. It
is inserted on the distal part of the convex surface of the processus
lateralis humeri.

_Supracoracoideus_ (Plate I., Figs. 1 and 2, spc)
(Supracoracoscapularis, Deltoideus, Schlüsselbeinhälfte, Theil der
Schulterblatthälfte des Hebers des Armes, Obergrätenmuskel, Hebemuskel
des Oberarmes, Epicoraco-humeralis). A muscle of considerable size
at the anterior region of the coracoid and the under region of the
scapula, which is divided into two parts: (a) the coracoid (inferior)
division is the stronger and arises from the whole anterior half of the
coracoid, from its outer and inner surfaces; it is inserted, together
with the second part, on the proximal, little-developed part of the
processus lateralis humeri; (b) the scapularis (superior) division
is the weaker of the two and is covered by the deltoides scapularis
inferior muscle; it arises from the surface of the under third of the
scapula, behind the spine; it unites with the preceding part to form a
single muscle and inserts itself, as said above, on the proximal part
of the processus lateralis humeri.

_Coraco-brachialis_ (Brevis) (Plate I., Figs. 4, 5, and 6, cbb)
(Theil des grossen Brustmuskels oder Hakenarmmuskel, Pectoralis II.,
Pectoralis minor). A fairly strong muscle. It arises from the outer
surface of the coracoid, except the median edge and the anterior
section, and runs to the flexor surface of the upper arm where it is
inserted on the proximal third between the lateral and median processes.

_Coraco-antebrachialis_ (Plate I., Figs. 2 and 5, b¹) (Biceps,
Coracoideus, Langer Kopf des langen Beugers, Langer Kopf des Biceps,
Biceps humeri, Biceps brachii, Coraco-radialis). A slender and
rather weak muscle on the flexor side of the upper arm. It arises by
a fairly broad but thin tendon from the outer surface of the coracoid
immediately before the coraco-brachialis. As a weak bundle it passes
between the lateral and median processes, lying medially near the
brachialis inferior muscle, with which, at the end of the upper arm, it
unites; after their union the two muscles continue as a broad tendon
that splits into two parts, which are inserted on the proximal end of
the radius and of the ulna.

_Humero-antebrachialis Inferior_ (Plate I., Figs. 2 and 6, hai)
(Brachialis inferior, Caput breve m. bicipitis, Kurzer Kopf des Biceps,
Brachial interne, Brachialis anticus, Erster vom Oberarm ausgehender
Beuger, Portion of Brachiæus). Springs from the lateral flexor side of
the humerus, from the distal end of the lateral process to the distal
end of the bone, except the epiphysis; at the end of the upper arm it
unites with the biceps and with it is inserted, by two tendons, to the
radius and ulna.

_Dorso-humeralis_ (Plate I., Fig. 1, dh) (Latissimus dorsi, Breiter
Rückenmuskel, Humero-dorsalis). It springs as an aponeurosis from
the back at the level of the first four or five dorsal vertebræ, and
passes, with converging fibers, cephalo-ventrad to unite with the teres
major muscle; in common with the latter it extends along the extensor
surface of the humerus to be inserted between the lateral and median
processes.

_Dorsalis Scapulæ_ (Plate I., Fig. 1, dss) (Deltoides scapularis
superior, Unterer Theil des äusseren Schulterblattmuskels,
Untergrätenmuskel, Suprascapularis, Infraspinatus, Supraspinatus).
Springs from the anterior half of the outer surface of the scapula,
passes between the deltoides scapularis inferior and the caput
scapulare laterale externum m. anconæi, as a narrow band, to be
inserted on the lateral side of the humerus.

_Deltoides scapularis Inferior_ (Plate I., Figs. 1 and 2, dsi)
(Deltoideus superior, Supra- and Infraspinatus, Theil der
Schulterhälfte des Hebers des Armes, Theil der oberen [Schulterblatt-]
Abtheilung des Deltoideus, Zweiter Hebemuskel des Oberarmes, Theil des
Deltoides). A strong muscle on the side of the shoulder. It springs
from the spine of the scapula, passes back with slightly converging
fibers, and ends chiefly on the outer surface of the processus
lateralis humeri, while a number of superficial fibers end in the
humero-radialis muscle.

_Scapulo-humeralis Profundus_ (Plate I., Fig. 4, shpr) (Teres minor,
Erster Teres major, Scapulo-humeralis). A small muscle that springs
from the posterior border of the lower third of the scapula, and
passes, with converging fibers, to its insertion on the humerus just
distal to the medial process.

_Teres Major_ (Grosser runder Muskel oder kleiner Rückwärtszieher des
Oberarmbein, Zweiter teres major). Springs from the posterior half of
the upper region of the outer surface of the scapula. It passes down,
with converging fibers, to unite with the latissimus dorsi muscle to
form a strong tendon that is inserted on the extensor surface of the
humerus.

_Subscapularis_ (Unterschulterblattmuskel). Springs from the inner
surface of the scapula, except from the suprascapula, goes with
converging fibers directly over the capsule of the shoulder joint to be
attached to the medial process of the humerus.

_Anconæus._ This strong muscle lies on the extensor side of the upper
arm. It is made up of two layers: the superficial comes from the
pectoral girdle in two heads: (a) the caput scapulare laterale externum
and (b) caput coraco-scapulare; the deeper layer originates on the
humerus by three heads, (c) caput humerale laterale, (d) caput humerale
posticum, and (e) caput humerale mediale. These five heads of the
anconæus muscle with their synonyms will now be described.

(a) _Caput Scapulare Laterale Externum_ (Plate I., Figs. 1 and 4,
asl) (Brevi proximum caput m. tricipitis, Gewöhnlicher [äusserer]
langer Kopf des dreiköpfigen Streckers, Portion scapulaire externe
du triceps-brachial, Erster langer Kopf des Triceps, [Zweiter]
abducirender vom Schultergerüst entstehender Kopf des Streckmuskels
des Vorderarmes, Triceps Nr. 1, Triceps longus). This muscle springs
as a tendon from the hinder border of the scapula directly beneath
the articular cavity, and extends back, between the scapulo-humeralis
profundus and the dorsalis scapulæ muscles, into the muscle belly.

(b) _Caput coraco-scapulare_ (Plate I., Figs. 2, 4, 5, 6, acs)
(Externum caput m. tricipitis, Innerer langer Kopf des dreiköpfigen
Streckers, Portion scapulaire interne du triceps-brachial, Zweiter
langer Kopf des Triceps, [Erster] abducirender vom Schultergerüst
entstehender Kopf des Streckmuskels des Vorderarmes, Triceps Nr. 2,
Triceps longus secundus). Arises by two distinct tendinous tips--the
upper, weaker one from the hinder border of the scapula, the lower,
broader one from the hinder border of the coracoid.

(c) _Caput Humeri Laterale_ (Plate I., Figs. 1 and 4, ahl) (Brevius
caput m. brachiei interni, [Aeusserer] kurzer Kopf des dreiköpfigen
Streckers, Portion huméral externe du triceps brachial, Aeusserer vom
Humerus ausgehender Kopf des Streckmuskels des Vorderarmes, Theil des
Triceps Nr. 3, Triceps externum). Springs from the lateral part of the
extensor surface of the humerus dorsal to the lateral process and the
origins of the humero-radialis and brachialis superior.

(d) _Caput Humerale Posticum_ (Plate I., Fig. 4, ahp) (Longissimum
caput m. brachiei internum, Theil des inneren [kurzen] Kopfes des
dreiköpfigen Streckers, Theil des Triceps Nr. 3, Theil des Triceps
internus, Theil der Portion humérale interne du triceps brachial,
[Mittler] vom Humerus ausgehender Kopf des Streckmuskels des
Vorderarmes). Springs from the middle of the extensor surface of the
humerus between the lateral and medial heads.

(e) _Caput Humerale Mediale_ (Longius caput m. brachiei interni, Theil
des [inneren] kurzen Kopfes des dreiköpfigen Streckers, Theil der
Portion humérale interne du triceps brachial, [Innerer] vom Humerus
ausgehender Kopf des Streckmuskels des Vorderarmes, Theil des Triceps
Nr. 3, Theil des Triceps internus). This head originates on the medial
part of the extensor surface of the upper arm at the end of the medial
process where it is united with the scapulo-humeralis profundus muscle.

The muscle mass formed by the union of all the above heads goes over,
as a broad and somewhat thick tendon, to become inserted on the
proximal part of the ulna.

_Humero-radialis_ (Plate I., Figs. 1 and 4, hr) (Caput longum m.
bicipitis, Eigener kurzer Beuger, [Zweiter] vom Oberarm ausgehender
Beuger, Brachialis externus, Portion _a_ of Brachiæus). A fairly large
muscle on the outer side of the upper arm, lying between the brachialis
inferior and caput humerale laterale muscles, with both of which it
is, at the beginning, united. It originates with its deeper and chief
mass from the outer surface of the humerus, just distal to the lateral
process; while its superficial layer, especially the upper fibers, come
directly from the deltoides scapularis inferior and therefore have
their origin on the scapula. In the middle of the upper arm it becomes
a slender round tendon that extends, through a tendinous loop, to the
radius, on whose outer side, at the end of the proximal third, it is
inserted.

MUSCLES OF THE FOREARM

_Humero-radialis Internus_ (Radialis internus, Lange Vorwärtswender,
Pronateur, Pronator teres, Pronator quadratus, Oberflächlich gelegener,
langer runder Einwärtsdreher). This muscle arises from the condylus
internus (C. ulnaris s. medialis) and attaches itself to the radius
throughout almost its entire length. It is a fairly strong muscle.

_Ulno-radialis_ (Carré pronateur, Pronator teres, Pronator quadratus,
Muskel welcher dem Pronator quadratus entsprecht). A strongly developed
muscle. It springs from the upper part of the flexor surface of the
ulna and is inserted on the lower part of the flexor surface of the
radius.

_Humero-radialis Longus_ (Plate II., Figs. 1 and 2, _1_) (Supinator
longus, Long supinateur, Lange Rückwärtswender, Supinator radii
longus). Among the Crocodilia this and the following muscle are well
developed. This one springs from the condylus externus humeri and is
inserted on the outer side of the entire length of the radius.

_Humero-radialis Brevis_ (Plate II., Fig. 4, _d_) (Supinator brevis,
Kurze Rückwärtswender, Extensor carpi-radialis brevis [?]). Arises near
the preceding from the external condyle of the humerus and is inserted
at the upper end of the radius.

_Humero-carpi-radialis_ (Plate II., Fig. 2, _a_) (Aeusserer oder langer
Speichenmuskel, Musculus quem parti superiori extensoris digitorum
communis respondere videbat, Extensor carpi-radialis longus, Abductor
pollicis longus). Towards the ulna, near the supinator longus muscle.
It springs from the external condyle of the humerus, covers the
supinator brevis muscle, and is inserted on the proximal end of the
carpi-radialis.

_Humero-carpi-ulnaris_ (Plate II., Fig. 2, _c_) (Extensor
carpi-ulnaris, Ulnaris externus). Originates on the external condyle of
the humerus, is inserted on the proximal end of the os carpi-ulnare.

_Humero-metacarpalis III., IV., V._ (Plate II., Fig. 2, _b_) (Extensor
digitorum longus, Aeusserer Speichenmuskel or Speichenstrecker der
Hand, Extenseur commun, Extensor radialis longus, Extensor digitorum
communis). This muscle lies between the humero-carpi-radialis and the
humero-carpi-ulnaris muscles. It springs from the condylus externus
humeri and divides, on reaching the carpus, into three thin, flat
tendons, which in part fuse with the carpo-phalangei muscle, and in
part are inserted on the carpal bones of the third, fourth, and fifth
fingers.

_Carpo-phalangei_ (Plate II., Fig. 2, _d_). (Extensor digitorum brevis,
Extenseurs courts, Gemeinschaftlicher Strecker der Hand, Extensor
digitorum communis brevis). Springs from the carpal and, in part, from
the metacarpal bones and is inserted on the terminal phalanges of the
five fingers.

_Ulno-carpi-radialis_ (Ein dem Strecker und Abzieher des Daumens
analoger Muskel, Extensor pollicis longus, Extensor carpi-radialis
brevior[?]). Springs from the under half of the ulna, and is inserted
on the os carpi-radiale.

_Carpo-phalangeus I._ (Extensor pollicis brevis). This is a small,
thick muscle that originates on the distal part of the os carpi-radiale
and is inserted on the phalanx of the thumb.

_Humero-radialis Lateralis_ (Plate II., Fig. 1, _6_) (Flexor
carpi-ulnaris, Innerer Ellenbogenmuskel, Ulnaris internus). A fairly
strongly developed muscle. It springs from the internal condyle of the
humerus, extends along the ulna, and is inserted on the proximal part
of the os carpi-ulnare, and the nearby pisiform bone.

_Humero-radialis Medialis_ (Plate II., Fig. 1, _2_) (Flexor
carpi-radialis, Radialis internus). A strongly developed muscle. It
springs from the internal condyle of the humerus, receives fibers from
almost the entire length of the radius, and is inserted on the proximal
end of the os carpi-radiale and with a thin tendon to the metacarpal
bone of the thumb. Rüdinger was not able to find this muscle in
_Alligator cynocephalus_.

_Carpo-phalangei_ (Plate II., Fig. 1, _4_) (Flexor digitorum communis
brevis, Oberflächlicher gemeinschaftlicher Fingerbeuger, Fléchisseur
sublime, Flexores sublimis a profundo perforati, Lange Flexoren der
Finger, Flexor digitorum communis sublimis s. brevis, Flexor digitorum
sublimis). A small thick muscle. It springs from the ligamentum
carpi-volare proprium and from the ulnar border of the distal end of
the os carpi-radiale and is divided into eight muscle-bellies which
pass over to the proximal ends of the first phalanges as thin tendons
that are penetrated by those of the humero-ulno-phalangei muscle.

_Humero-ulno-phalangei_ (Plate II., Figs. 1 and 2, _5_) (Flexor
digitorum communis profundus, Fléchisseur profond, Tiefer
gemeinschaftlicher Fingerbeuger, Flexor digitorum profundus, Flexor
profundus). Arises with three heads. The first head takes its
origin from the internal condyle of the humerus, runs between the
humero-radialis lateralis muscles, and passes as a tendon over to the
carpus where it unites with the other two heads of this muscle. The
second, deep head comes from almost the entire length of the ulna.
These two heads may be called the long heads. The third, short head
springs from the proximal ends of the two large carpal bones of the
first row, and becomes united radially with the thick flat tendon
ending the first two heads. The common terminal tendon splits into four
points which pass among the tendons of the carpo-phalangei muscle and
are inserted on the terminal phalanges. From the terminal tendons of
this muscle spring the lumbricales muscles.

_Carpo-phalangeus_ (Plate II., Fig. 1, _8_) (Abductor pollicis).
Springs from the os carpi-radiale; is inserted on the first phalanx of
the thumb.

_Carpo-metacarpalis I._ (Plate II., Fig. 1, _9_) (Opponens pollicis).
Originates from the os carpi-radiale and is inserted on the radial side
of the entire first metacarpus.

_Metacarpo-phalangeus I._ Originates from the base of the metacarpus of
digit III.; is inserted on the ulnar side of the first phalanx of the
thumb.

_Pisiformi-phalangeus primus digiti V._ (Plate II., Fig. 1, _7_)
(Abductor digiti minimi, Abducteur du petit doigt, Abductor digiti
quinti). Springs from the pisiform bone, and is inserted on the medial
border of the first phalanx of the fifth finger.

_Carpo-metacarpalis V._ (Opponens digiti minimi, Opponens primus).
Springs from the carpi-ulnare bone and is inserted on the metacarpal
bone of the fifth digit.

_Carpo-phalangeus primus digiti V._ (Plate II., Fig. 1, _3_) (Flexor
digiti minimi brevis, Opponens secundus). Arises from the ulnar border
of the proximal part of the carpi-radiale bone and is inserted on the
proximal end of the first phalanx of the fifth finger.

_Metacarpo-phalangeus I. digiti V._ (Adductor digiti minimi). Springs
from the metacarpal bones of the second and third fingers and is
inserted on the radial side of the first phalanx of the fifth finger.

THE ABDOMINAL MUSCLES

_Obliquus Abdominis Externus_ (Grand oblique, Aeusserer schiefer
Bauchmuskel, Obliquus externus, Obliquus externus + internus +
Serrati, Oblique descendens). Springs, with a flat prong, from the
uncinate processes of the true ribs, thence it extends as a tendinous
aponeurosis, near the lateral boundary of the ileo-costalis muscle,
caudal-ward to the region of the twenty-third (crocodile) vertebra.
From this fairly straight line of origin the muscle takes a sharply
distoventral course and is inserted, at least in part, on the outer
surface of the sternal part of the ribs of the tenth to sixteenth
vertebræ, but does not reach the mid-ventral line. Under this chief
part of the outer layer of the abdominal muscle lies a second, more
band-like muscle mass which is also strong but of considerably less
extent. It takes its origin from the outer surface of the middle third
of the ribs. In the region of the twentieth vertebra it fuses with the
upper layer, but inwardly reaches nearer the median line than the upper
layer.

_Obliquus Abdominis Internus_ (Petit oblique, Obliquus internus,
Subcostalis). Arises as a flat muscle layer first with a strong
tendinous portion from the anterior dorsal border of the os pubis and
from the there-located cartilaginous inscriptio tendinea of the rectus;
second, by a dorsal portion, with a short tendon, from the anteromedial
surface of the pubo-iliac articulation from the pubis and ilium
equally; third, from the dorsal anterior ends of the last named bones.
It is inserted somewhat mediad to the lateral border of the rectus
ventralis muscle that covers it on the outside.

_Transversus Abdominis_ (Transverse, Oblique Bauchmuskel, Innerer
Bauchmuskel, Transversus ventralis). This muscle springs by short,
flat, indistinct forks from the inner surface of the proximal ends of
the dorsal ribs but does not reach the centra of the vertebræ because
of the long, broad transverse processes. Caudally the origin passes
dorsalward to the lateral border of the quadratus lumborum muscle
between which and the ileo-costalis muscle it is attached to the end of
the transverse process.

_Rectus Abdominis_ (Gerader Bauchmuskel + pyramidenförmiger Muskel,
Pyramidalis, Rectus abdominis + pyramidalis). This muscle consists, in
the Crocodilia, of several very distinct parts:

I. The rectus ventralis, the chief part, arises as a fleshy tendon from
the sternum and from the ventral part of the last rib that reaches
the sternum, and extends with direct longitudinal fiber-bundles of
equal mass over the ventral third of the body back to the pelvis. It
is inserted as a fleshy tendon on the anterior border of the pubis and
more laterally is united, together with the obliquus internus muscle,
chiefly to the last abdominal ribs which arise as an ossification of
the last strongly developed inscriptio tendinea. This muscle-band,
which unites with that of the opposite side to form the linea alba, is
divided metamerically by seven distinct inscriptiones tendinea. These
inscriptiones are the above described abdominal ribs which consist
of bony connective-tissue without a trace of cartilage cells. These
so-called abdominal ribs, then, are not true ribs but are ossifications
of the tendinous structures.

II. From the anterior border of the os pubis and the last strong
inscription, also, to some extent, as a process of the preceding
part, begins a new fleshy layer which, extending in diminishing size
backward, is inserted by a strong tendon on the distoventral end of the
ischium somewhat laterad to the symphysis. It is the muscle that is
called by different authors the pyramidalis.

III. Rectus lateralis. About in the region of the twentieth vertebra,
or at the level of the fifth inscription, a fleshy band-like muscle
separates itself from the edge of the rectus muscle and the obliquus
internus muscle and passes over to fuse with the ischio-coccygeus
muscle.

IV. Rectus internus. On the inner surface of the rectus ventralis, from
which it is separated by the intervening aponeurosis of the rectus
muscle, appears a muscle lying on the outside of the diaphragmatic
muscle. It extends as a broad band from the breast to the anterior
border of the os pubis, with longitudinally directed fibers, to half
the width of the rectus ventralis muscle.

_Intercostales_ (Zwischenrippenmuskeln). The intercostal muscles in
the Crocodilia are, in proportion to the strength of the ribs, of
slight structure; they extend only from rib to rib and are, therefore,
very short, though fairly thick. They, as usual, consist of the outer
muscles with a direction like that of the external oblique, and of an
inner muscle extending in the opposite direction, _i.e._, at right
angles. The internal muscles are especially well developed in the
breast region and pass over into the internal oblique muscle.

_Quadratus Lumborum_ (Carré des lombes, Viereckiger Lendenmuskel, Psoas
major). A strong, thick muscle that springs from the inner surface
of the transverse processes and bodies of the last six presacral and
the first sacral vertebræ. The muscle diminishes as it passes in a
caudoventral direction and is inserted with a strong tendinous band to
the trochanter femoris.

_The Diaphragm_ (Diaphragmaticus, Zwerchfell, Bauchfellmuskel).
Closely inclosed between the skin and muscle of the abdomen, in the
Crocodilia, is a pair of muscles; they are, as a whole, thin muscles
that are widely separated and extend in an anteroposterior direction.
Each arises by two parts which, however, are united at the pelvis.
One of these parts is small at its beginning, is fairly thick, and is
attached by a short tendon, immediately over the pubis in front of the
hip joint, to the ilium. The other part is not a very thick layer,
and is attached, by a fairly long line, partly on the inner surface
of the hindermost abdominal rib and partly on the outer border of the
pubis. After the union of these two portions the muscle extends farther
forwards and the fibers of the stronger portion spread out like a fan,
becoming wider and thinner as they go forward and are at last attached
partly to the pericardium, partly to the lobes of the liver of that
side of the body. To be more exact, the fibers of the diaphragmaticus
that lie nearest the middle line of the belly-wall extend forward as a
fairly broad band to fuse with the pericardium. Most of the fibers of
this muscle, however, are in close connection with a fibrous membrane
which surrounds the liver parenchyma; this membrane is mostly very
thin but it gradually becomes thicker towards the hinder border of
the liver. Other muscle bands do not reach so far as the liver but
are located near the middle line of the back; they are all, however,
attached to an aponeurosis which passes over the upper, hinder border
of the liver lobes to fuse with the fibrous capsule of the liver.

To the sternum as to the ribs is only a small part of this muscle
attached.

Between the two above described muscles is found a space which is
filled, in great part, with a fibrous membrane that binds the two
muscles together. This membrane begins very thin and without a marked
boundary behind the kidneys; it runs forward directly under them and
the dorsal wall of the body, becoming gradually thicker, though never
very thick, and fuses, laterad to the kidneys, with the above-mentioned
aponeurosis of the two diaphragmaticus muscles. Thence this aponeurosis
goes to the upper, hinder side of the liver where it becomes fairly
thick. One thus finds in front of the stomach a fibrous membrane,
belonging to the diaphragmaticus, which is pierced by the œsophagus and
by a fairly large space that extends around the œsophagus and between
it and the liver. This membrane fastens the liver to the œsophagus.

The muscle of the right side is covered, on almost its entire inner
surface (from its hinder end to the liver) by the belly-like skin, and
is fairly closely united with it. The left muscle, on the other hand,
is only covered by this skin from the hinder border of the stomach
forwards; farther forward it lies immediately on the under and left
side of the stomach and is united with it by loose connective-tissue.
Outwardly both muscles are united by a thin layer of connective-tissue
to the true abdominal muscles. So far as yet known this muscle is not
present in other reptiles.

MUSCLES OF THE POSTERIOR APPENDAGES

_Ambiens_ (Plate III., Figs. 1 and 2, amb, Plate IV., Figs. 2 and
4, amb, Plate V., Figs. 2 and 3, amb) (Part I., Rectus femoris and
Sartorius partim, Vastus internus, Innere Streckmuskelmasse) (Part II.,
Gracilis, Rectus femoris, Sartorius). Arises by a short tendon from the
anterior spine of the ilium, near its union with the pubis. The muscle
swells quickly to a thick belly which, lying under the skin on the
forward and inner side of the upper thigh, is again reduced to a small,
flat tendon which extends abruptly over the anteromedial surface of the
knee joint to its outer side; it then passes through the complex of
tendons of the femoro-tibialis muscle, beneath which it unites with the
tendon of origin for the peroneus posterior muscle.

To this muscle is the following strange muscle to be ascribed (Part
II): it springs, small in extent, from the inner surface of the os
pubis near the acetabulum, extends thence forward around the pubis, and
runs into a long, thin tendon which unites with the insertion tendon of
the subcutaneous extensor ilio-tibialis muscle.

_Extensor Ilio-tibialis_ (Plate III., Fig. 2, ex. il. tb.) (Part I.,
Rectus femoris, Adductor flexor, Glutæus maximus; M. du facia lata,
Vastus externus, Tensor faciæ latæ, Tensor femoris vaginæ, Glutæus
minimus, Tensor faciæ femoris). This muscle, in the Crocodilia,
consists of two parts:

I. The chief part is long and broad, and springs as a tendon from more
than the anterior half of the lateral border of the dorsal crest of the
ilium, covering the origin of the ilio-fibularis muscle. Its insertion,
by a broad, flat tendon, overlying the femoro-tibialis muscle, together
with the tendon of this muscle, is on the anterior surface of the head
of the tibia.

II. The second, significantly smaller and narrower, part arises outside
of the quadratus lumborum by a short tendon from the most dorsal end of
the ilium; it goes over, medially, along the nearer head of the ambiens
muscle, then to the anteromedial side of the upper leg, into the
deeper-lying femoro-tibialis muscle.

_Femoro-tibialis_ (Plate III., Fig. 1, fm. tb., Plate IV., Fig. 2, fm.
tb., Plate V., Figs. 1 and 3, fm. tb.) (Cruræus et Vasti, Cruralis).
Arises by an anterior inner and a posterior outer head; both heads
arise from the outer-anterior and inner surfaces of the femur, and
unite with each other and with the extensor ilio-tibialis and ambiens
muscles as a strong tendon, which extends over the knee and is inserted
on the anterior border of the head of the tibia; this tendon incloses,
like a sheath, the end-tendon of the ambiens muscle.

_Ilio-fibularis_ (Plate III., Fig. 2, il. fib., Plate IV., Fig. 2,
il. fib., Plate V., Fig. 3, il. fib.) (Biceps cruris, Semitendinosus +
Semimembranosus, Glutæus maximus, Abductor fibularis, Flexor abductor
cruris). This consists, in the Crocodilia, of two entirely separate
small, band-like muscles. The first springs by a short tendon from
the lateral surface of the middle ilium, very near the origin of the
caudali-ilio-femoralis and extensor ilio-tibialis muscles. The chief
part of the end-tendon is inserted at the end of the first sixth of the
fibula, on its outer-forward corner near the origin of the peroneus
anterior muscle; a shorter tendon-branch goes to the tendon of the
peroneus posterior muscle; and a third, still smaller branch goes to
the caput femorale of the gastrocnemius muscle, by which it contributes
to the structure of the lateral part of the tendo-communis externus.

The second part springs, by an equally short tendon, very near the
first, from the hinder end of the dorsal crest of the ilium, goes
directly over the preceding to the knee, where its tendon unites with
that of the extensor ilio-tibialis muscle.

_Ilio-femoralis_ (Plate III., Fig. 2, il. fm., Plate IV., Fig. 2, il.
fm., Plate V., Fig. 1, il. f.) (Glutæus, Quadratus femoris [?], Glutæus
medius). This muscle is inwardly fused with the caudali-ilio-femoralis,
whose anterior part it forms.

_Caudali-ilio-femoralis_ (Plate III., Fig. 2, cd. il. fm., Plate
IV., Fig. 1, cd. il. fm.) (Zweiter Auswärtsroller, Extensor femoris
caudalis accessorius, Glutæus minimus). This forms a thick mass that
springs directly from the lateral surface of the anterior and middle
parts of the ilium, is covered outside by the ilio-fibularis muscle,
and, pushing between the two heads of the femoro-tibialis muscle, is
inserted on the whole outer surface of the middle third of the femur.

_Caudi-femoralis_ (Plate III., Figs. 1 and 2, cd. fm., Plate
IV., Fig. 1, cd. fm.) (Pyriformis, Pyriformis + Subcaudalis,
Femoro-peroneo-coccygeus, Extensor femoris caudalis). This muscle in
the Crocodilia consists of two parts:

I. The chief part extends from the first postsacral (the twelfth)
vertebra caudad; it springs from the roots of the caudal ribs
(transverse processes) and the whole lateral surface of the vertebral
arches. Since the first postsacral vertebra has no ventral process, the
muscles of the opposite sides fuse in the mid-line. Towards the caudal
region it gradually increases in strength. Its fibers converge in a
lateroventral direction to form a short, thick tendon which attaches
itself to the inner surface of the femur mediad and somewhat below the
trochanter. At right angles from this tendon extends a round, long
tendon which, lying parallel to the hinder side of the thigh, pushes
in between the chief parts of the ischiadicus and later between the
origin-tendon of the gastrocnemius and peroneus posterior muscles, and
is inserted on the posterior surface of the head of the fibula.

II. This is a more anterior and inner muscle, small in size, which
has a fleshy origin from the bodies and ribs (transverse processes)
of the second sacral and first caudal vertebræ at a distance from the
posteromedian border of the ischium. It extends caudad, lying near the
hinder part of the pubi-ischio-femoralis externus, and is inserted on
the trochanter.

_Flexor Tibialis Externus_ (Plate III., Figs. 1, 2, 3, fl. tb. ext.
or f. t. ext., Plate IV., Fig. 4, fl. tb. ext., Plate V., Figs. 1
and 3, fl. tb. ext. and f. t. ext.) (Triceps flexor cruris partim,
Biceps). A strong, spindle-shaped muscle that arises, together with the
ilio-fibularis, by a short tendon, from the side of the portio dorsalis
of the ilium, and in the neighborhood of the bend of the knee is split
into two tendons, of which the short one is inserted on the fibular
side of the neck of the tibia, while the other, running along near the
caput femoris of the gastrocnemius muscle, unites with the tendon of
the caput tibiæ of the gastrocnemius muscle just beyond the ankle joint.

_Flexor Tibialis Internus_ (Plate III., Figs. 1, 2, 3, fl. tb. int., or
f. t. int., Plate IV., Fig. 4, fl. tb. int., Plate V., Fig. 1, fl. tb.
int.) (Demi-nerveux + Demi-membraneux, Triceps flexor cruris partim,
Gracilis, Adductor flexor tibialis, Semimembranosus, Semitendinosus
+ Gracilis partim, Gracilis + Semimembranosus + Semitendinosus). A
three-headed muscle whose heads arise separately and first unite in
the region of the lower leg as a short, strong tendon. Their origins
are as follows: (1) As a band from the anterior margin of the ischium,
pushing between the ischio-femoralis and the pubi-ischio-femoralis
externus muscles; (2) From the posterior margin of the ischium as
a narrow, tendinous band near the insertion of the ischio-caudalis
muscle; (3) from the portio-dorsalis posterior of the ilium,
ventralward, near the origin of the flexor tibialis externus muscle.

_Ischio-femoralis_ (Plate III., Fig. 1, is. f.) (Adductores, Adductor
longus, Adductor primus). Springs directly (without tendon) from the
entire anterior border of the ischium. It is band-like and, running
over the tendon of the pubo-femoralis internus and externus muscle, is
inserted as a broad, fleshy tendon on the middle third of the inner,
posterior surface of the femur.

_Pubi-ischio-femoralis Externus_ (Plate III., Fig. 1, p. is. f. int.,
Plate IV., Fig. 1, p. is. f. int.) (as a whole: Quatuor pectinei
[partim]; in parts: I. Marsupialis externus, Obturator externus; II.
Quadratus femoris). This muscle arises in two parts. The anterior part
is broad and comes from the entire ventral and inwardly turned outer
surface of the pubis. Its insertion is on the femur in conjunction
with the first part of the pubi-ischio-femoralis internus. The
second part is shorter but thicker than the first and springs from
almost all of the outer surface of the ischium that is not covered
by the origins of the ischio-femoralis, pubi-ischio-tibialis, and
flexor tibialis internus muscles. It is inserted, by a strong, short
tendon, on the trochanter, somewhat caudad to the insertion of the
pubi-ischio-femoralis internus.

_Pubi-ischio-femoralis Internus_ (Plate III., Fig. 1, p. is. f. int.,
Plate IV., Figs. 1 and 2, p. is. f. int.) (as a whole: Iliacus internus
+ Quatuor pectinei [partim]; in parts: I. and II.: Kamm-Muskeln,
Pectineus inferior [I.] + superior [II.], Marsupialis internus,
Obturator internus; III., Iliacus [Darmbeinmuskel], Iliacus internus).
This muscle arises by two or three parts: I. The anterior arises,
without tendon, from the greater part of the inner and anteriorly
directed surface of the pubis; median to this, in the alligator, is
a small bundle, II., which unites with part I. These unite with the
pubi-ischio-femoralis externus to form a strong tendon that is inserted
on the trochanter.

III. This is a larger muscle that springs without tendon from the inner
surface of the body and transverse processes of the twenty-fifth and
twenty-sixth (in alligator) vertebræ, from the part of the ilium that
lies between these transverse processes and the ischium, and from a
small part of the ischium. It is inserted on the whole inner surface of
the proximal third of the femur.

_Pubi-ischio-femoralis Posterior_ (Plate III., Fig. 1, p. is. f. post.,
Plate IV., Figs. 1 and 4, p. is. f. post. and pb. is. f. m. post.)
(Adductus [partim], Gemellus, Obturator internus). Springs without
tendon from the whole caudally directed border of the ischium. It is
inserted, by a short tendon, near and laterad to the tendon of the
ischio-femoralis muscle, on the hinder surface of the upper leg.

_Extensor Longus Digitorum_ (Plate V., Figs. 2 and 3, ext. l. [long.]
dig.) (Long extenseur commun, Gemeinschaftlicher Fussheber oder Beuger,
Extensor communis digitorum). Springs, together with the tibialis
anticus, from the external condyle of the femur, goes with this muscle
under the ligamentum tibio-fibulare and after union with this divides
into four short tendons. Three of these tendons are inserted on the
fibular side of the bases of the first three metatarsal bones; the
fourth goes over into the muscle of the third toe.

_Tibialis Anticus_ (Plate III., Fig. 1, tib. ant., Plate IV., Fig. 4,
tib. ant., Plate V., Figs. 2 and 3, tib. ant.) (Jambier antérieur,
Vorderer Schienbeinmuskel). Springs by a fleshy tendon from the
anterior surface of the head and neck of the tibia and quickly unites
with the following muscle.

_Peroneus Anterior_ (Plate V., Figs. 2 and 3, peron. ant.) (Peroneus
longus). In the alligator. Its origin: it extends from the insertion
of the ilio-fibularis muscle distalwards by the whole outer surface of
the fibula, under the skin and over the ligamentum tibio-fibulare, and
gives off a broad, tendinous portion to help strengthen the tendon
Achilles, which portion may be followed to the rudiment of the fifth
toe. On the outer side of this toe rudiment is attached a tendon from
the tibial border of the muscle.

In the crocodile. This muscle is here divided into two parts, of
which the one that springs from the anterior surface of the fibula
is inserted on the toe rudiment, while the greater and outward part
extends over the calcaneum bone and has the same distribution as in the
alligator.

_Peroneus Posterior_ (Plate IV., Figs. 3 and 4, peron. post., Plate
V., Figs. 2 and 3, peron. post.) (Plantaris). Springs chiefly from the
tendon of the ambiens muscle running over the knee and forms the direct
continuation of this muscle. Besides this come tendinous fibers from
the insertion tendons of the femoro-tibialis and extensor ilio-tibialis
muscles; and finally supporting fibers from the outer, end-tendon of
the ilio-fibularis muscle. The fibers of this muscle pass partly into
the fibular portion of the caput femoralis of the gastrocnemius, while
the chief mass of the muscle is inserted on the posterior surface of
the calcaneum.

_Gastrocnemius_ (Plate III., Figs. 1 and 3, cap. int. gastr., Plate
IV., Fig. 4, cap. int. gastr. and cap. ext. gastr., Plate V., Figs. 2
and 3, cap. ext. gastr. and cap. int. gastr.) (Solenmuskel, Outer head
of gastrocnemius). This is the strongest superficial flexor muscle on
the posterior surface of the lower leg; it consists of two heads: I.
The caput femorale (externum) originates by a strong, short tendon from
the lateral and posterior surface of the external condyle of the femur.
This head has a double insertion: (1) from the outer, fibular border of
the muscle separates off a tendon that spreads out in the first layer
of the plantar tendon-muscle; (2) the chief part of caput I. becomes
a broad, flat, subcutaneous tendon which is covered by the tendon
Achilles and serves as the origin of the short flexors of the toes.

II. The caput tibiale (internum) springs without a tendon from the
posterior surface of the head and the proximal third of the tibia. The
broad and somewhat flat muscle has two insertions: (1) on the plantar
and medial border of the first basis metatarsi; (2) the chief insertion
on the outer border of the rudimentary fifth toe, after forming, with
the tendon of the flexor tibialis externus, the tendon Achilles.

_Flexor Longus Digitorum_ (Plate IV., Fig. 4) (Langer durchbohrender
gemeinschaftlicher Zehenbeuger). A many-headed muscle visible on
the posterior side of the lower leg after removal of the preceding
muscle. (a) Caput externum: a flat, fairly broad muscle which springs
from the outer and posterior surface of the fibula. Arriving at the
astragalo-scaphoid bone, it forms a very strong tendon which unites
with the still stronger tendon of the other head and both together
form the broad initial tendon of the flexor digitorum communis brevis
muscle. (b) Caput internum: this springs without a tendon from the
whole posterior surface of the upper half of the tibia, sometimes, as
in crocodiles, uniting with the caput femoralis of the gastrocnemius
muscle. The common tendon splits into three points for the first,
second, and third toes.

_Tibialis Posticus_ (Plate V., Fig. 2, tib. post.) (Jambier postérieur,
Hinterer Schienbeinmuskel). Originates without tendon from the whole
fibular side of the tibia, on the one hand, and from the whole inner
and forward side of the fibula on the other hand, occupying the
whole space between these two bones on the hinder side of the lower
leg. At its proximal end it is united with the caput internum of the
preceding muscle which completely covers it from behind. It narrows
down to a very strong tendon which divides into two equally strong,
round tendons; of these the one on the tibial side is inserted on the
basis ossis of the first metacarpal, the one towards the fibula goes
immediately to the second metacarpal.

_Interosseus Cruris_ (Kniekehlmuskel). A small muscle stretching
between the distal ends of the fibula and tibia with almost transverse
fibers; it is covered dorsally by the tibio-fibulare ligament and
appears as a distal division of the tibialis posticus muscle.

_Flexor Digitorum Brevis_ (Flexor longus accessorius, Flexor brevis
perforatus). Springs from the bones of the foot and from the strong
tendon of the flexor digitorum longus muscle. It divides into three
bellies for the second, third and fourth toes. The first two are
pierced by the above-mentioned tendon of the flexor digitorum longus
and are inserted on the next to last phalanx of the second and third
toes; the third, on the contrary, is inserted on the claw joint of the
fourth toe and is not perforated; there is no tendon to the fourth toe
from the flexor digitorum longus muscle.

_Extensor Hallucis Proprius_ (Plate V., Fig. 3) (Kurzer
gemeinschaftlicher Zehenstrecker, Extensor hallucis). This muscle
springs by a short, flat, fairly strong tendon from the outer dorsal
border of the distal half of the fibula. It is inserted: (1) on the
proximal half of the first metatarsal bone, (2) a second much weaker
part is united with the tendon of insertion of the tibialis anticus
extensor longus digitorum to the first metatarsal bone.

THE TAIL MUSCLES

The muscles of the tail have, as shown by Gadow, the character of
the primitive body muscles, with their primitive metameric division,
fairly plainly preserved. This musculature is arranged in four rows
of trumpet-shaped cones, one projecting into the other, by which
arrangement each metamere exhibits a transverse zigzag line of four
anteriorly and three posteriorly directed points.

_Ilio-ischio-caudalis_ (Plate III., Figs. 1 and 2, is. cd., Plate
IV., Figs. 1 and 2, is. cd.) (Ischio-coccygeus). The crocodile is the
nearest to the typical condition in the musculature of the tail. The
lateral and ventral part of the tail musculature forms a broad mass
that extends to the end of the tail; it lies immediately under the skin
and springs from the caudal ribs (transverse processes--Gadow) and
from the spinous processes of all the caudal vertebræ. The entire side
musculature of the tail ends cephalad in several portions; the most
ventral and medial of these bound the cloaca as an at least slightly
developed, morphological sphincter; the lateral portion is attached
to the posteroventral border of the ischium; while the dorsal portion
is inserted by two heads on the first caudal rib and on the posterior
spine of the ilium.

PLATE I.

SHOULDER MUSCLES OF CROCODILUS ACUTUS. (From Bronn after Fürbringer.)

FIG. 1. SHOULDER MUSCLES AFTER REMOVAL OF THE SPHINCTER COLLI MUSCLE
(_sphr_).

FIG. 2. SHOULDER MUSCLES AFTER REMOVAL OF THE SPHINCTER COLLI MUSCLE
(_sphc_).

FIG. 3. DEEP LAYER OF THE INNER SHOULDER MUSCLES AFTER REMOVAL OF
THE HUMERUS AND ITS MUSCULATURE AS WELL AS THE COLLO-SCAPULARIS
SUPERFICIALIS MUSCLES (_cssp_) AND THORACI-SCAPULARIS SUPERFICIALIS
MUSCLES (_thcsp_).

FIG. 4. SHOULDER MUSCLES AFTER REMOVAL OF THE PARS SCAPULARIS OF THE
SUPRA-CORACO-SCAPULARIS (_sps_) AND OF THE BICEPS MUSCLE (_b_).

FIG. 5. DIFFERENT VIEW OF FIG. 4.

FIG. 6. SHOULDER MUSCLES AFTER REMOVAL OF THE PARS CORACOIDEA OF THE
SUPRA-CORACO-SCAPULARIS (_spc_) AND DELTOIDES SCAPULARIS SUPERIOR
(_dss_) MUSCLES.

LETTERING FOR ALL FIGURES OF THIS PLATE.

_acs_, _ahl_, _ahp_, _asl_, coraco-scapular, humerale laterale,
humerale posticum, and scapulare laterale externum heads of
the anconæus muscle; _b_, coraco-antebrachialis (biceps); _c_,
coracoid; _cbb_, coraco-brachialis; _cc_, costo-coracoideus; _Cl_,
clavicle; _cssp_, collo-scapularis superficialis (levator scapulæ
superficialis); _cst_, capiti-sternalis (sterno-mastoideus);
_cthspr_, collo-thoraci-scapularis profundus (levator scapulæ and
serratus profundus); _cu_, dorso-scapularis (cucullaris); _dh_,
dorso-humeralis (latissimus dorsi); _dsi_, deltoides scapularis
inferior; _dss_, dorsalis scapulæ (deltoides scapularis superior);
_Ec_, epicoracoid; _Est_, episternum; _esthy_, episterno-hyoideus;
_H_, humerus; _hai_, humero-antebrachialis inferior (brachialis
inferior); _hr_, humero-radialis; _p_, pectoralis; _PL_, processus
lateralis humeri; _PM_, processus medialis humeri; _R_, radius;
_rh_, rhomboideus; _S_, scapula; _sbsc_, subscapularis; _shpr_,
scapulo-humeralis profundus; _spc_, supra-coraco-scapularis; _sphc_,
sphincter colli; _SpS_, spina scapulæ; _SS_, suprascapulare; _St_,
sternum; _Sta_, anterior part of sternum; _Stp_, posterior part
of sternum; _thssp_, thoraci-scapularis superficialis (serratus
superficialis); _U_, ulna; _V₅_, _V₆_, 5th and 6th vertebræ.

NERVES SHOWN IN THIS PLATE.

3_a_. thoracicus VII.
7. posterior branch of the thoracicus superior VII for the
collo-thoraci-suprascapularis profundus and rhomboideus
muscles.
7_a_. proximal.
7_b_. distal thoracicus superior VII.
10_a_. thoracicus inferior.
12. supra-coracoideus.
15. integumental, (13 and 14), muscular branch of the
supra-coracoideus.
19. pectoralis.
21. brachialis longus inferior.
29_b_. teres major.
31. dorsalis scapulæ (posterior).
32. cutaneus brachii and antebrachii superior lateralis.
32_a_. humero-radialis.
33. deltoides inferior, (25 and 42), cutaneus brachii and
antebrachii medialis.

[Illustration]

PLATE II.

FIGS. 1-4. MUSCLES OF THE FOREARM OF THE ALLIGATOR.

(From Bronn.)

FIGS. 5-7. (From Bronn after Rathke.)

FIG. 1. _1_, humero-radialis longus (supinator longus);
_2_, humero-radialis medialis (flexor carpi radialis); _3_,
carpo-phalangei I digiti V; _4_, carpo-phalangei (flexor digitorum
communis brevis); _5_, humero-ulno-phalangei (flexor digitorum
communis profundus); _6_, humero-radialis longus; _s._ flexor
carpi ulnaris; _7_, pisiforme-phalangeus primus digiti V; _8_,
carpo-phalangeus I; _9_, carpo-metacarpalis I.

FIG. 2. 1-5 as in FIG. 1; _a_, humero-carpi-radialis; _b_,
humero-metacarpalis III, IV, V (extensor digitorum longus); _c_,
humero-carpi-ulnaris; _d_, carpo-phalangei (extensor-digitorum
brevis).

FIG. 3. _a_, _b_, humero-ulno-phalangei (flexor digitorum communis
profundus).

FIG. 4. _a_, ulno-carpi-radialis; _b_, ulna; _c_,
humero-ulnaris-lateralis (flexor carpi ulnaris); _d_, humero-radialis
brevis (supinator brevis).

FIG. 5. HEAD, NECK, AND A PART OF THE BODY OF A CROCODILUS VULGARIS.
(Ventral View).

_a_, lower jaw; _b_, upper jaw; _c_, arch of palate; _d_, fold of
palate; _h_, pterygoideus internus (Rathke); _i_, pterygoideus
externus (Rathke); _k_, longus colli (Rathke); _m_, rectus capitis
anticus major (Rathke); _n_, sterno-mastoideus (Rathke); _o_, levator
scapulæ (Rathke); _p_, scalenus (Rathke).

FIG. 6. PART OF A SIMILAR PREPARATION OF C. RHOMBIFER.

_a_, the hindermost of the superior maxillary teeth; _b_, lower
jaw; _c_, wings of palate; _d_, pterygoid; _e_, quadrate; _g_,
intertransversalis (Rathke); _h_, trachelomastoideus (Rathke);
_i_, levator scapulæ (Rathke); _k_, longus colli (Rathke); _m_,
pterygoideus externus (Rathke); _n_, rectus capitis anticus major
(Rathke).

FIG. 7. A PART OF THE HEAD AND NECK OF ALLIGATOR LUCIUS.

_a_, pterygoideus externus (Rathke); _b_, digastricus (Rathke);
_c_, rectus capitis anticus major (Rathke); _d_, sterno-mastoideus,
anterior belly (Rathke); _e_, sterno-mastoideus, posterior belly
(Rathke); _f_, levator scapulæ (Rathke); _g_, cervicalis adscendens
(Rathke); _h_, longus colli (Rathke); _i_, intertransversalis
(Rathke); _k_, trachelomastoideus (Rathke); _l_, biventer cervicis
(Rathke); _m_, splenius colli (Rathke); _n_, splenius capitis
(Rathke).

[Illustration]

PLATE III.

FIG. 1. MUSCLES OF THE POSTERIOR EXTREMITY OF ALLIGATOR
MISSISSIPPIENSIS. LEFT SIDE, VENTRAL (PLANTAR) SURFACE.

FIG. 2. THE SAME, DORSAL AND LATERAL SURFACES.

FIG. 3. A. MISSISSIPPIENSIS; THE TENDONS OF THE FLEXOR TIBIALIS
MUSCLE IN THEIR RELATION TO THE GASTROCNEMIUS MUSCLE. RIGHT LEG,
INNER SURFACE. (FIGS. 1-3 from Bronn, after Gadow.)

_amb_, ambiens; _cap_, _ext_, _gastr_, external head of
gastrocnemius; _cap_, _int_, _gastr_, internal head of
gastrocnemius; _cd_, _fm_, caudali-femoralis; _cd_, _il_, _fem_,
caudi-ilio-femoralis; ex. il. tb. extensor ilio-tibialis; _ext_,
_l_ (_long_), _dig_, extensor longus digitorum; _fl_, _tb_, _ext_,
flexor tibialis externus; _fl_, _tb_, _int_, flexor tibialis
internus; _fm_, _tb_, femoro-tibialis; _il_, _cd_, ilio-caudalis;
_il_, _cost_, ilio-costalis; _il_, _fib_, ilio-fibularis; _il_, fm
(_il_, f), ilio-femoralis; _il_, _s_, _cd_, ilio-sacro-caudalis;
_is_, _cd_, ischio-caudalis; _is_, _f_, ischio-femoralis; _ob_,
_ext_, obliquus externus; _pb_, _cd_, pubi-caudalis; _pb_, _is_,
_tb_, pubi-ischio-tibialis; _pb_, _tb_, pubi-tibialis; _peron_,
_ant_, peroneus anterior; _peron_, _post_, peroneus posterior; _p_,
_is_, _f_, _ext_, pubi-ischio-femoralis externus; _p_, _is_, _f_,
_int_, pubi-ischio-femoralis internus; _p_, _is_, _f_, _post_,
pubi-ischio-femoralis posterior; _qudr_, _lb_, quadratus lumborum;
_rect_, rectus abdominis; _tib_, _ant_, tibialis anticus; _tib_,
_post_, tibialis posticus; _trans_, transversus abdominis; _tr_,
_per_, transversus perinei; _m_, _post_, _il_, posterior border of
ilium; _ob_, foramen in pubis for the obturator nerve; _o_, _il_,
ilium; _o_, _is_, ischium; _o_, _pb_, pubis; _o_, _cl_, cloacal bone;
_pr_, _l_, _pb_, lateral process of pubis; _pr_, _tr_, transverse
process; _sp_, _ant_, _il_, anterior spine of ilium; _Sy_, _p_,
symphysis pubis; _Sy_, _is_, symphysis of ischium; _tb_, _is_,
tubercle of ischium.

[Illustration]

PLATE IV. (From Bronn, after Gadow.)

FIG. 1. ALLIGATOR MISSISSIPPIENSIS. INNER SURFACE OF THE PELVIC
REGION, LEFT SIDE. THE PUBIS, ISCHIUM, AND VERTEBRÆ ARE CUT THROUGH
THE MEDIAN PLANE, XXVIII, 28th VERTEBRA.

FIG. 2. HATTERIA PUNCTATA.

FIG. 3. A. MISSISSIPPIENSIS. THE DEEPEST MUSCLES ON THE PLANTAR
SURFACE OF THE LEFT HIND FOOT. ROMAN NUMERALS IX-XII, SHORT TOE
MUSCLES.

FIG. 4. A. MISSISSIPPIENSIS. LEFT LEG FROM THE POSTERO-MESIAL
ASPECT. THE PLANTAR FLEXOR MUSCULATURE IN SITU, AFTER REMOVAL OF THE
GASTROCNEMIUS MUSCLE AND THE ASSOCIATED MUSCLES. ROMAN NUMERALS,
VI-X, SHORT TOE MUSCLES.

_amb_, ambiens; _cap_, _ext_, _gastr_, external head of
gastrocnemius; _cap_, _int_, _gastr_, internal head of
gastrocnemius; _cd_, _fm_, caudali-femoralis; _cd_, _il_, _fem_,
caudi-ilio-femoralis; _ex_, _il_, _tb_, extensor ilio-tibialis;
_ext_, _l_ (_long_), _dig_, extensor longus digitorum; _fl_, _tb_,
_ext_, flexor tibialis externus; _fl_, _tb_, _int_, flexor tibialis
internus; _fm_, _tb_, femoro-tibialis; _il_, _cd_, ilio-caudalis;
_il_, _cost_, ilio-costalis; _il_, _fib_, ilio-fibularis; _il_, _fm_
(_il_, _f_), ilio-femoralis; _il_, _s_, _cd_, ilio-sacro-caudalis;
_is_, _cd_, ischio-caudalis; _is_, _f_, ischio-femoralis; _ob_,
_ext_, obliquus externus; _pb_, _cd_, pubi-caudalis; _pb_, _is_,
_tb_, pubi-ischio-tibialis; _pb_, _tb_, pubi-tibialis; _peron_,
_ant_, peroneus anterior; _peron_, _post_, peroneus posterior; _p_,
_is_, _f_, _ext_, pubi-ischio-femoralis externus; _p_, _is_, _f_,
_int_, pubi-ischio-femoralis internus; _p_, _is_, _f_, _post_,
pubi-ischio-femoralis posterior; _qudr_, _lb_, quadratus lumborum;
_rect_, rectus abdominis; _tib_, _ant_, tibialis anticus; _tib_,
_post_, tibialis posticus; _trans_, transversus abdominis; _tr_,
_per_, transversus perinei; _m_, _post_, _il_, posterior border of
ilium; _ob_, foramen in pubis for the obturator nerve; _o_, _il_,
ilium; _o_, _is_, ischium; _o_, _pb_, pubis; _o_, _cl_, cloacal bone;
_pr_, _l_, _pb_, lateral process of pubis; _pr_, _tr_, transverse
process; _sp_, _ant_, _il_, anterior spine of ilium; _Sy_, _p_,
symphysis pubis; _Sy_, _is_, symphysis of ischium; _tb_, _is_,
tubercle of ischium.

[Illustration]

PLATE V. (From Bronn, after Gadow.)

FIG. 1. HATTERIA PUNCTATA. (Ventral View.)

FIG. 2. ALLIGATOR MISSISSIPPIENSIS. LEFT POSTERIOR EXTREMITY; FOOT
IN PRONATION, HENCE SEEN FROM THE DORSAL SIDE. _lg_, _t_, _f_,
LIGAMENTUM TIBIO-FIBULARE.

FIG. 3. A. MISSISSIPPIENSIS. MUSCLES OF THE DORSAL SURFACE OF THE
LOWER LEG AND FOOT.

_amb_, ambiens; _cap_, _ext_, _gastr_, external head of
gastrocnemius; _cap_, _int_, _gastr_, internal head of
gastrocnemius; _cd_, _fm_, caudali-femoralis; _cd_, _il_, _fem_,
caudi-ilio-femoralis; _ex_, _il_, _tb_, extensor ilio-tibialis;
_ext_, _l_ (_long_), _dig_, extensor longus digitorum; _fl_, _tb_,
_ext_, flexor tibialis externus; _fl_, _tb_, _int_, flexor tibialis
internus; _fm_, _tb_, femoro-tibialis; _il_, _cd_, ilio-caudalis;
_il_, _cost_, ilio-costalis; _il_, _fib_, ilio-fibularis; _il_, _fm_
(_il_, _f_), ilio-femoralis; _il_, _s_, _cd_, ilio-sacro-caudalis;
_is_, _cd_, ischio-caudalis; _is_, _f_, ischio-femoralis; _ob_,
_ext_, obliquus externus; _pb_, _cd_, pubi-caudalis; _pb_, _is_,
_tb_, pubi-ischio-tibialis; _pb_, _tb_, pubi-tibialis; _peron_,
_ant_, peroneus anterior; _peron_, _post_, peroneus posterior; _p_,
_is_, _f_, _ext_, pubi-ischio-femoralis externus; _p_, _is_, _f_,
_int_, pubi-ischio-femoralis internus; _p_, _is_, _f_, _post_,
pubi-ischio-femoralis posterior; _qudr_, _lb_, quadratus lumborum;
_rect_, rectus abdominis; _tib_, _ant_, tibialis anticus; _tib_,
_post_, tibialis posticus; _trans_, transversus abdominis; _tr_,
_per_, transversus perinei; _m_, _post_, _il_, posterior border of
ilium; _ob_, foramen in pubis for the obturator nerve; _o_, _il_,
ilium; _o_, _is_, ischium; _o_, _pb_, pubis; _o_, _cl_, cloacal bone;
_pr_, _l_, _pb_, lateral process of pubis; _pr_, _tr_, transverse
process; _sp_, _ant_, _il_, anterior spine of ilium; _Sy_, _p_,
symphysis pubis; _Sy_, _is_, symphysis of ischium; _tb_, _is_,
tubercle of ischium.

[Illustration]

CHAPTER IV

THE NERVOUS SYSTEM

SPINAL CORD

The spinal cord extends the whole length of the vertebral canal and
ends near the end of the tail as a thin, round thread. It varies in
thickness and shape in cross section, being nearly always elliptical,
but at places approaching a circle. Large, spindle-formed thickenings
of about equal diameter are present in the cervical and lumbar regions.

A cauda equina is absent in the alligator, the nerves of the large tail
leaving the cord like the intercostals.

On its ventral surface the cord has a deep perpendicular fissure, the
fissura ventralis, that extends almost to the center; it extends even
along the reduced region in the tail. A vascular membrane extends into
this fissure.

A shallow but distinct furrow extends along the dorsal side of the
cord, parallel to which, on either side, is a fine, linear furrow.

The first two spinal nerves have no dorsal roots.

BRAIN

The cervical cord passes insensibly into the medulla, the dorsal furrow
becoming wider and more shallow as it merges into the fourth ventricle.

A dorsal view of the brain is shown in Figure 30, A. The most prominent
structures here seen are the cerebral hemispheres, VH, whose combined
transverse diameter is greater than their longitudinal. The tapering,
cephalic end of each hemisphere forms an olfactory tract, I, which
extends cephalad to form the olfactory bulb, B. ol. Lying between the
caudal ends of the hemispheres is a small conical body, G.p., called by
Bronn and others the pineal body. The writer has found (62), however,
that this body is the paraphysis rather than the epiphysis. Caudad to
the cerebra-hemispheres and in contact with them are the optic lobes,
MH; they have about the same shape and position as in the frog, but are
much smaller in proportion to the size of the hemispheres. Immediately
caudad to the optic lobes is the cerebellum, HH, somewhat elliptical in
outline as seen from above.

Extending caudad from beneath the cerebellum is the medulla, NH, with
its triangular fourth ventricle. The outlines of the medulla are
somewhat obscured by the numerous roots of the eighth to eleventh
cranial nerves, VIII-XI, which arise along its dorsal border. The
medulla, as was said above, passes, as is usually the case, without
any line of demarcation into the spinal cord, the obex filling in the
apex of the fourth ventricle at the anterior end of the median dorsal
fissure.

[Illustration: FIG. 30. BRAIN OF ALLIGATOR. (A, dorsal; B, ventral; and
C, lateral view.) (From Wiedersheim, slightly altered.)

_VH_, cerebral hemispheres, each of which gives rise
postero-laterally to a hippocampal lobe partially overlying the
corresponding optic tract, _Tr.opt_; _ZH_, thalamencephalon; _MH_,
optic lobes; _HH_, cerebellum; _NH_, medulla oblongata; _I-XII_,
cranial nerves; _1_, _2_, first and second spinal nerves; _B.ol_,
olfactory bulb; _Tro_, olfactory tract; _G.p_, paraphysis; _Jnf_,
infundibulum; _Hyp_, hypophysis; _Med_, spinal cord.]

A lateral view of the brain is shown in Figure 30, C. The hemisphere,
VH, is conical in outline, with a small projection from the
posteroventral region; its continuation forwards as the olfactory
tract, Tro., and bulb, B. ol., is plain. Beneath it and extending
forwards are the prominent optic nerve, II, and tract. Caudad to the
latter and projecting ventrad and caudad are the infundibulum, Inf.,
and hypophysis, Hyp.

Caudad to the cerebrum are seen the end of the paraphysis, G.p., the
optic lobes, MH, and the cerebellum, HH. From the cerebral peduncles
(ventrad to the optic lobes) arises the oculomotor nerve, III, and
dorsocaudad to this, between the optic lobe and the cerebellum, arises
the trochlear nerve, IV. From the middle zone (in a dorsoventral
direction) of the medulla, ventrad to the cerebellum, arises the very
large trigeminal nerve, V; while from its usual place, on the ventral
surface of the medulla, the abducens nerve, VI, takes its origin by
several roots. At some distance caudad from the trigeminal, from the
dorsal surface of the medulla, as noted above, the very large acoustic
nerve, VIII, arises; and immediately ventrad to this, on the side of
the medulla, the facial nerve, VII, may be seen. Commencing just
caudad to the acoustic and extending along the upper border of the
medulla and beginning of the spinal cord, are seen a dozen or more
small nerve roots, which unite to form the glossopharyngeal, IX, vagus,
X, and spinal accessory, XI, nerves. Ventral to the roots of the
last, on the ventral surface of the medulla, arise the roots of the
hypoglossal nerve, XII. A short distance caudad to this nerve are seen
the first two spinal nerves, 1 and 2, which have, as noted above, no
dorsal roots.

A ventral view of the brain is shown in Figure 30, B. The cerebral
hemispheres, VH, have the same outline, of course, as in the dorsal
view, but the rounded projection from the caudal end of each is here
seen on each side of the infundibulum, Inf. The infundibulum is in
close contact with the chiasma anteriorly, and lies close between the
converging optic tracts, Tr. opt. From the chiasma the optic nerves,
II, extend, in an antero-lateral direction, almost at right angles to
each other. The appearance of the olfactory tracts, I, is the same
as in the dorsal view. Caudad to the infundibulum, from the cerebral
peduncles, ZH, arise the rather small oculomotor nerves, III. Caudad
to these, from near the ventral fissure, on the middle region of the
medulla, arise the abducens nerves, VI, and from the ventral side of
the posterior part of the medulla and of the anterior end of the cord
arise the hypoglossal, XII, and the first two spinal nerves, 1 and 2.
The origins of the other cranial nerves were described in connection
with the lateral view of the brain, where they show more clearly. On
each side of the cerebral peduncles is seen the ventrolateral edge of
the corresponding optic lobe. The pyramidal tracts are seen, extending
caudad from the region of the peduncles, as a swelling on each side of
the median ventral fissure.

THE CRANIAL NERVES (CROCODILE)

The origin of each of the cranial nerves was described in connection
with the lateral and ventral views of the brain. A full description
of the distribution of these nerves would require more space than the
limits of this book will allow, but a brief account will now be given.

I and II. The olfactory and optic nerves. These two large nerves go
immediately to their respective sense organs, so that no further
discussion of them need be here given.

III. The oculomotor nerve. The single stem divides into three branches:
a median, going to the externus rectus muscle; a lateral, going to
the inferior rectus muscle; and an intermedial, going to the inferior
oblique muscle.

IV. The trochlear (pathetic) nerve leads to the superior oblique muscle.

V. The trigeminal nerve. The distribution of this nerve is very
complicated. It has three main divisions: (1) the ophthalmic branch,
(2) the superior maxillary branch, and (3) the inferior maxillary
branch. (1) The ophthalmic in turn divides into two branches: the
smaller, frontal, going to the integument of the upper and lower
eyelids; the larger, nasal, going chiefly to the nasal cavity but also
sending some small branches to the upper and lower eyelids. (2) The
superior maxillary branch separates into a number of divisions: (a) a
branch that, in the neighborhood of the auditory capsule, fuses with
the facial nerve; (b) a twig to the integument of the forehead and to
the upper and lower eyelids; (c) a branch to the Harderian gland and
the conjunctiva; (d) a branch to the neighborhood of the cheek, to the
angle of the mouth, and to the palatine branch of the facial nerve; (e)
a branch to the palate; (f) a branch to the integument of the upper
jaw; (g) a branch to the teeth of the upper jaw. (3) The inferior
maxillary branch divides into four branches: (a) this division supplies
the skin of the cheek region; (b) a branch to the chewing muscles; (c)
a branch that divides into two nerves--the first going to the skin of
the lower jaw, the second dividing again into two nerves, both of which
lead to the integument of the lower jaw; (d) the fourth division of the
inferior maxillary, known as the inferior alveolar, itself divides into
two twigs--(a′) the first twig divides into two parts, a larger and a
smaller, both of which lead, by different paths, to the inner skin of
the mouth; (b′) the second twig divides into four parts--two leading to
the mylohyoid muscle and to the integument at the corner of the mouth,
one to the integumental glands at the corner of the mouth, and one to
the floor of the mouth cavity.

VI. The abducens nerve leads to the retractor oculi muscle and to the
muscle for the nictitating membrane.

VII. The facial nerve gives off three main branches: (1) the first
divides again into three twigs--(a) connecting with a branch of the
trigeminal nerve, (b) and (c) connecting with the trigeminal and also
leading to the palate; (2) the second branch divides into two twigs
that connect with the glossopharyngeal nerve; (3) the third branch
divides into two parts, a muscular twig, and the chorda tympani.

VIII. The auditory or acoustic nerve leads, of course, to the sensory
regions of the ear.

IX. The glossopharyngeal nerve divides into four main branches,
as follows: (1) to the larynx, (2) to the œsophagus, (3) to the
hyomaxillary and sterno-maxillary muscles, and (4) to the tongue. There
are also certain communicating twigs with the facial and vagus nerves.

X. The vagus or pneumogastric nerve gives off four branches: (1)
and (2) communicate with each other and supply the pharynx, larynx,
œsophagus, and trachea; (3) goes to the œsophagus; (4) goes to the
heart, lungs, and stomach.

XI. The spinal accessory nerve. There seems to be some doubt as to the
exact identity and distribution of this nerve, but Bronn says that,
according to Fischer, it gives twigs to the lower head-muscles and then
divides into fine branches in the atlanti-mastoideus muscle.

XII. The hypoglossal nerve, going to the region of the tongue,
divides into three branches: (1) the median and smallest goes to the
sterno-maxillary muscle; (2) the inner and larger goes to the same
muscle and also to the coraco-hyoid and sterno-hyoid muscles; (3) the
outer and largest divides into three twigs of which the first two
lead to the hyomaxillary and sterno-maxillary muscles respectively,
while the third divides into two twigs that lead to the hyoglossal and
genioglossal muscles respectively.

THE SPINAL NERVES

As was noted above, the dorsal roots of the first two spinal nerves are
lacking.

I, II, and III. The ventral branches of these three nerves supply the
smaller, ventral neck muscles.

IV. The ventral branch of this nerve innervates with its chief
divisions the ventral muscles, the sphincter colli, and the integument
of the neck, and sends a small branch to the levator scapulæ
superficialis muscle.

V. The ventral branch of this nerve sends branches to the ventral
muscles of the neck, to the levator scapulæ superficialis; a large
branch goes to the sterno-mastoid; and the rest of the nerve
distributes itself in the sphincter colli and the integument and
ventral muscles of the neck.

VI. The sixth nerve distributes itself to the ventral musculature and
to the integument of the neck, and sends a fairly strong branch to the
levator scapulæ superficialis muscle and to the most anterior part of
the collo-thoraci-suprascapularis profundus muscle.

VII. The seventh nerve is the first to enter, by a small branch, into
the brachial plexus (Figure 31). It also sends a branch to the ventral
muscles and the integument of the neck, and three branches to various
shoulder muscles.

VIII. The ventral branch of the eighth nerve (Figure 31) is the
second largest nerve of the brachial plexus. It gives some twigs
to the ventral muscles and then gives one or two nerves to the
collo-thoraci-suprascapularis profundus and the serratus superficialis
muscles. The rest of the nerve divides into an inferior and a superior
branch which unite with the ninth nerve.

IX. The ninth and tenth nerves are the largest of the brachial plexus.
The former, after giving off some twigs to the ventral musculature
and to the serratus superficialis and the hinder regions of the
collo-thoraci-suprascapularis profundus muscles, unites with the tenth
nerve just after giving off the small thoracicus inferior nerve to
the costo-coracoideus muscle. After uniting with the tenth nerve the
ninth nerve immediately divides into two branches that form loops with
branches of the eighth nerve, the whole making a very complicated
plexus.

X. The tenth nerve, as noted above, is one of the two largest nerves
of the brachial plexus. After giving off a single nerve to the ventral
musculature, this nerve unites with the eleventh nerve; it then gives a
branch to the costo-coracoideus muscle and forms a loop with the ninth
nerve. After giving off a couple of nerves it again divides into two
equal branches which unite with similar branches of the eighth nerve.

XI. The eleventh nerve is next to the smallest of the plexus.
Besides branches to the trunk musculature it gives a fine twig to
the integument of the axilla and unites with the tenth nerve in the
brachial plexus. This is the last nerve that enters into the brachial
plexus.

[Illustration: FIG. 31. BRACHIAL PLEXUS OF C. ACUTUS. (From Bronn,
after Fürbringer.)

VII-XI. ventral branches of seventh to eleventh spinal nerves.
3_a_. thoracicus anterior VII.
4. thoracicus superior V.
7. thoracicus superior VI.
7_a_. proximally-leading thoracicus superior.
7_b_. distally-leading thoracicus superior VIII.
9. thoracicus superior IX.
10_a_, 10_a₁_, 10_a₂_, 10_a₃_. thoracicus inferior.
18. cutaneus pectoralis.
19. pectoralis.
21. brachialis longus inferior.
22. coraco-brachialis.
22_c_. branch for the distal belly of biceps muscle.
24. muscular branch for the humero-antebrachialis inferior.
(25 + 42). cutaneus brachii and antebrachii medialis.
29. subscapularis.
31. dorsalis scapulæ (posterior).
32. cutaneus brachii superior lateralis.
33. deltoides inferior.
34. brachialis longus superior.
36. anconæus.
36_a_. scapulo-humeralis profundus.]

The distribution of the nerves of the brachial plexus is as follows
(Fig. 31): (a) supracoracoideus to the muscle of that name and to
the integument of the breast; (b) thoraci inferiores nerves (10a)--a
complex of nerves from the eighth, ninth, and tenth spinal stems--lead
to the costo-coracoideus muscles and to the anterior part of the
transversus abdominis muscle; (c) the pectoralis (19), a large nerve
leading to the muscle of that name; (d) cutaneus pectoralis (18), fine
branches from the XIth spinal nerve to the integument of the axilla and
the neighboring parts of the breast; (e) coraco-brachialis (22) to the
like named muscle; (f) cutaneus brachii et antebrachii medialis (25 +
42) to the medial side of the integument of the upper and fore arm; (g)
brachialis longus inferior (21), a large nerve that supplies the biceps
and humero-antebrachialis inferior muscles, and then divides into the
medianus and ulnaris inferior nerves; (h) subscapularis (29) to the
like named muscle; (i) scapulo-humeralis profundus (36a) to the like
named muscle; (j) axillaris, a large stem that divides into two main
twigs that lead to the skin of the lateral side of the upper arm, to
the proximal part of the forearm, to the humero-radialis muscle, and to
the deltoides coraco-sternalis muscle; (k) dorsalis scapulæ (posterior)
(31) to the deltoideus scapularis muscle; (1) teres major (29b),
one (alligator) or two (crocodile) middle-sized nerves to the teres
major muscle; (m) latissimi dorsi (29b) to the like named muscle; (n)
brachialis longus superior (radialis) (not shown in Figure 31) to the
extensor side of forearm and the hand.

Of the spinal nerves between the brachial and crural plexuses Bronn
gives no description for the Crocodilia. The most posterior nerve of
the former plexus is the eleventh and the most anterior nerve to take
part in the latter is the twenty-third, so that there are eleven nerves
that are doubtless distributed to the regions not supplied by the two
plexuses.

[Illustration: FIG. 32. CRURAL PLEXUS AND ISCHIADIC PLEXUS OF THE LEFT
SIDE OF A. MISSISSIPPIENSIS. THE NERVE BRANCHES ARE SHOWN AS FAR AS
THEIR ENTRANCE INTO THE MUSCLES. THE CRURAL PLEXUS IS MADE UP OF THE
PRESACRAL STEMS _a_, _b_, _c_. THE OBTURATOR NERVE IS BUILT OF TWO
BRANCHES FROM STEMS A & B. (FROM BRONN, AFTER GADOW.)

_a_, _b_, _c_. presacral nerves.
_α_ & _β_. postsacral nerves.
_s_ = _XXVI_. sacral nerve (26th spinal nerve).
2. to extensor ileo-tibialis muscle.
3. to femoro-tibialis muscle.
4. to ileo-fibularis muscle.
5. to ileo-femoralis muscle.
6. to caudi-ileo-femoralis muscle.
7. caudi-femoralis muscle.
8. to flexor tibialis externus muscle.
9. to flexor tibialis internus muscle.
11. to ischio-femoralis muscle.
13. pubo-ischio-femoralis internus.
14. pubo-ischio-femoralis externus muscle.
15. to pubo-ischio-femoralis posterior muscle.]

The crural-ischial plexuses (Fig. 32) are made up of branches from five
nerves, three presacral (a, b, and c), the sacral (s = xxvi), and one
postsacral (α); the second postsacral shown in the figure apparently
does not enter into the plexus.

The first and second presacrals terminate chiefly in the abdominal and
thigh muscles, though the second sends a large branch to fuse with a
branch from the third to form the large obturator nerve (N. obt.) that
leads to the muscles of the thigh and knee.

The third presacral sends a branch back to fuse with the large sacral
(s = xxvi), and these two, together with a branch from the first
postsacral, form a complicated network that sends numerous branches
to the muscles of the pelvic and femoral regions, to the skin, legs,
and tail, as shown in Figure 32. The large muscles of the tail are
innervated by the regular, metameric nerves of that region, and since
there are usually thirty-nine caudal vertebræ, there are probably about
that many pairs of caudal nerves, although the last few vertebræ and
the muscles of that region are so small it may be that some of the
nerves are lacking.

SPECIAL SENSE ORGANS

It is not possible in a work of this size to give much space to
the discussion of the anatomy of the special sense organs. A few
of the main features will be given here, taken mainly from Bronn’s
_Thierreich_, but for details of structure the reader is referred to
that larger work.

_The Eye._ As might be expected, the Crocodilia have the usual upper
and lower eyelids and the nictitating membrane. Except along their
thickened rims the lids are usually rather faintly pigmented, and near
the thickened border numerous goblet cells are found.

The structure of the upper and lower _lids_ is similar except that in
the former a bony formation is present, as a support to that lid, even
in very young animals. The arrangement of the muscles, which are of
both smooth and striped fibers, and the histological structure cannot
be described here.

The _nictitating membrane_ is strongly developed in the Crocodilia. Its
outer surface is marked by two fairly high folds that are conspicuously
pigmented. The cartilage described in the nictitating membrane of
Lacerta is wanting, according to Bronn, in the Crocodilia.

The glands of the eye are of three types: the lachrymal glands proper,
the Harderian glands, and the conjunctival glands. The _lachrymal
gland_ is small in proportion to the size of the eye. It is an
elongated, almost band-like structure situated in the roof of the
eye-socket, near its border; its long axis lies in an antero-posterior
direction. It is so closely inclosed by and united with connective
tissue that it is difficult to find.

The _Harderian gland_ is much larger than the lachrymal gland proper
and is easily found. It lies in the forward part of the eye-socket
and is of a somewhat three-cornered shape. From its outer and forward
base it sends a short, delicate duct to open between the nictitating
membrane and the eyeball.

The _lachrymal canal_ is well developed in the Crocodilia. Near the
forward angle of the eye, on the inner side of the lower lid, are found
from three to eight tear dots, lying in a row from behind forward.
Each of these dots opens into a small elongated sac. This sac opens
downwards and forwards into a common canal, which canal, at first
narrow but soon widening, extends for a time parallel to the free
border of the eyelid and then enters the opening in the hinder side of
the lachrymal bone. Rathke found none of these tear dots on the upper
eyelid so concluded that the lachrymal fluid could escape only through
the lower lid. This canal, which might correspond to the lachrymal sac
of higher forms, is rather narrow until it enters the lachrymal bone,
then it becomes considerably wider and forms a sort of reservoir that
Rathke calls the “_saccus naso-lachrymalis_.” This reservoir is of
irregular form and opens forwards into the base of the nasal cavity
proper.

The third type of gland mentioned above, the _conjunctival_, is found
on the lower eyelid where the conjunctiva passes from the lid to the
eyeball. The gland is of a “scattered acinose” type.

The usual muscles of the eyeball are found in the Crocodilia. The four
_rectus_ and two _oblique_ muscles have about the usual arrangement and
are attached to the eyeball by very short aponeuroses. The _retractor
oculi_ muscle is only weakly developed. It consists of two separate
bundles which, lying behind the optic nerve, arise from the forward
bony wall of the socket and are inserted on the sclera very near the
optic nerve.

The _eyeball_ consists of the usual layers, including, as might be
expected from the nocturnal habits of the Crocodilia, a typical
_tapetum lucidum_.

In the _sclera_, instead of the bony ring common to the saurians, is
found a well-developed cartilage covered with the fibrous layer of the
sclera; the fibers of this layer are arranged into two more or less
distinct layers.

While not worked out in detail the _cornea_ consists of the usual five
layers.

In the _iris_ the musculature is less developed than in the birds;
Bronn thinks this may be compensated for by the greater development of
the “vascular structures.”

The _pupil_ is a vertical slit.

The _choroid_ is very closely united on the outside with the sclera; on
the inside it is less closely attached to the retina except at the ora
serrata. It consists of an outer fibrous coat, an inner, unstratified
pigmented epithelium derived embryologically from the pigmented layer
of the retina, and the ground substance which is a network of irregular
and very vascular cells.

As in probably all reptiles there is present in the Crocodilia a
vascular pigmented fold of the choroid, the _pecten_, which projects
into the middle of the cavity of the eyeball.

In the _retina_ Bronn describes the following ten layers, which are
those commonly given in other vertebrate retinas: (1) the inner
limiting membrane, (2) optic fiber layer, (3) ganglion cell layer,
(4) inner granular layer, (5) inner nuclear layer, (6) outer granular
layer, (7) outer nuclear layer, (8) outer limiting membrane, (9) cone
layer, (10) pigmented layer. The Crocodilia differ from probably all
other reptiles in having rods as well as cones in the retina. The rods
are more numerous except in the neighborhood of the fovea centralis
where the cones predominate; in the fovea itself only cones are found.

The _lens_ does not show any characteristics unusual enough to warrant
special description.

_The Ear._ The ear is of special interest here because it is in the
Crocodilia that are first found the three distinct regions of the ear
that are seen in the Aves and Mammalia: the external auditory meatus,
the tympanic cavity, and the labyrinth. It is the presence of the
meatus that lifts the Crocodilia above the other Reptilia.

Two strong folds of integument, one above and one below, completely
cover the outer ear and allow it to open as a mere slit on the lateral
surface of the head a little back of the corner of the eye. By lifting
the upper valve one may perceive the lower half of the meatus and
the bottom of the tympanic membrane. The upper valve is the larger
and is sickle shaped; the lower is smaller and more three cornered.
Both spring from the outer surface of the squamosal bone, from its
posterior obtuse angle to its anterior union with the postfrontal.
The lower fold is raised highest behind the corner of the eye and is
lost in the middle of the rima auditoria; by this Hasse indicates the
position of the outer opening of the external auditory meatus. The
form of the _meatus_ may be compared to a wedge whose base is directed
dorso-medio-caudad and whose edge points in a ventro-latero-cephalic
direction; its side walls are either soft or bony; its outer end is
covered by the folds; at its inner end is the tympanic membrane or drum.

The _drum_ is a round, soft, elastic membrane in which a radial
arrangement of its constituent fibers may be seen. It is funnel shaped
from without and above, and the fibers radiate from the apex to which
the columella is attached. The membrane is stretched taut and while
it does not, as in the higher vertebrates, lie in a bony groove, it
possesses around its periphery a strong thickening of circular fibers,
the _annulus tympanicus_, by means of which it is closely united with
the lining membrane of the outer ear passage. The drum is attached
chiefly to the quadrate but in part to the squamosal bone.

The _middle ear_ is divided into an outer part, the _tympanic cavity
proper_, and a part next to the labyrinth, the _recessus cavi tympani_.
Within the tympanic cavity, besides blood-vessels and nerves, is
found the _columella_ with its appendage (found in all Reptilia), the
_recessus scalæ tympani_. The tympanic cavity is formed mainly by the
quadrate, though the exoccipital and squamosal bones take some part.
In outline it might be compared to a truncated, four-sided pyramid,
with its base below, its truncated apex above, and with an anterior, a
posterior, a mesial, and a lateral side.

From the floor of each tympanic cavity a _Eustachian tube_ leads
towards the throat. These tubes unite and connect with the throat by a
single small opening just behind the posterior nares, as shown in the
figures of the skull.

The _semicircular canals_ with their _ampullæ_ lie in the usual
positions as seen in other vertebrates: the anterior vertical,
posterior vertical, and horizontal. The details in structure of the
inner ear cannot be given here. The nervous epithelium is said to have
the same characteristics as in other vertebrates.

CHAPTER V

THE DIGESTIVE SYSTEM

THE ORAL CAVITY

The _mouth_ in the Crocodilia is large, as is well known, and may be
opened very wide. It is bounded anteriorly and laterally by the teeth
of the two jaws; these teeth were described in connection with the
skull. The mucous membrane of the roof of the mouth and of the dorsum
of the tongue, especially the former, exhibits numerous small papillæ
(see page 160), and among these, in the posterior region of the mouth,
are the ducts of mucous glands.

[Illustration: FIG. 33. INTERIOR OF THE MOUTH OF A. MISSISSIPPIENSIS

_f_, transverse fold at the base of the tongue; _v_, _v_, velum
palatinum.]

The _tongue_ extends from just back of the mandibular symphysis to the
glottis. It is attached throughout its entire ventral side except for
a short distance at its tip, so that it may be elevated and depressed
but not protruded. Among the papillæ on its dorsal surface are sense
organs, said to be tactile and gustatory corpuscles (see page 165). The
posterior margin of the tongue is elevated as a transverse fold that
meets a corresponding fold, the _velum palatinum_, from the lower side
of the palate and completely shuts off the mouth from the openings of
the trachea and gullet (Fig. 33). Into this hinder chamber open the
posterior nares, so that the animal can open its mouth under water
without getting water into its trachea; or it may, while holding its
prey in its mouth, come to the surface to breathe, without danger of
letting water into its trachea. The nasal passages, leading from the
nostrils to the posterior nares, are, of course, completely inclosed by
bone, as described in connection with the skull. Ventral to the larynx
and posterior part of the mouth is the large, shield-shaped hyoid
apparatus, Fig. 25, h, also described in connection with the skull.

THE ŒSOPHAGUS

The œsophagus, Fig. 34, _e_, is long and of about the same diameter
throughout except possibly for a slight enlargement of the anterior
region where it leaves the pharynx. The two “olivary enlargements”
mentioned by Chaffanjon (15) are not always present, and when seen were
found to contain either food or small stones or both.

The outside of the œsophagus is smooth and muscular while the lining
is thrown into numerous longitudinal folds that in the empty œsophagus
nearly obliterate the lumen; where distended by food or pebbles the
longitudinal folds may be almost obliterated. In a thirty-inch animal
the œsophagus is about six inches long, and opens suddenly, but
without any apparent valve, into the large chamber of the stomach. The
histology of the œsophagus and the other regions of the digestive tract
will be described later.

[Illustration: FIG. 34. DIGESTIVE SYSTEM OF A. MISSISSIPPIENSIS.

_bd_, bile duct; _bs_, bile sac; _c_, cloaca; _e_, œsophagus; _f_,
larger or fundic region of stomach; _h_, hyoid apparatus; _l_, liver;
_p_, smaller or pyloric region of the stomach; _pa_, pancreas; _r_,
rectum; _s_, small intestine; _t_, tongue; _tr_, trachea.]

THE STOMACH

The stomach, as is well known, is made up of two distinct parts;
that on the animal’s left, into which the œsophagus opens, is many
times larger than the part from which the small intestine leads. The
larger or _fundic region_, Fig. 34, _f_, has, as will be described,
very heavy muscular walls. When empty the lining of this part of
the stomach is thrown into a few comparatively large folds, but when
greatly distended with food, as it sometimes is, the internal folds are
completely obliterated and the muscular layers are stretched until they
have scarcely an eighth of their original thickness. In Figure 34 the
stomach is considerably distended.

The large region of the stomach frequently contains a number of stones,
and for that reason, probably, is sometimes spoken of as the gizzard.
In one thirty-inch alligator fourteen pebbles of irregular shape,
varying in largest diameter from four to seventeen mm. and aggregating
six grams in weight, were found. Voeltzkow (78) says that gastroliths
of two to three cm. diameter are found in the stomach of the adult
Madagascar crocodile.

Neither the transverse fold nor the smooth, lateral disks (or shields)
described by Chaffanjon could be seen in either the empty or in the
distended stomach.

The _smaller part_ of the stomach, Fig. 34, _p_, lies to the right
and somewhat ventrad to the anterior region of the larger part, near
the entrance to the œsophagus. It connects by a fairly large opening
with the larger part of the stomach, and by a smaller opening with the
duodenum. The former opening apparently has no valve, unless it be a
slight sphincter muscle; the latter is guarded by a pair of thickened
lips, called by Chaffanjon “semilunar valves.”

The walls of the smaller part of the stomach are, as might be
expected, much thinner than those of the larger region, but they are
proportionately fairly thick and are internally thrown into numerous
folds.

THE INTESTINE

In the intestine three regions may be distinguished: a long,
considerably coiled small intestine; a wide, nearly straight rectum;
and a short, wide cloaca.

The _small intestine_, Fig. 34, _s_, is of moderate and rather uniform
diameter, though somewhat thicker near the stomach, and is not coiled
so extensively as figured by Chaffanjon. Near the stomach it receives
the ducts of the liver and pancreas. The bile duct, Fig. 34, _bd_,
is a continuation of an elongated bile sac, _bs_, which lies between
the large right and smaller left lobes of the liver, _l_. The two
main lobes of the liver, which appear smaller than in reality because
of foreshortening in drawing, are connected, across the base of the
œsophagus, by a narrow transverse band.

The pancreas, _pa_, which is of fair size, lies partly dorsal to and
partly in a narrow loop of the intestine, so that it is not very
evident in a ventral view of the animal.

The small intestine has heavy muscular walls whose histological
structure will be described elsewhere. It opens abruptly, without any
indication of a cæcum, into the large intestine or rectum.

The _rectum_, _r_, is of about twice the diameter of the small
intestine, though this, of course, varies with the amount of fecal
matter it contains; it is nearly straight and possesses much thinner
walls than the small intestine, though this, again, varies with the
state of collapse or distention.

At the posterior end of the rectum is a heavy sphincter valve
separating that part of the intestine from the cloaca.

The _cloaca_, _c_, is widest anteriorly where it is about as wide as
the rectum; it gradually diminishes in diameter caudad, and appears
flattened laterally. Its wall has the same general structure as the
rectum, as will be described below. The mucous membrane posterior
to the openings of the genital ducts is thrown into a more or less
complete, ring-like transverse fold (Fig. 55 g.). In some species there
may be a second, half-ring-like fold in the dorsal wall caudad to the
more complete ring. The cloaca is divided by this fold into a larger
anterior portion, g, and a shorter posterior portion, h; in the former
the mucous membrane is thrown into a large number of small folds that
in places form a network; in the latter the mucous membrane has a hard,
thick epithelium, with a smooth surface and only a few longitudinal
folds.

The _ureters_ open, Fig. 55, _d_, _e_, at a moderate distance from each
other, into the anterior region of the cloaca (about where the dorsal
and lateral walls of this region come together). The _genital ducts_
(oviducts or vasa deferentia), _c_, _f_, on the other hand, open close
together through the ventral wall of the posterior half of the cloaca,
just in front of the copulatory organ.

Into the cloaca, very near the anus, open two glands of fairly large
size that Rathke called _musk glands_. These glands lie outside of
the pelvis between the side walls of the cloaca and a large muscle
that surrounds this part of the body. They have an oval form and open
usually from their anterior end, sometimes just caudad to this, by a
short, fairly wide, slit-like opening which has an anteroposterior
direction. The walls of the glands are made up of three closely
associated layers of connective tissue, the inner one being thrown into
folds. Since these layers contain no muscle fibers the secretion of the
gland is probably squeezed out by contraction of the circular muscles
of the cloaca. Usually the cloacal glands are stretched full by a
thick, yellowish mass that smells strongly of musk.

The part of the cloaca caudad to the pelvic opening has a differently
arranged musculature from the more anterior region. It consists of two
separate pairs of striped muscles that surround the musk glands on
the outer side. The first pair form a fairly broad, moderately thick
ring muscle next to the anus that is attached anteriorly to the pubis
and posteriorly to the second hæmal process. When these muscles draw
together they narrow or completely close the anal slit. The muscles
of the other pair are broader but thinner, and extend in a general
dorso-ventral direction. Anteriorly, above the cloaca, they are united
with each other, but posteriorly they separate and, with the above
ring muscle, are inserted on the second hæmal arch. Judging from their
attachment they widen the anal opening laterally.

THE HISTOLOGY OF THE ENTERON OF THE FLORIDA ALLIGATOR

It has long been known that the sea lamprey, _Petromyzon marinus_,
during the spawning season, when the body is distended with eggs, takes
no food, and that the digestive tract during this period shrivels up
until it is reduced to a mere thread. This condition doubtless obtains
in other forms as well, though it has not been actually observed by the
writer elsewhere.

A number of small alligators that were kept alive in the laboratory
for a year or more caused the writer to wonder whether any very marked
change had taken place in their digestive tracts during the months they
took no food.

In captivity, especially if the water in their tank be kept cold,
alligators may refuse food for five or sixth months. Whether, during
the winter months, in their native haunts, they entirely cease feeding,
the writer has had no opportunity to observe, though it is popularly
reported that such is the case.

The first alligator from which tissues were taken was about a year and
a half old, and measured eighteen inches in length. It was killed in
March after a fast of several months, probably four or five, possibly
more, though it was not in the writer’s possession for so long a time.

[Illustration: FIG. 35. OUTLINE OF DIGESTIVE TRACT

Fig. 35. A diagrammatic outline of the digestive tract of the
alligator from the beginning of the œsophagus to the cloaca, to
show the planes of the sections that were studied, _a.oes._,
anterior œsophagus; _a.r._, anterior rectum; _a.s.i._, anterior
small intestine; _c.st._, cardiac stomach; _f.st._, fundic stomach;
_m.s.i._, middle small intestine; _p.oes._, posterior œsophagus;
_p.r._, posterior rectum or cloaca; _p.s.i._, posterior small
intestine; _p.st._, pyloric stomach.]

Although carefully fixed in the usual fluids, the epithelial
structures from this animal were not as clearly defined in most cases
as could be desired; this rather unsatisfactory fixation may have
been due to some physiological condition characteristic of the period
of hibernation. That this was the case seems likely from the better
fixation obtained by the same methods in the case of animals killed
during the feeding season.

The other animals from which tissues were taken were considerably
smaller than the one mentioned above. They were killed early in the
fall, after having been fed regularly for about five months upon bits
of meat, both raw and cooked.

_The Tongue._ The covering of the tongue was studied in two regions,
near the free end, and towards the base.

A section of the former region, drawn under high power, is shown in
Figure 36. It consists of a dense mass of fibrous tissue, _a_, and
small scattered cells, overlaid by a stratified epithelium of eight
or ten layers. Only a small part of the fibrous base, just beneath
the epithelium, is here shown. It is a dense areolar tissue with the
elastic fibers apparently predominating.

The epithelium, _e_, consists, as has just been said, of about eight
or ten layers of cells, those at the base being generally cuboidal
in shape, while towards the surface the cells become more and more
flattened until at the surface they form a thick horny layer, _h_,
in which no nuclei can be seen. The cells of the horny layer are
flattened into mere fibers, which, at places, are seen projecting from
the surface. The boundary between the horny cells and those beneath
is quite distinct, though perhaps not quite so sharp as shown in the
figure under discussion.

[Illustration:

FIG. 36. The covering of the anterior region of the tongue of the
hibernating animal, under fairly high magnification; the plane of
this section is not shown in Figure 35; _a_, areolar tissue; _e_,
epithelium; _h_, horny layer of epithelium.]

In a previous paper, the writer noted that the dorsum of the tongue
is covered with small, evenly distributed papillæ, easily seen by aid
of a hand lens. These so-called papillæ are here seen to be hardly
papillæ at all, but small folds or wrinkles, although the epithelium
is somewhat thickened at intervals. No glands are to be seen in this
region of the tongue.

The only difference between the anterior region of the tongue during
hibernation and during the feeding season seems to be in the scaly
layer of the epithelium. Instead of the compact, sharply differentiated
layer of scaly cells seen in Figure 36, the anterior region of the
tongue during feeding is covered with a layer of rather loose, scaly
cells, in most of which the nuclei may be seen. No difference in
the amount of sloughing off can be noticed as is the case with the
epithelium of the roof of the mouth.

Figure 37 represents a section, under very low magnification, of the
covering of the base of the tongue. The areolar tissue, _a_, is about
the same as in the preceding section, except that it is more compact
just under the epithelium than it is in its deeper regions. It seems
also more vascular than in the preceding section.

[Illustration:

FIG. 37. Covering of the posterior region of the tongue of the
hibernating animal showing glands, under low magnification; _a_,
areolar tissue; _bv_, blood-vessels; _g_, glands; _e_, epithelium.]

The epithelium, _e_, is of the stratified squamous variety, but
consists of many more layers of cells than in the preceding section and
is hence several times as thick. While its cells are flattened towards
the surface, after the manner of this kind of epithelium, they do not
form the definite horny layer described above.

[Illustration:

FIG. 38. One of the glands from the posterior region of the tongue
of the hibernating animal, under high magnification; _a_, areolar
tissue; _av_, alveolus.]

The most marked difference between the two regions of the tongue is
the presence, in the posterior or basal region, of numerous glands,
_g_, probably mucous- or slime-secreting. They are thickly scattered
through the areolar base, close beneath the epithelium. Two large
glands and one small one are shown in the figure under discussion.
Each gland opens to the surface by an apparently wide duct, but since
no good section of such a duct was obtained it is not shown in the
figure. Although the rest of the tissue was well preserved and showed
cell structure clearly, it was with difficulty that the details of the
glands could be determined.

A high-power drawing of a portion of one of the glands is shown
in Figure 38. The large alveolus, _av_, to the left, is from the
peripheral region of the gland and is surrounded, on its free side, by
the areolar tissue described above. The inter-alveolar spaces, which
are somewhat exaggerated in the figure, are filled with fibers which
are arranged more or less in layers and hence appears different from
the surrounding areolar tissue. The alveoli are circular or elongated
in section, and have fairly wide lumina. They are lined with a single
layer of columnar or cuboidal cells which are very granular, so that
their walls are difficult to determine. Each cell contains, near its
base, a very large, usually spherical nucleus. These nuclei stain
darkly and give the dark appearance to the glands as seen under low
magnification, especially in rather thick sections.

During feeding the epithelium of this region of the tongue consists
of fewer layers of cells than during hibernation but is otherwise
unchanged from what is described above. The glands consist, at least in
all of the material examined, of much fewer alveoli than are shown in
Figure 37. One of these glands is shown in Figure 39.

Although no more care was used in fixation than in the corresponding
tissue of the hibernating animal the glands here show their cell
details far more clearly than in the former tissue; this may have been
partly due to the latter sections being thinner.

The glands are of a compound, tubulo-alveolar type; although numerous
sections through ducts were obtained (as in Fig. 39), no details of
these ducts could be seen.

[Illustration:

FIG. 39. One of the glands from the posterior region of the tongue
of the feeding animal, under somewhat higher magnification than used
in Figure 37; _av_, alveolus; _d_, duct of gland; _e_, stratified
epithelium.]

As noted above, and as may be seen by comparing Figures 37 and 39, the
gland during hibernation, at least in the animals studied, consists
of many more alveoli than during the feeding season; this, of course,
might not prove to be always the case if larger numbers of animals were
studied; the difference in the ages of the animals might have caused
this difference in the glands. In the material studied the largest
glands from the hibernating animals consist of more than twice as many
alveoli as the glands in the feeding animals. As seen under high
magnification there is no noticeable difference in the glands at the
two seasons.

Rathke has given the name of “Geschmackwärzchen” to the conical
projections found on the dorsum of the crocodilian tongue; they are
distinguished by their softness and thinner epithelial covering from
the cones that, in many of these animals, bear the openings of the
mucous glands.

These taste papillæ generally have the form of a truncated cone and
often are surrounded by a shallow circular pit, outside of which, in
turn, is sometimes a small low wall. They are distributed over the
entire dorsum of the tongue, usually at considerable distance from each
other in comparison to the size of the tongue, and are not so numerous
as the taste papillæ of the Mammalia. Rathke found their absolute
number greatest in _A. lucius_.

Rathke mentions other larger and harder projections on the tongue of
certain Crocodilia which, though not perforated by a mucous duct, he
thinks are of questionable relation to the sense papillæ. They usually
have more the form of a flattened than of a truncated cone, and are
very numerous in some species.

_The Roof of the Mouth._ In the paper mentioned above the author notes
that the papillæ on the roof of the mouth are evenly distributed and
are more distinct than those of the dorsum of the tongue. One of these
papillæ as seen under fairly high magnification is shown in Figure 40.

[Illustration:

FIG. 40. The covering of the roof of the mouth of the hibernating
animal, under fairly high magnification; _a_, areolar tissue; _e_,
epithelium; _h_, horny layer; _f_, fibers of horny layer.]

The areolar tissue, _a_, forming the base of the section is of about
the same character as seen in the section of the tongue. Less than one
tenth of the thickness of the entire areolar base is shown in this
section.

The epithelium, _e_, where not thrown into papillæ, has also about the
same character as that of the anterior region of the tongue--the same
number of cell layers and the same distinct horny layer.

At intervals the thickness of the cellular part of the epithelium
is greatly increased, and at the same time the horny layer is also
thickened, to form distinct papillæ like the one shown in the figure.
These, as has been said, are comparatively small and have the shape
of a blunt cone. The center of the cone is, of course, made up of the
cellular epithelium, while the outside is covered with the thickened
horny layer from which fibers, _f_, are often seen projecting. Near the
apex of the cone the nuclei are larger and more widely scattered than
those at the base.

No glands were seen in the roof of the mouth of the hibernating animal,
but since the entire roof was not sectioned it is probable that they
may exist in some regions; in fact, as noted below, sections through
the posterior region of the roof of the mouth of the feeding animal do
show numerous glands.

As might be expected there is comparatively little difference between
this region of the enteron during hibernation and during the feeding
season. The only noticeable difference is in the stratified epithelium;
that of the feeding animal not only has less sharp papillæ but has also
a much thinner scaly layer of cells. As is seen in the figure of the
roof of the mouth during hibernation the scaly cells make up, except on
the papillæ, nearly or quite half of the thickness of the epithelium,
while in the feeding animal they make up not more than one fourth or
one third of the entire epithelium. Very few cells are seen sloughing
off as in Figure 40; possibly the act of feeding keeps the superficial
scaly cells rubbed off smooth.

In the extreme posterior region of the roof of the mouth the epithelium
consists of a greater number of layers (though the number is very
variable) than in the region shown in Figure 40. In this posterior
region, as noted above, glands are found. These glands have the same
structure as those described in connection with the posterior region of
the tongue.

_The Œsophagus._ Sections of the œsophagus were made from two regions,
an anterior, half-inch caudad to the pharynx, and a posterior region,
half-inch cephalad to the opening of the œsophagus into the stomach
(Fig. 35).

The general structure of the wall of the œsophagus, as seen under a
low magnification, will first be described, after which the minute
structure of the epithelium, as seen under high magnification, will be
discussed.

In the _anterior region_ the usual layers of the vertebrate enteron are
present, except, possibly, the muscularis mucosa.

The epithelium, to be described later, is, together with the submucosa,
thrown into complicated folds; its closely arranged and darkly stained
nuclei cause it to stand out in strong contrast to the other tissues of
the section (Fig. 41, _e_).

The submucosa, _sm_, is of considerable thickness. It is composed
of a fairly dense mass of connective tissue, mainly elastic fibers,
through which are scattered small blood-vessels, _bv_, and small dark
areas, _mb_, that are apparently longitudinal bundles of involuntary
muscle fibers. These few and scattered fibers probably represent the
muscularis mucosa that is so well developed in the posterior region of
the œsophagus.

Outside of the mucosa is a thick circular layer of involuntary muscle
fibers, _cm_, the fibers being collected into irregular bundles,
between which are narrow spaces filled with connective tissue that
contains a few small blood-vessels.

Surrounding the circular layer is a thinner and less clearly defined
layer of longitudinal muscle fibers, _lm_. The muscle bundles are
more definite than in the circular layer and are separated from each
other by a considerable amount of connective tissue with a few small
blood-vessels.

[Illustration:

FIG. 41. A transsection through the anterior region of the
œsophagus of the hibernating animal under low magnification; _bv_,
blood-vessels; _mb_, muscle bundles; other letters as in Figure 42.]

The serosa, _s_, is here quite indistinct. It consists of a slightly
vascular connective tissue which cannot be distinctly differentiated
from the connective tissue of the longitudinal layer.

[Illustration:

FIG. 42. A transsection through the posterior region of the œsophagus
of the hibernating animal, under low magnification; _e_, epithelium;
_cm_, circular muscles; _lm_, longitudinal muscles; _mm_, muscularis
mucosa; _sm_, submucosa; _s_, serosa.]

In the _posterior region_ of the œsophagus, as may be seen by
comparison of figures 41 and 42, the wall as a whole is about one
third thicker than in the anterior region just described, though how
much of this difference is due to different degrees of distension or
contraction it is hard to say.

The epithelium, _e_, is in the tissue studied thrown into less
complicated folds than in the anterior region, and is not so thick.

The submucosa, _sm_, if the entire layer may be so called, has about
the same thickness and structure as in the more anterior region; but
instead of the small and widely scattered bundles of longitudinal
muscle fibers there is a distinct layer of muscle which may be called
the muscularis mucosa, _mm_, lying about midway between the epithelium
and the circular muscle layer.

The muscularis mucosa is somewhat variable in thickness and is thrown
into folds that correspond to the larger folds of the epithelium and
the submucosa; one of these folds is shown in Figure 42. The fibers of
the muscularis mucosa are apparently all longitudinal in position.

Outside of the submucosa is a layer of circular muscle fibers, _cm_; it
is here somewhat wider and more dense than in the anterior region.

The longitudinal muscle layer (Fig. 42, _lm_) is much wider and more
compact than in the anterior region. The fibers are indistinctly
divided into large irregular masses as shown in the figure.

The serosa (Fig. 42, _s_) is a varying but fairly thick layer that is
quite distinct from the longitudinal muscle layer. It consists of the
usual connective tissue groundwork with scattered blood-vessels.

The epithelium, as was said above, is thicker and somewhat more folded
in the anterior than in the posterior region, and in the former region
is partially ciliated while in the latter cilia are entirely wanting.
With these exceptions the epithelium is practically the same in the two
regions.

Figure 43 represents the epithelium from the anterior region as seen
under high magnification. The outlines of all the cells could not be
determined but if each nucleus represents a cell there are twenty-five
or thirty layers of cells. The nuclei are arranged in two dense,
irregular groups, one along the base of the epithelium, the other about
two thirds of the distance from the base to the free border. The basal
nuclei are perhaps slightly larger and more rounded than those of the
distal group. Between these two groups are numerous more scattered
nuclei; while scattered through the epithelium, except near the free
border, are smaller, round nuclei that stain somewhat darker than the
rest; these, from their size and appearance, seem possibly to belong
to an invisible network of connective tissue that has penetrated the
epithelium from the surrounding mucosa.

[Illustration:

FIG. 43. The epithelium of the anterior region of the œsophagus of
the hibernating animal, under high magnification.]

The free border of the epithelium consists of long, ciliated, columnar
cells in which the cell walls may be easily seen. The cilia are of
average length and even in this anterior region are not everywhere
present; possibly they are arranged in bands, but the material at hand
was not sufficient to determine this. As was noted above, cilia are
wanting in the posterior region.

The only differences noted in the anterior region of the œsophagus
between the feeding and the hibernating conditions are in the
muscularis mucosa and the epithelium. As was noted above, the
muscularis mucosa is practically absent in the hibernating stage, being
represented only by a few small, scattered bundles of longitudinal
muscle fibers; while in the feeding stage there is a narrow but fairly
distinct layer to represent the muscularis mucosa.

[Illustration:

FIG. 44. The epithelium of the anterior region of the œsophagus of
the feeding animal, under high magnification.]

The difference in the appearance of the epithelium is not striking. The
nuclei are somewhat larger in the feeding stage and, instead of being
crowded into a basal and a median zone, as noted in the hibernating
conditions, they form a dense basal zone, but show no indication of
medial zone. From the dense basal zone the nuclei become more scattered
towards the free surface and are rarely found closer to the surface
than is shown in Figure 44. The smaller nuclei scattered among the
larger ones, noted in connection with the hibernating stage, are not
here seen.

As in the hibernating stage cilia are present on some but not all cells
of this region.

The only noticeable difference between the feeding and hibernating
conditions of the posterior region of the œsophagus is in the
epithelium, which, as in the feeding condition of the anterior
œsophagus, exhibits but one zone of closely set nuclei, that at the
base of the epithelium.

_The Stomach._ The stomach was sectioned in three regions, as shown
in Figure 35: (1) in the cardiac region very near the opening of the
œsophagus; (2) in the middle or fundic region; and (3) in the region
near the opening of the pylorus. The first two sections are in the
first or large region of the stomach; the third section is in the
second or small region of the stomach (Fig. 35).

The wall as a whole is thickest in the fundus, being there practically
twice as thick as in the pyloric and half again as thick as in the
cardiac region. This great thickening is due mainly to a thickening
of the middle or oblique layer of muscle, which is here remarkably
developed. The mucosa is of nearly uniform thickness in the different
regions and will be described later.

Since there is no striking difference beside that of thickness in the
general structure of the wall of the different regions, the pyloric
region, as seen under low magnification, will now be described (Fig.
45).

The mucosa, _m_, consists of fairly long glands underlaid by a
well-marked muscularis mucosa, _mm_, the latter exhibiting a compact
circular layer over a wider but more scattered layer of longitudinal
fibers. A considerable amount of fibrous connective tissue lies among
the muscle fibers. The circular layer of the muscularis mucosa sends
towards the surface numerous strands or septa between the glands;
six or eight of these are seen in the figure. These strands are not
nearly so numerous in the large region of the stomach. As was said, the
outer or longitudinal layer of the muscularis mucosa is wider but less
compact than the circular and its bundles of fibers are seen in the
figure as a layer of large, scattered dots just beneath the circular
layer.

The submucosa, _sm_, is of average thickness and density. In the fundic
and cardiac regions it seems to extend between the circular and oblique
layers; at any rate, there is a considerable layer of connective tissue
between these two muscular layers.

The circular muscular layer, _cm_, is of only moderate thickness and
is of rather a loose character. In the pyloric region it is not very
distinct from the underlying oblique layer, but in the other regions,
as has just been said, it is separated from the oblique layer by a
considerable layer of connective tissue like that of the submucosa.

The oblique layer, _om_, even in this section of the pyloric region
is the thickest of the three muscle layers; while in the cardiac, and
especially in the fundic, regions it is of great thickness, as was
noted above, and is made up of larger bundles with less intervening
connective tissue.

[Illustration:

FIG. 45. A transsection through the wall of the pyloric region of the
stomach of the feeding animal, under low magnification; _m_, mucosa;
_om_, oblique muscles; other letters as in Figure 42.]

The outer or longitudinal muscle layer, _lm_, is comparatively
little developed and consists of small rather scattered bundles of
muscles with a correspondingly large amount of connective tissue.
This connective tissue passes insensibly into that of the surrounding
serosa, _s_, a loose, vascular layer of varying thickness and density,
shown very thick in Figure 45, but often much thinner.

So far as could be determined, the mucous membrane has the same
structure in both anterior and middle regions of the stomach. That of
the pyloric or small region, although fixed, stained, et cetera, just
as carefully as the rest, did not show cell details sufficiently well
to draw; the ducts of the glands in this region are fairly distinct but
the deeper parts of the glands have the appearance of series of alveoli
or large adipose cells. What the significance of this condition may be
the writer is not able to say, but since the structure of this region
of the gastric mucous membrane is not clear no attempt will be made to
describe its appearance under higher magnification than was employed in
the figure above. However, as will be noted below, there is probably
no great difference between the pyloric mucosa and that of the other
regions of the stomach.

[Illustration:

FIG. 46. The glands of the middle or fundic region of the stomach of
the hibernating animal, under high magnification; _A_, through duct;
_B_, through body of gland; _C_, through fundus of gland.]

Figure 46 shows portions of typical glands from the mucosa of the
middle region of the stomach, the posterior border of the large stomach
cavity; _A_ is a longitudinal section through two ducts where they
open to the surface; _B_ is a similar section through the body of a
gland below the region of the duct; _C_ is a transsection through the
bottom or fundus of a gland; all are drawn with a camera under the same
magnification.

As is seen in Figure 45, under low magnification, the duct is about
one third of the entire length of the gland. The lumen of the duct is
fairly wide, that of the body of the gland is reduced to a mere slit,
while that of the fundus is quite wide.

One, two, or possibly more, glands may open to the surface through one
duct, as is shown in Figure 46. There is nothing peculiar about the
epithelium of these glands. Near the opening of the duct the cells are
of a typical columnar character with finely granular cytoplasm, each
with a nucleus at its basal end.

In the deeper parts of the duct the cells become shorter until in the
body of the gland (Fig. 46, _B_) they are cuboidal in outline.

The bodies of the glands are so closely packed together that it is
difficult to pick out an individual tube that will show details clearly
enough to draw with a camera lucida. So far as could be observed all of
the cells of this region of the gland are alike.

The bottom or fundus of the gland, as seen in Figure 46, _C_, is
somewhat enlarged and has a wide lumen. The cells are of the same
general character as in the more distal parts of the gland except that
they are somewhat more columnar or pyramidal than in the body of the
gland. The nuclei of the body and fundus are usually somewhat larger
and more nearly spherical than in the columnar cells of the duct.

The feeding animals from which tissues were taken were considerably
smaller than the hibernating specimen, so that the stomach walls were
proportionately thinner; but, so far as could be discovered, there was
no difference in structure.

The relative thickness of the entire wall in each of the three regions
sectioned was about the same as described above.

As has been said, the mucosa on the pyloric or small region of the
stomach from the hibernating animal was so poorly fixed that its
structure could not be made out. In the feeding stage the mucosa of
this region was as well fixed as any of the other tissues and showed
that its structure is essentially like that shown in Figure 46, except
that the glands are proportionately not quite so long as in the fundic
and cardiac regions, and are somewhat more open--that is, they have
wider lumina; their lining cells are all of one kind and are unchanged
from what was seen in the hibernating condition.

_The Small Intestine._ Three regions of the small intestine will be
described: (1) an anterior, just caudad to the stomach; (2) a middle;
and (3) a posterior, one half inch cephalad to the rectum or large
intestine (Fig. 35).

As might be expected, the general structure of the wall of the
intestine is essentially the same in all three regions, the slight
differences noticeable being due mainly to variations in the thickness
of the various layers.

The middle and posterior regions have about the same diameter, while
the diameter of the anterior region is considerably greater, due
partly to the greater diameter of the lumen but mainly to the greater
thickness of the constituent layers, especially the mucosa. The mucosa
is also thrown into more numerous and complicated folds in the anterior
than in the middle and posterior regions; the complexity of the mucosa
seems to diminish as the intestine is followed caudad. In the anterior
region the mucosa may form at least one half of the entire thickness of
the wall, while in the posterior region it may form less than one third
of the thickness of the intestinal wall. The minute structure of the
intestinal epithelium will be described below.

The chief peculiarity of the intestinal wall is the apparent total
absence of a submucosa (Fig. 47). As will be described later, the
mucosal epithelium is laid upon the usual bed of fibrous and lymphatic
tissue, the tunica propria (Fig. 47, _tp_).

At the outer border of the tunica propria, and with no tissue
corresponding to a submucosa between it and the circular muscular
layer, is a thin and indistinct layer that has the appearance of a
longitudinal layer of muscle fibers; this should correspond to the
muscularis mucosa (Figs. 47, 48, 49, and 51, _mm_).

[Illustration:

FIG. 47. A transsection of the wall of the anterior region of the
small intestine of the hibernating animal, under low magnification;
_ln_, lymph node; _tp_, tunica propria; other letters as in Figure
42.]

The circular, _cm_, and longitudinal, _lm_, muscle layers are compact,
and are distinct from the other layers of the wall; the former is
approximately twice the thickness of the latter. The relative thickness
of all the layers in the three regions of the intestine may be seen by
comparing Figures 47, 48, and 49.

[Illustration:

FIG. 48. An outline of a transsection of the wall of the middle
region of the small intestine of the hibernating animal, under low
magnification; lettering as in Figure 42.]

The serosa, _s_, which is of about the same character in the three
regions under discussion, is a distinct and fairly dense layer of
connective tissue with numerous blood-vessels.

The general appearance of the mucous membrane as a whole is
sufficiently clear in the low-power drawing described above, so that
all that need be shown under a higher magnification is the epithelium
(Fig. 50). The upper part of this figure represents the lower end of
one of the intestinal glands cut longitudinally, below which is the end
of another gland in transverse section. Between the two sections is the
compact tunica propria of lymphatic tissue.

The section from which this particular figure was drawn was in the
anterior region, but the corresponding part of a section in either of
the other regions would have practically the same appearance.

The epithelium is of the stratified columnar type. The superficial
cells are very tall and narrow, with the nuclei generally at or near
the bases, though an occasional nucleus may be seen near the free end
of a cell. Below the tall columnar cells are four or five rows of
nuclei which represent smaller, irregular cells, though the cell walls
could not always be determined between the closely packed nuclei. No
goblet cells are to be seen at any place.

[Illustration:

FIG. 49. An outline of a transsection through the wall of the
posterior region of the small intestine of the hibernating animal,
under low magnification; lettering as in Figure 42.]

The relative diameters of the three regions of the small intestine in
the feeding condition are about the same as noted for the hibernating
stage; that is, the anterior region has the greatest diameter and the
other regions are smaller and have about the same average diameter.

The most marked difference between the intestine during hibernation and
feeding is in the relative thickness of the mucosa and muscular layers.
As described for the hibernating stage, so in the feeding stage,
the mucosa is relatively the thickest in the anterior regions and
diminishes in thickness caudad; but while, in the hibernating stage, it
forms, in the anterior region, as much as half of the entire thickness
of the wall, in the feeding condition it forms, in the same region, at
least two thirds of the entire wall and in the middle and posterior
regions more than half of the wall.

[Illustration:

FIG. 50. Part of the mucous membrane of the anterior region of the
small intestine of the hibernating animal, under high magnification.
The upper part of the figure shows a part of a gland cut
longitudinally, the lower part of the figure shows another gland cut
transversely; _e_, epithelium; _tp_, tunica propria.]

The feeding animals being the smaller, the diameter of the intestine
was considerably less than in the hibernating stage; but the actual
thickness of the mucosa was practically the same, so that the
difference in diameter was due to the difference in the thickness of
the muscular and fibrous layers. It is therefore probable that the
differences noted above are due rather to the differences in the size
of the animals from which the tissues were taken than to the different
conditions of hibernation and feeding. The point to be noticed is
that the increase in the diameter of the intestine is due almost if
not entirely to an increase in thickness of the connective tissue and
muscle layers.

No difference in the complexity of the folds of the mucosa of the two
stages can be noticed.

The thickness of the fibro-muscular part of the wall of the intestine
varies considerably on different sides of the same region, but it
consists of the same layers in about the same relative amounts.

[Illustration:

FIG. 51. An outline of a transsection of the wall of the middle
region of the small intestine of the feeding animal, under low
magnification; _m_, mucosa; other letters as in Figure 42.]

Figure 51 represents in outline the wall of the middle region of the
small intestine during feeding.

The epithelium is of the same thickness in the two stages, and the only
difference in its character that can be seen under a high magnification
is that, in the middle region at least, the nuclei are not crowded so
close together at the basal ends of the cells as in the hibernating
stage but are scattered more towards their free ends.

[Illustration:

FIG. 52. A transsection of the wall of the anterior region of the
rectum or large intestine of the hibernating animal, under low
magnification; _tp_, tunica propria; other letters as in Figure 42.]

Altogether, the differences in microscopic structure between the small
intestine of an alligator at the end of the hibernating period and at
the end of a period of regular feeding are very slight.

_The Large Intestine._ The planes of the two sections studied are shown
in Figure 35; a low-power drawing of the posterior region is shown in
Figure 52. The anterior and posterior regions of the large intestine
do not differ from each other sufficiently to make it worth while
to represent both by drawings. Had an entire section through either
region been drawn it would be seen that the wall is of very different
thickness in different places, as was noted in connection with the
small intestine; the posterior section was drawn where the wall was
thin.

It might be supposed that in the feeding season the fecal matter in the
posterior region of the rectum would stretch the walls sufficiently to
obliterate largely the prominent folds seen in Figure 52, but such does
not seem to be the case. The usual layers of the vertebrate intestine
are present.

[Illustration:

FIG. 53. The epithelium of the anterior region of the rectum of the
hibernating animal, under high magnification; _e_, epithelium; _tp_,
tunica propria.]

The epithelium, shown under high magnification in Figure 53, is of
the same character and thickness throughout, except that as the anal
aperture is approached the columnar epithelium changes into the
stratified variety. It consists of very tall and narrow columnar cells
apparently in one layer, though it is difficult to be sure of this.
With an occasional exception, near the top, all of the nuclei are
arranged in a fairly wide zone below the middle of the epithelium.
The nuclei are oval in shape and lie so close together that it is
difficult, as has been said, to be sure that the cell to which each
belongs extends throughout the entire thickness of the epithelium.

Beneath the epithelium (Fig. 52, _e_) is a dense tunica propria, _tp_,
underlaid, in turn, by the muscularis mucosa, _mm_, and a submucosa,
_sm_, of the usual character, which is thrown into marked folds.
The circular, _cm_, and longitudinal, _lm_, layers are of the usual
character except that they vary more in thickness, as noted above, and
in density than is usually the case.

The serosa, _s_, is comparatively thin and compact in both regions, and
varies somewhat in thickness at different places.

The large intestine of the feeding animal was sectioned in the same
regions as in the hibernating. As has been said, the feeding animals
used were much smaller than the hibernating, so that, as might be
expected, the diameter of the large intestine was much less in the
former than in the latter. Except for this difference in diameter there
was no noticeable difference between the two stages. In the case of the
small intestine, it will be remembered, the greater diameter of the
intestine of the larger animal was mainly due to the greater thickness
of the muscular and connective-tissue layers and not to any increase
in thickness of the mucous membrane. In the large intestine the mucosa
varies in thickness in the animals of different size as do the other
layers of the wall.

The glandular character of the lining of the large intestine seems to
indicate that this region of the intestine must have some digestive or
absorptive function and that it does not act merely as a receptacle
for fecal matter; this makes it all the more strange that there should
not be some change produced in its structure by five or six months of
feeding or of fasting.

_Summary._ The material used in this investigation was taken from
young animals at the end of a feeding period of about five months, and
towards the end of the hibernating period after fasting for four or
five months.

The regions of the enteron that were studied were as follows: the tip
and base of the tongue; the anterior and posterior regions of the roof
of the mouth; the anterior and posterior regions of the œsophagus; the
cardiac, fundic, and pyloric regions of the stomach; the anterior,
middle, and posterior regions of the small intestine; the anterior and
posterior regions of the large intestine. Since the work was started
at the end of the hibernating period, the tissues of that period were
studied and drawn first.

The only difference between the structure of the tip of the tongue
during hibernation and during the feeding season is that the scaly
epithelium with which it is covered is somewhat thicker and more
compact in the former than in the latter condition, though even this
difference may have been due to differences in the ages of the animals
used. The base of the tongue differs from the tip in having a thicker
epithelium and in having compound tubulo-alveolar glands. These glands
in the hibernating animal have many more alveoli than in the feeding
animal, though this, again, may have been due to the difference in age.

The lining of the roof of the mouth is essentially the same as that
of the tongue. The glands are found only in the posterior region. The
slight differences in the papillæ here found may easily be due to the
difference in age.

The œsophagus shows the usual layers for that region. Its epithelium
is partly ciliated in the anterior part. The muscularis mucosa is very
scant in the anterior region. The only difference between the two
stages is that in the feeding the muscularis mucosa in the anterior
region is much more strongly developed than in the hibernating stage;
and in the former the nuclei of the epithelium are not arranged in two
zones as in the latter.

The stomach has the usual layers, and has essentially the same
structure in the three regions studied, except that the wall in the
fundic region is much the thickest, due mainly to the great thickness
of the middle muscle layer. Only one kind of cell is found in the
gastric glands. No difference is to be noted between the hibernating
and feeding conditions.

The chief peculiarity of the small intestine is the apparent entire
absence of the submucosa. Goblet cells are also wanting. The greater
diameter of the anterior region is due both to the greater diameter
of the lumen and to the greater thickness of the walls. The middle
and posterior regions have about the same diameter, though the mucosa
becomes thinner and less complicated caudad. There is practically no
difference between the hibernating and feeding stages.

The anterior and posterior regions of the large intestine have
essentially the same structure. No difference can be seen between the
hibernating and feeding conditions.

CHAPTER VI

THE UROGENITAL ORGANS

Figure 54 represents the urogenital apparatus of a thirty-inch
female specimen of _Alligator mississippiensis_. Figure 55 shows the
corresponding organs of a male _A. lucius_; reproduced from Bronn.

The urogenital organs in the young animal are so similar in the two
sexes that one might easily be mistaken for the other; of course in
sexually mature animals, especially during the breeding season, this is
not the case.

The _kidneys_, Fig. 54, k, Fig. 55, a, are flattened, lobulated organs
lying against the dorsal body wall. The large anterior lobe of each
kidney is pointed at its anterior end and lies at some little distance
from its fellow; it is partially divided into secondary lobes and
is traversed on its ventral surface by branching blood-vessels. Its
antero-medial border is sometimes partially concealed, in a ventral
view, by the elongated gonad of that side. Caudad to the main lobe of
the kidney is a smaller, usually distinct, lobe in contact mesially
with its fellow of the opposite side.

A fairly wide _ureter_, Fig. 54, u, Fig. 55, d, extends from the
posterior end of each kidney to open (Fig. 54, u¹, Fig. 55, e) into
the anterior region of the cloaca, as described in connection with the
digestive system.

The _ovary_, Fig. 54, o, as noted above and as seen in Figs. 54 and 55,
in the young animal is of practically the same shape as the testis. The
ova at this stage are of microscopic size and are hence not visible
to the naked eye. The ovary, even at this stage, is more or less
distinctly marked off into lobules by a series of small grooves.

[Illustration: FIG. 54. FEMALE UROGENITAL SYSTEM.

_f_, oviduct; _f¹_, opening of oviduct; _k_, kidney; _m_, mesentery;
_o_, ovary; _u_, ureter; _u¹_, opening of ureter.]

The _oviduct_, Fig. 54, f, which at this stage is, of course, of small
diameter, extends across the ventral surface of its corresponding
kidney and opens, f¹, into the posterior part of the cloaca as has
already been described. Its peritoneal opening is some distance
cephalad to the head of the ovary. Its course from this opening is
straight until about the anterior end of the ovary; it then becomes
somewhat convoluted for a short distance, but gradually straightens
out, to pass to its posterior end as a nearly straight duct. The
anterior straight portion of the oviduct is connected with the head of
the ovary by a narrow band of mesentery.

Each _testis_, Fig. 55, b, like the ovary, lies along the ventro-mesial
border of its corresponding kidney and is connected with the posterior
region of the cloaca by a slender vas deferens, Fig. 55, c, f.

According to Rathke (in _C. acutus_) a small, slender epididymis lies
along the outer side of the posterior half of each testis.

_The Copulatory Organs._ The _penis_, Fig. 56, usually lies completely
hidden in the cloaca; with its glans projecting backwards it is
strongly arched; along the convex side of the arch, which is directed
towards the upper wall of the cloaca, runs a groove, which serves as a
penial urethra to conduct the semen.

According to Rathke there may be recognized in connection with the
penis two fibrous strands (resembling the corpora cavernosa of
mammals), a corpus cavernosus urethrea, and a covering derived from the
mucous membrane of the cloaca. The two fibrous strands arise from the
pubis as two thick plates that soon completely fuse together by their
adjacent sides to form the shaft, c, of the penis. These fused strands
taper gradually towards the glans, in which they end in a point. From
their mode of fusion there is left between them, on the side towards
the upper wall of the cloaca, a fairly deep furrow that extends to the
tip. According to Rathke these shafts are not of cavernous tissue, but
the tube is lined by a layer of this tissue.

[Illustration: FIG. 55. MALE UROGENITAL APPARATUS OF ALLIGATOR LUCIUS.
(After Bronn.)

_a_, kidney; _b_, testis; _c_, vas deferens; _d_, ureter; _e_,
opening of the ureter into the cloaca; _f_, opening of the vas
deferens into the cloaca; _g_, upper region of the cloaca; _h_,
hinder region of the cloaca; _i_, rectum.]

[Illustration: FIG. 56. MALE ORGAN OF ALLIGATOR LUCIUS, XI. (From
Bronn, after Rathke.)

_a_, the right crus penis; _b_, the mucous membrane of the cloaca
that covers the organ; _c_, shaft of the penis; _d_, base of glans;
_e_, point of glans; _f_, part of the ring muscle of the cloaca.]

The _glans_, e, consists of two parts between which, where they leave
the shaft of the penis, is a funnel-shaped hole, wider towards the free
end of the penis and divided into similar lateral halves by a fold
of skin. The glans is much shorter than the shaft of the penis. The
covering of the penis is much thinner than the mucous membrane of the
cloaca and is thinnest along the groove; it extends from the shaft over
the glans without forming a foreskin.

The base of the penis is attached to the pubis near its symphysis.
With this base the most anterior part of the strong ring-muscle of the
cloaca is closely attached by a fairly large mass of fibrous tissue.
Rathke fails to find any muscles that are concerned alone with the
copulatory organs.

In the copulation of the crocodile, according to Rathke, the penis
is erected, though how this is caused is difficult to say since the
corpora cavernosa consist only of fibrous tissue and the cavernous
tissue lining the groove is very thin. The penis can, therefore,
project only a short distance from the cloaca. The cavernous tissue
is capable of causing only a slight elongation of the shaft, but the
glans is considerably elongated by the strong influx of blood into that
structure. According to Voeltzkow (78) the penis in the Madagascar
crocodile is 20 cm. in length.

The clitoris of the sexually mature female crocodile is very much
smaller than the penis of a male of the same size, but, according to
Rathke, it varies greatly in size in different species. It is built on
exactly the same plan as the penis.

According to Bronn the clitoris as well as the penis projects from the
cloaca, out through the anus, in the embryo of the crocodile; this
was not observed by the present writer in the embryo of the Florida
alligator, _A. mississippiensis_.

CHAPTER VII

THE RESPIRATORY ORGANS

_The Larynx and Trachea._ In the Crocodilia the framework of the larynx
consists of three cartilages, of which two represent the arytenoids of
the Mammalia; the third represents the thyroid and cricoid of mammals.
The last is considerably larger than the first and is a broad closed
ring, differing in form in the different species. In spite of the fact
that some of them have a voice, the vocal cords, according to Bronn,
are wanting in the Crocodilia. According to Henle the vocal apparatus
is produced by the projection into the laryngeal cavity of the inner
border of the small arytenoid cartilages and by the infolding, under
these cartilages, of the mucous membrane of the larynx; this forms the
thick but fairly free folds that, when the glottis is narrowed, are
well adapted to produce the harsh tone of the animal.

The epiglottis is absent in the Crocodilia.

In many Crocodilia (_C. vulgaris_, for example) the trachea, Fig. 57,
tr, forms a loop which begins in some species before hatching, in other
species not until long after hatching. In the genus _Alligator_ the
trachea is straight.

More universal than this looped structure there is found another
peculiar structure in the crocodilian trachea. It is a short vertical
partition in the stem just before its division into the two branches.
This partition is partly membranous and possesses one or more
stiffening cartilaginous strands which are outgrowths of so many
cartilaginous rings of the trachea. The number of the stiffening fibers
varies in the different species.

[Illustration: FIG. 57. RESPIRATORY ORGANS.

_b_, bronchus; _e_, œsophagus; _g_, glottis; _l_, lung; _t_, tongue;
_tr_, trachea.]

The number of the tracheal rings varies not only in the different
species but also in different individuals of the same species. There
are between fifty and sixty in _A. mississippiensis_. According to
Rathke the number of rings in the individual animal almost certainly
does not increase with age. The number of rings is smallest in the
gavials and greatest in the crocodiles (genus _Crocodilus_). The number
of rings in the two divisions of the trachea does not increase with
age except, perhaps, in _C. acutus_ and _biporcatus_. The lateral bend
that the tracheal stem of so many Crocodilia exhibits is not due to the
greater number of rings because in some species (gavials) where the
bend is present the number of rings is smaller than in the Crocodilia
where the bend is absent.

According to Rathke and others most of the tracheal rings are closed,
but a varying, though at most small, number are open on the dorsal
side. These openings become wider as the larynx is approached. The
transverse muscle fibers which are found in the most anterior and
largest of these breaks in the tracheal rings were found, says Rathke,
in embryos after the middle period of incubation.

The cartilaginous rings of the bronchi, b, are also apparently open
for a time after their formation, but soon close. Not infrequently in
embryos and in young animals are found rings that are split like a
fork, with one or both branches fused with neighboring branches.

_The Lungs._ The lungs, Fig. 57, l, are more highly developed among
the Crocodilia than among any other Saurian or Hydrosaurian group. The
histological groundwork of the whole lung tissue is a connective tissue
of fine elastic fibers. In the lungs, on the canal that appears as the
elongation of the bronchus, cartilage appears, according to Rathke, as
bands lying one behind the other; some of these bands form complete,
others partial rings; some of the latter are forked. The hindermost
appear to be the broadest and most irregular. Their number is different
in different species and varies in different individuals of the same
species. They range in number, according to Rathke, from nine, in _A.
lucius_, to twenty-five, in _C. acutus_.[7]

[7] Miller (45c) says: “In the crocodile and alligator the bronchus
enters the lung near its center, and passes somewhat obliquely into
the lung until it reaches the junction of the lower middle third; here
it breaks up into eight to fifteen tubular passages. These tubular
passages are studded with a great many air-sacs. In these animals the
lung for the first time gives the structure as it is found in mammals.
There are many air-sacs which communicate with a common cavity, or
atrium, all of which in turn communicate with a single terminal
bronchus. A single lobule of the mammalian lung is simply enlarged
to form the lung of the crocodile; the lung of the former is only a
conglomerate of that of the latter.”

The arterial branch, carrying venous blood to the lungs, develops a
capillary network close to the alveolar walls, which leads away over
the low alveolar septa, while over the tops of the higher septa and
on the inner surface of the tube-like bronchial processes it forms a
wide-meshed network of capillaries that are apparently chiefly nutrient.

All the respiratory capillaries are attached by only one side to the
alveolar wall; the free side that projects into the air space of the
alveolus is covered by a continuous pavement epithelium.

While the respiratory surfaces are covered with an alveolar epithelium
of large polygonal cells, the free borders of all high septa and
ridges, as well as the inner surfaces of the bronchial processes, are
covered with ciliated cylindrical cells.

CHAPTER VIII

THE VASCULAR SYSTEM

The account given by Bronn in his _Thierreich_ is apparently the only
published description of the circulatory organs in the Crocodilia. This
account, even when translated, is not very satisfactory, especially
because it contains no diagrams of the circulation. It was, therefore,
deemed worth while to work out the circulation in the Florida alligator
in order that we might have not only a written description, but also a
series of more or less accurate diagrams of the veins and arteries.

A number of departures from the description of Bronn were found, some
of which are noted below.

Most of the work was done upon animals of about thirty inches length,
which were obtained alive from the Arkansas Alligator Farm at Hot
Springs, Ark.

The arteries were injected with a colored starch mass by inserting
a two-way cannula into the dorsal aorta. With the blood thus forced
into them from the arteries, the veins could, in most cases, be traced
without difficulty.

In the diagrams the outlines of the more important organs are
accurately shown by dotted lines, and the relative diameters of the
blood-vessels are shown as accurately as possible by the solid black
lines.

THE HEART

In the Crocodilia, as is well known, the heart is four-chambered and
has about the same general shape as in the higher vertebrates, Fig. 58.

The venous blood is emptied into a thin-walled sinus venosus on the
dorsal side of the heart by three large vessels and one small one. The
largest of these, the postcava, empties into the posterior side of the
sinus venosus and brings blood from the posterior regions of the body;
it is quite wide, but is exposed for a very short distance between the
liver and the heart. Two large hepatic veins empty into the postcava
so near the sinus venosus that they practically have openings into the
sinus, as is shown in a somewhat exaggerated way in Fig. 59. Near the
postcaval and hepatic openings is the distinct coronary vein, lying in
a slight depression between the right and left ventricles.

From the anterior regions of the body the blood is brought back through
two fairly wide but very thin-walled precaval veins which pass across
the dorsal surface of the heart to enter the sinus venosus.

[Illustration: FIG. 58. HEART OF A. LUCIUS. (Dorsal View.)

(From Bronn, after Fritsch.)

A_o_, _d_, _Ao_, _s_, right and left aortæ; _At_, _d_, _At_, _s_,
right and left atria; _Au_, _d_, _Au_, _s_, right and left auricles;
_Ca_, _pr_, primary carotid; _P_, _d_, _P_, _s_, right and left
pulmonary arteries; _Sc_, _d_, _Sc_, _s_, right and left subclavians;
_Sv_, sinus venosus; _V_, _cc_, coronary vein; _V_, _c_, _d_, _V_,
_c_, _i_, right _precava_, and inferior cava; _Ven_, _d_, _Ven_, _s_,
right and left ventricles; _V_, _h_, hepatic vein; _V_, _p_, _d_,
_V_, _p_, _s_, right and left pulmonary veins.]

The arterial blood is brought from the lungs by two wide, thin-walled
pulmonary veins, Fig. 58, V.p.s., V.p.d. They leave the lungs somewhat
caudad to their middle region, near the point of entrance of the
bronchii and the pulmonary arteries, pass mediad in a direction almost
at right angles to the long axis of the body, and enter the left
auricle at the same point.

Blood leaves the heart through five large vessels: (1) the pulmonary
artery, (2) the two aortic arches, (3) the right subclavian, (4) the
primary carotid.

The pulmonary leaves the small right ventricle as a single stem, which
soon branches into two arteries that pass cephalad and laterad to the
lungs, along with and close to the main bronchi. The other arteries
that carry blood into the systemic circulation are fused at their base
to form a sort of conus arteriosus which may be distended in injected
specimens until it is larger than the two ventricles together. When
opened this conus is found to contain two chambers that lead into the
left ventricle; the larger chamber gives origin to the right systemic
arch, the right subclavian, and the primary carotid; the smaller
chamber is the basal part of the left systemic arch.

The two systemic vessels, Fig. 58, Ao.s, Ao.d, pass, in the usual
manner, as two arches to the dorsal region, just posterior to the
ventricles, where they form the dorsal aorta in the manner to be
described in connection with the arterial system.

The further course of the primary carotid and of the right subclavian
will also be described in connection with the arterial system.

The auricles are very large in proportion to the ventricles, though
their relative sizes will, of course, vary with the amount of contained
blood.

[Illustration:

FIG. 59. The veins of the posterior region of the Florida alligator.
The postcaval system and its associated veins are shown in the main
figure; the hepatic portal system is shown in the smaller figure to
the left.--For lettering, see pages 224-25.]

THE VENOUS SYSTEM

_The Posterior Vena Cava and its Branches._ The _postcava_, Fig. 59,
pc, as noted above, is a wide, thin-walled vessel seen extending
across the short space between the anterior face of the right lobe of
the liver and the sinus venosus. As was also noted above, the hepatic
veins, vh,--at any rate that from the left lobe of the liver,--enter
the postcava so close to the heart that they may be considered to have
one or more distinct openings into the sinus venosus. Followed caudad,
the postcava may be traced through the large right lobe of the liver,
from which it receives several branches. Emerging from the posterior
border of the liver, it is seen to extend caudad, in the median
line, as a rather inconspicuous vessel that receives blood from the
reproductive organs and the kidneys that lie close on either side of it.

The _hepatic portal_ vein, h, has the usual distribution for that
vessel. Entering the liver in the neighborhood of the bile duct, it
receives first (_i.e._, nearest the liver) a small branch from the
pancreas, pv; near the pancreatic are one or two branches from the
stomach, g, and a branch from the spleen, sp. A short distance caudad
to these vessels are two or three mesenteric veins, m, leading from the
mesentery and small intestine. Caudad to the mesenterics, the portal
system may be seen as a vein of diminished caliber, i, leading from the
posterior part of the small intestine and from the large intestine.

The connection mentioned by Bronn between the rectal branch of the
portal vein and the caudal vein could not be demonstrated. After
entering the liver, the portal, of course, breaks up into capillaries,
and the blood thus distributed is re-collected by the capillaries of
the hepatic veins above mentioned.

The _internal epigastric_ veins, ep, are, perhaps, the most conspicuous
vessels of the postcaval system. When the ventral abdominal wall of the
animal is removed, they may be seen extending forward from the pelvic
region, on each side of the body, to enter the posterior edge of the
liver. The epigastric of the right side enters the large or right lobe
of the liver, where it breaks up into capillaries; the left epigastric
sends its main branch into the left lobe of the liver, but also sends a
branch over to enter the right lobe.

Following the epigastrics caudad, they are seen to receive vessels
from nearly all parts of the posterior region of the body. The left
epigastric, which extends across the ventral side of the stomach,
receives from that organ four or five branches, g¹; while the farther
removed right epigastric receives only one or two branches from the
stomach. Posterior to these gastric veins the epigastrics receive one
or more veins, b, from the body wall and skin. Posterior, again, to
the last-named veins, each epigastric receives, in the pelvic region,
a large vein, the _iliac_, il, which receives, in turn, a vein from
the pelvis, pl, and continues down the thigh and lower leg to the foot
as the _femoral_, f, the chief vein of the posterior appendage. After
receiving small branches from the muscles of the thigh, the femoral
receives near the knee a small branch from the posterior surface of
the lower leg, fb, and a larger one, t, that leads from the anterior
surface of the lower leg and foot.

The veins of the pes were so small, in the comparatively small
animals it was necessary to use, that their distribution could not be
determined with certainty, though they seemed to parallel very closely
their corresponding arteries to be described below.

A short distance caudad to the iliac veins, each epigastric receives
one or two fairly large branches from the pelvic region, called by
Bronn the _ischiadic_ veins, is. Caudad to the ischiadics and dorsal
to the cloaca, each epigastric is united with a short but wide _renal
portal_ or renal advehente vein, rp, leading to the posterior border of
its respective kidney and receiving, on the way, a short branch from
the pelvic region, shown just cephalad to the reference lines rt and rp.

Very close to its junction with the renal portals each epigastric gives
off a small branch which unites with its fellow of the opposite side to
form a median vein, rt, the _rectal_ leading from the posterior part
of the large intestine. A very short distance caudal to these last
veins, in the region just dorsal to the anal opening, the epigastrics
are formed by the division of the _caudal_ vein, cv, which, of course,
brings blood from the tail and is, on account of the large size of
that organ, of considerable caliber.

_The Anterior Venæ Cavæ and their Branches._ The entrance of the
precaval veins into the heart was mentioned above; their branches, in
order from the heart cephalad, will now be described. Since the two
precavæ are alike, it will be necessary to describe the branches of
only one side of the body. After leaving the heart, the precava may
be traced forward, for a short distance, at the side of the trachea
and œsophagus, as a wide, thin-walled trunk, Fig. 60, vca. The first
tributaries that it receives are the internal mammary and vertebral
veins, which join it at the base of the neck at almost the same place.

The _internal mammary_, Fig. 60, im, is a rather small vein, bringing
blood from the ventral wall of the thorax. It may be followed along
the inner surface of the ribs, near the sternum, in company with its
corresponding artery.

The _vertebral_ vein, Fig. 60, v, is also of small diameter and extends
to the dorsal body wall near the spinal column, from which region it
returns blood to the anterior vena cava; it is drawn too large in the
figure.

[Illustration: FIG. 60. The veins of the anterior region of the
Florida alligator. The veins of the left foreleg are shown at A.--For
lettering, see pages 224-25.]

Just cephalad to the vertebral and internal mammary, the _internal
jugular_, j, enters the precava. The internal jugular may be followed
directly forward, close to the side of the trachea and œsophagus,
from which it receives numerous branches. Near its point of entrance
to, or rather exit from, the skull, it anastomoses, by two or three
short branches, with the external jugular, ej, to be described later.
Its distribution in the cranial cavity could not be determined in the
available material. At the point of entry of the internal jugular the
precava passes laterad for a short distance and then divides into two
more or less equal branches, the above-mentioned external jugular, ej,
and the subclavian, s, of which the latter will first be described.

The _subclavian_, s, of course, returns blood from the regions of
the shoulder and arm. On reaching the body wall, where it might be
called the _axillary_, ax, it receives, on its posterior side, a large
_thoracic_ vein, t, which returns blood from the thorax, shoulder, and
skin. The thoracic receives a branch from the posterior surface of the
arm, which might be called the _postbrachial_, pb; this postbrachial
may be traced, as a rather small vessel, to the hand; at the elbow it
is connected, by one or more small branches, with the brachial.

Just distal to the thoracic the axillary vein receives two fairly large
vessels, the _subscapulars_, sc, that return blood from the shoulder
and upper arm. After receiving the subscapulars, the axillary may be
followed into the upper arm as the _brachial_, br. As has been said,
the brachial and postbrachial anastomose near the elbow, and in this
region the former receives a small vessel that extends parallel to it
from the manus.

In the forearm the brachial may be called the _radial_, Fig. 60, A, ra;
on the back of the manus the radial receives branches from the various
digits and from a rather complex plexus of vessels in the carpal region.

The _external jugular_, Fig. 60, ej, after separating from the
subclavian, may be traced cephalad, close beneath the skin, to the base
of the skull, where it is connected with the internal jugular by short
branches, as has already been noted. It receives several small branches
from the skin and muscles of the neck and shoulder regions. At the
region of its anastomosis with the internal jugular it receives a large
branch, the _muscular_, ms, from the massive muscle at the angle of the
jaw and from the skin of that region.

A short distance cephalad to the muscular the external jugular
receives, on its mesial side, two or three branches from the trachea,
larynx, and œsophagus, tr. Anterior to these vessels the external
jugular is formed by the union of two chief veins, the _lingual_,
l, from the ventro-lateral surface of the tongue, and the _inferior
dental_, id, from the mesial surface of the lower jaw. The connection
of the _superior dental_ (extending along the bases of the maxillary
teeth) with the jugular could not be determined with certainty, hence
that vessel is not shown in the figure. The same is true of the small
veins in the region of the cranium.

THE ARTERIAL SYSTEM

_The Abdominal Aorta and its Branches._ The right and left aortic
arches, Fig. 61, Aod, Aos, arising from the heart in the manner already
described, form a rather long loop and approach each other in the
middorsal line. Here they are united by a short, wide connective in
such a way that the left arch seems continued into the cœliac artery
and the right into the dorsal aorta proper. Each arch, anterior to the
connective, gives off two fairly large branches, oe, to the posterior
region of the œsophagus.

The _cœliac_ artery, Fig. 61, c, is the largest branch of the
abdominal aortic system. After giving off a couple of small branches,
oe, to the posterior region of the œsophagus, it gives off a large
_spleno-intestinal_ artery, si, to the spleen and small intestine.

The cœliac then breaks up into three arteries of about the same size:
the _gastro-hepatico-intestinal_, ghi, carrying blood to the stomach,
liver, and small intestine; the _pancreo-intestinal_, pi, leading to
the pancreas and small intestine; and the _gastric_, ga, to the greater
part of the stomach.

From the _dorsal aorta_ proper, da, which, as has been said, seems
to be the direct continuation of the right aortic arch, several
arteries are given off; these will be described as they occur in an
antero-posterior direction.

At about the point of union of the two aortic arches arises the most
anterior of seven or eight pairs of _lumbar_ arteries, lu 1-7; this
first lumbar artery is continued cephalad for some distance as a
longitudinal trunk that gives off several lateral branches to the walls
of the thoracic region. The other six or seven lumbars are distributed
to the dorsal body wall, and arise, at more or less regular intervals,
as far caudad as the sacrum, or even back of that point.

[Illustration:

FIG. 61. The arteries of the posterior region of the Florida
alligator.--For lettering, see pages 224-25.]

The first large branch of the aorta is the unpaired _mesenteric_
artery, m¹, which is given off in about the region of the fourth pair
of lumbars; it carries blood through the mesentery to the greater
part of the small intestine and also sends a small branch to the large
intestine.

Posterior to the mesenteric, the aorta gives off four or five pairs of
short arteries, the _urogenitals_, _u 1-4_, that lead to the nearby
reproductive organs and kidneys.

About the middle region of the kidneys, a short distance anterior to
the sacrum, is given off a pair of rather large arteries, called by
Bronn the _ischiadicæ_, is¹; each ischiadica, after giving off a couple
of small branches to the back, passes laterad and divides into three
main branches: (1¹) to the ventral body wall, (3¹) to the anterior
border and deeper region of the thigh, and (2¹) to the pelvis.

In the region of the sacrum is given off a pair of _iliac_ arteries,
il¹. Each iliac is of about the same diameter as the ischiadica and
gives off, soon after leaving the aorta, an artery, ab, that apparently
leads chiefly to the abdominal muscles. Distal to the origin of the
abdominal, the iliac gives off a small _pelvic_ artery, pa, which
leads, as the name would indicate, to the pelvis. The iliac then passes
into the thigh, where it gives off several large branches and may be
called the _sciatic_, sc. At the knee the sciatic gives off two rather
small branches: one, the _fibular_ artery, f¹, extends down along the
posterior side of the lower leg; the other is parallel to the first
and may be called the _tibial_ artery, tb, since it extends along the
anterior or tibial side of the shank. These two arteries give off
numerous branches to the muscles of the lower leg. After giving off the
fibular and tibial arteries, the sciatic passes, as a large vessel,
through the lower leg, to which it gives but few branches, and may here
be called the _crural_ artery, cr. At the tarsus it divides rather
suddenly and, perhaps, variably, into four chief branches, leading to
the toes.

[Illustration:

FIG. 62. The arteries of the anterior region of the Florida
alligator. The arteries of the left foreleg shown at A.--For
lettering, see pages 224-25.]

A short distance caudad to the origin of the iliacs the dorsal aorta
gives off a pair of small _pelvic_ arteries, pa¹, going to the
muscles of that region. Caudal to these pelvic arteries is given off
the unpaired _first hæmorrhoidal artery_, he¹, which divides into a
_rectal_, rt¹, and a _cloacal_, cl, branch.

Caudal to the first hæmorrhoidal arises the _second hæmorrhoidal_, he²;
also unpaired, leading to the cloaca.

Posterior to the second hæmorrhoidal, the aorta continues into the tail
as the large _caudal_ artery, ca.

_The Anterior Arteries._ The origin of the great arterial trunks--the
pulmonary, aortic arches, primary carotid, and right subclavian has
already been given and the distribution of the pulmonary arteries and
aortic arches has been described, so that it now remains to describe
the distribution of the right subclavian, Fig. 62, sc.d., and the
primary carotid, capr.

The _right subclavian_, sc.d., since it has an independent origin from
the heart, instead of arising as a branch of the primary carotid,
will be described first. After leaving the heart it passes cephalad
and laterad and gives off the following branches in order, beginning
at the heart: an _œsophageal_ artery, oe, a small, caudally directed
vessel carrying blood to the posterior region of the œsophagus. Close
to the œsophageal arises another small, caudally directed vessel, the
_pleural_ artery, plu, extending to the pleura and possibly to the
pericardium. From the same region as the preceding two arteries, but
extending cephalad along the trachea and œsophagus, arises the much
larger branch of the right subclavian, the right _collateralis colli_,
cc, whose course and distribution will be described later.

Close to the distal side of the collateralis colli arises the very
small _thyroid_ artery, th, leading to the oval thyroid gland that lies
against the ventral surface of the trachea a short distance anterior to
the heart.

A short distance distal to the thyroid artery the subclavian gives off
a fairly large artery, the _internal mammary_, im¹ (shown too large
in the figure), that passes to the inner surface of the ribs near the
sternum and lies parallel to the vein of the same name, described above.

A short distance distal to the internal mammary arises an artery of
about the same diameter, the _vertebral_, v¹; it passes dorsad and
caudad to the region of the thoracic vertebræ.

After giving off the five vessels just described, the subclavian artery
passes into the shoulder where it divides into three main branches: (a)
the _subscapular_, sc¹, going to the skin and muscles of the shoulder;
(b) the _thoracic_, t¹, carrying blood to the posterior muscles of
the shoulder and to the posterior region of the upper arm; (c) the
_brachial_, br¹, which is really the continuation of the subclavian and
is the chief artery of the anterior appendage.

After sending several branches to the upper arm the brachial divides,
in the region of the elbow, into two main vessels, the _radial_, ra¹,
and _ulnar_, ul¹, arteries, Fig. 62, A. The radial artery, in the
carpal region, divides in a complicated way into five main vessels that
extend into the digits. The ulnar artery gives off several branches to
the forearm, but apparently does not connect directly with the branches
to the digits.

The _primary carotid_, capr. After leaving the heart, this very large
vessel passes cephalad and laterad for some distance on the left side
of the body and then gives off, from its anterior side, the large left
subclavian artery, sc.s., to be described later. After giving off the
subclavian artery, it makes a short loop, still farther to the left,
and then turns sharply mediad to pass to the head in the median plane
directly dorsal to the œsophagus. Its distribution in the cervical and
cephalic region will be described later. The mate to the œsophageal
branch, oe (near heart), of the right subclavian which was mentioned
above is apparently sometimes given off from the primary carotid near
its base (as shown in Fig. 62) and sometimes as a branch of the left
pleural artery.

The left _subclavian_ artery, sc.s., although it has a different
origin, has the same branches as described in connection with the right
subclavian. The exact order in which the first of these (the thyroid,
th; the internal mammary, im¹; the collateralis colli, cc; the pleural,
plu; and the vertebral, v¹) are given off is, as might be expected,
subject to some variation.

The _collateralis colli_, cc (following Bronn’s nomenclature), whose
origin was noted above, will now be discussed; since the two are alike
only one need be described. After leaving the subclavian, it passes
cephalad, at the side of the trachea and œsophagus, in company with the
internal jugular vein, so that in this part of its course it would seem
to be the internal carotid artery. It gives numerous small twigs to the
trachea and œsophagus, oe. In the region of the posterior part of the
huge jaw muscle it is connected directly, x, with the adjacent branch,
cm (called by Bronn the common carotid) of the primary carotid, and
indirectly, x¹, with a complicated group of branches from the common
carotid. Cephalad to the connective x¹, which extends dorsad and is
hence foreshortened in the figure, the collateralis colli gives off a
small vessel, y (too large in Fig. 62), to the shoulder and skin; it
then sends a fairly large branch, jm, into the large jaw muscle, close
to which it now lies. Next a small branch, lg, is sent to the larynx.
Continuing cephalad and laterad (in Fig. 62 it is drawn farther to the
side than it actually lies) for a short distance farther, it divides
into three branches: (1) a short twig, mg, that goes to the musk
gland on the side of the mandible and to the skin of that region; (2)
a large branch, the _mandibular_, md, that enters the large foramen
on the mesial side of the mandible and extends in the cavity of that
bone throughout its entire length; (3) the _lingual_ artery, l¹,
which in turn divides, some distance cephalad, into two branches, one
extending along the lateral region, the other nearer the mid-ventral
surface of the tongue. It is seen, then, that the collateralis colli
arteries supply directly the lower side of the head--tongue, mandible,
etc.--though they may also send blood through the above-mentioned
connectives to the brain and dorsal regions of the skull.

The _primary carotid_, capr, as was noted above, makes a curve to the
left after leaving the heart and then passes back to the median plane,
where it may be seen lying against the ventral side of the neck muscles
and dorsal to the œsophagus; in this place it gives off a series
of unpaired _cervical_ arteries, Fig. 62, ce, each of which almost
immediately divides into an anterior and a posterior branch, that carry
blood to the cervical vertebræ. At the base of the skull, in the region
where it is united by the first connective, x, with the collateralis
colli, as described above, the primary carotid divides into two similar
branches, called by Bronn the _common carotids_, cm. The distribution
of these two vessels is symmetrical, so that only one need be
described. While the collateralis colli, as has been said, carry blood
chiefly to the tongue and lower jaw, the common carotids supply the
cranium and upper jaw.

Soon after its formation by the division of the primary carotid, the
common carotid is joined, as noted above, with the collateralis colli
of that side by the connective, x; since the common carotid and its
branches all lie dorsal to the collateralis colli and its branches,
the connectives x and x¹ extend in a more or less dorso-ventral
direction. The two common carotids, almost completely surrounded by
bone, in passing cephalad sweep first laterad, then mediad, so that
they together form almost a complete ellipse, as seen in Fig. 62; there
is, however, no apparent connection between them at the anterior region
where they lie so close together.

A short distance cephalad to the connective x the common carotid is
connected laterally, z, with a rather complicated plexus of vessels
lying at the base of the skull; it is through this plexus that the
common carotid is connected with the collateralis colli by the second
connective, x¹.

The short branch z quickly divides into three parts: (1) a small
anteriorly directed vessel which may be called the _internal carotid_,
ic, since it enters the skull through the most ventral of the three
foramina in the exoccipital, and probably supplies the brain, though
its further course could not be followed; (2) a somewhat larger
posteriorly directed artery, oc, going to the muscles at the occipital
region of the skull; (3) a short laterally directed stem, z¹. The
last-named branch, z¹, in turn, leads in three directions: (a) to
the collateralis colli artery through the connective x¹; (b) a short
anteriorly directed vessel, e, that passes into the skull, possibly to
the ear, through the large foramen that lies between the exoccipital
and quadrate bones; it gives off a small twig, q, to the muscles in the
region of the jaw articulation (quadrate); (c) the main stem of the
branch z continues laterad and cephalad as one of the chief arteries,
z², to the anterior region of the skull, giving off a fairly wide
branch, jm¹, to the large jaw muscle, and then two branches, o¹ and o²,
to the lateral surface of the eyeball and socket; it then anastomoses,
just cephalad and laterad to the eye, with the forward continuation,
cm¹, of the corresponding main stem, cm, of the common carotid, already
mentioned. The vessel cm¹, after almost meeting its fellow in the
middle line, passes cephalad and laterad across the ventral surface of
the eye to the union, above mentioned, with the lateral branch, z²;
at the posterior-mesial border of the eye it gives off a branch that
divides into two twigs, one, o³, for the posterior eye muscles, and
one, e¹ to the region of the ear and the top of the skull.

At the point of union of the branches cm¹ and z² a sort of simple
plexus may be formed from which two vessels, n, pass to the posterior
nasal region, and two vessels pass forward along the side of the upper
jaw. Of the latter two vessels one, which may be called the _inferior
dental_ of the maxilla, dm, is very small and extends along the maxilla
to its very tip, at the base of the teeth and ventral to the palatine
bone; the other, which is larger and may be called the _superior
dental_ of the maxilla, dm¹, extends cephalad along the mesial side of
the maxilla, dorsal to the palatine bone; it sends numerous twigs into
the maxillary bone among the roots of the teeth. After passing nearly
to the end of the snout, the superior dental, dm¹ suddenly forms a loop
towards the median line and passes as a straight branch, n¹, directly
caudad, near and parallel to the median plane. The branch n¹ extends
along the floor of the nasal cavity and, after giving off twigs to this
chamber, ends in a network of vessels, o⁴, on the anterior surface of
the eyeball and socket.

A pair of very small arteries, n², may be seen in the nasal chamber
between and parallel to the branches, n¹; they lie close to each
side of the nasal septum and supply the anterior nasal region. They
apparently arise, as shown by the broken lines, from the loop of the
superior dental artery, dm¹, though this could not be definitely
determined.

LETTERING FOR FIGURES 59-62

Aos., Aod., left and right aortic arches.
ab, abdominal artery.
ax, axillary vein.

b, veins from body wall.
br, brachial vein.
br¹, brachial artery.

c, cœliac artery.
ca, caudal artery.
capr, primary carotid.
cc, collateralis colli artery.
ce, cervical artery.
cl, cloacal artery.
cm, cm¹, common carotid artery.
cr, crural artery.
cv, caudal vein.

da, dorsal aorta.
dm, inferior dental artery of maxilla.
dm¹, superior dental artery of maxilla.

e, e¹, artery into skull, perhaps to ear.
ej, external jugular vein.
ep, internal epigastric vein.

f, femoral vein.
f¹, fibular artery.
fb, fibular vein.

g, gastric vein of portal.
g¹, gastric vein of epigastric.
ga, gastric artery.
ghi, gastro-hepatico-intestinal artery.

h, hepatic portal vein.
he¹, he², hæmorrhoidal arteries.

i, intestinal vein.
ic, internal carotid artery.
id, inferior dental vein.
il, iliac vein.
il¹, iliac artery.
im, internal mammary vein.
im¹, internal mammary artery.
is, ischiadic vein.
is¹, ischiadic artery.

j, internal jugular vein.
jm, jm¹, artery to jaw muscle.

l, lingual vein.
l¹, lingual artery.
lg, laryngeal artery.
lu, 1-7, lumber arteries (numbers on left side of figure).

m, mesenteric vein.
m¹, mesenteric artery.
md, mandibular artery.
mg, artery to musk gland.
ms, muscular vein.

n, artery to posterior nasal region.
n¹, artery to anterior and mid-nasal region.
n², artery to anterior nasal region.

o¹-o⁴, arteries to eye.
oc, artery to muscles at base of skull.
oe, œsophageal arteries.

pa, pelvic artery.
pa¹, second pelvic artery.
pb, post brachial vein.
pc, post cava.
pd, right pulmonary artery.
pi, pancreo-intestinal artery.
pl, pl¹, pelvic vein.
plu, pleural artery.
ps, left pulmonary artery.

q, artery to muscle at angle of jaw.

ra, radial vein.
ra¹, radial artery.
re, reproductive vein or artery.
rp, renal portal vein.
rt, rectal vein.
rt¹, rectal artery.
rv, renal vein.

s, subclavian vein.
sc, sciatic artery (Fig. 61).
sc, subscapular vein (Fig. 60).
sc¹, subscapular artery.
sc.d., sc.s., right and left subclavian arteries.
si, spleno-intestinal artery.
sp, splenic vein.
s.v., sinus venosus.

t, thoracic vein.
t¹, thoracic artery.
tb, tibial artery.
th, thyroid artery.
tr, tracheal vein.

u, 1-4, urogenital arteries (numbers on right side of figure).
ul¹, ulnar artery.

v, vertebral vein.
v¹, vertebral artery.
vca, anterior vena cava.
vh, hepatic vein.
vpd, vps, right and left pulmonary veins.

x, x¹, connectives between collateralis colli and carotid.

y, artery to shoulder and skin.

z, z¹, z², branches of common carotid.
1¹, 2¹, 3¹, branches of ischiadic artery.

CHAPTER IX

THE DEVELOPMENT OF THE ALLIGATOR

(_A. mississippiensis_)

INTRODUCTION

With the exception of S. F. Clarke’s well-known paper, to which
frequent reference will be made, practically no work has been done upon
the development of the American alligator. This is probably due to the
great difficulties experienced in obtaining the necessary embryological
material. Clarke, some twenty years ago, made three trips to the swamps
of Florida in quest of the desired material. The writer has also spent
parts of three summers in the Southern swamps--once in the Everglades,
once among the smaller swamps and lakes of central Florida, and once in
the Okefinokee Swamp. For the first of these expeditions he is indebted
to the Elizabeth Thompson Science Fund; but for the more successful
trip, when most of the material for this work was collected, he is
indebted to the Smithsonian Institution, from which a liberal grant of
money to defray the expenses of the expedition was received.

The writer also desires to express his appreciation of the numerous
courtesies that he has received from Dr. Samuel F. Clarke, especially
for the loan of several excellent series of sections, from which a
number of the earlier stages were drawn.

In preparing the material several kinds of fixation were employed,
but the ordinary corrosive sublimate-acetic mixture gave about the
most satisfactory results. Ten per cent. formalin, Parker’s mixture of
formalin and alcohol, etc., were also used. In all cases the embryos
were stained _in toto_ with borax carmine, and in most cases the
sections were also stained on the slide with Lyon’s blue. This double
stain gave excellent results. Transverse, sagittal, and horizontal
series of sections were made, the youngest embryos being cut into
sections five microns thick, the older stages ten microns or more in
thickness.

THE EGG

FIGURES 1, 1_a_ (PLATE VI.)

The egg (Fig. 1) is a perfect ellipse, the relative lengths of whose
axes vary considerably in the eggs of different nests and slightly in
the eggs of the same nest. Of more than four hundred eggs measured, the
longest was 85 mm.; the shortest 65 mm. Of the same eggs, the greatest
short diameter was 50 mm.; the least short diameter was 38 mm. The
average long diameter of these four hundred eggs was 73.74 mm.; the
average short diameter was 42.59 mm. The average variation in the long
axis of the eggs of any one nest was 11.32 mm., more than twice the
average variation in the short axis, which was 5.14 mm. No relation was
noticed between the size and the number of eggs in any one nest. Ten
eggs of average size weighed 812 grams--about 81 grams each.

Voeltzkow (78) states that the form of the egg of the Madagascar
crocodile is very variable. No two eggs in the same nest are exactly
alike, some being elliptical, some “egg-shaped,” and some “cylindrical
with rounded ends.” The average size is 68 mm. by 47 mm., shorter and
thicker than the average alligator egg.

When first laid, the eggs are pure white, and are quite slimy for a few
hours, but they generally become stained after a time by the damp and
decaying vegetation composing the nest in which they are closely packed.

The shell is thicker and of a coarser texture than that of the hen’s
egg. Being of a calcareous nature, it is easily dissolved in dilute
acids.

The shell membrane is in two not very distinct layers, the fibers of
which, according to S. F. Clarke, are spirally wound around the egg at
right angles to each other. No air-chamber, such as is found in the
hen’s egg, is found in any stage in the development.

In most--probably all normal--eggs a white band appears around the
lesser circumference a short time after being laid. This chalky band,
which is shown at about its maximum development in Fig. 1_a_, is
found, on removal of the shell, to be caused, not by a change in the
shell, but by the appearance of an area of chalky substance in the
shell membranes. Clarke thinks this change in the membrane is to aid
in the passage of gases to and from the developing embryo. Generally
this chalky area forms a distinct band entirely around the shorter
circumference of the egg, but sometimes extends only partly around it.
It varies in width from about 15 mm. to 35 mm., being narrowest at its
first appearance. Sometimes its borders are quite sharp and even (Fig.
1_a_); in other cases they are very irregular. If the embryo dies the
chalky band is likely to become spotted with dark areas.

The shell and shell membrane of the egg of the Madagascar crocodile are
essentially the same as those just described, except that the shell is
sometimes pierced by small pores that pass entirely through it. The
same chalky band surrounds the median zone of the egg (78).

The white of the egg is chiefly remarkable for its unusual density,
being so stiff that the entire egg may be emptied from the shell into
the hand and passed from one hand to the other without danger of
rupturing either the mass of albumen or the enclosed yolk. The albumen,
especially in the immediate neighborhood of the yolk, seems to consist
of a number of very thin concentric layers. It varies in color, in
different eggs, from a pale yellowish white, its usual color, to a very
decided green.

As might be expected, no chalazæ are present.

The yolk is a spherical mass, of a pale yellow color, lying in the
center of the white. Its diameter is so great that it lies very close
to the shell around the lesser circumference of the egg, so that it is
there covered by only a thin layer of white, and care must be taken in
removing the shell from this region in order not to rupture the yolk.
The yolk substance is quite fluid and is contained in a rather delicate
vitelline membrane.

The albumen and yolk of the crocodile’s egg, as described by Voeltzkow,
differ from those of the alligator only in the color of the albumen,
which in the crocodile is _normally_ light green (78).

As pointed out by Clarke, the position of the embryo upon the yolk is
subject to some variation. During the earliest stages it may occur
at the pole of the yolk nearest the side of the egg; later it may
generally be found toward the end of the egg; and still later it shifts
its position to the side of the egg. It is probable, as Clarke says,
that the position at the end of the egg secures better protection
by the greater amount of white, at that point, between the yolk and
the shell; while the later removal to the side of the egg, when the
vascular area and the allantois begin to function, secures a better
aëration of the blood of the embryo.

Around the embryo, during the stages that precede the formation of
the vascular area, is seen an irregular area of a lighter color and a
mottled appearance. This area is bounded by a distinct, narrow, white
line, and varies in size from perhaps a square centimeter to one third
the surface of the yolk.

During the earliest stages of development the embryo is very
transparent; so that, as there is no fixed place upon the yolk at which
it may be expected to occur, it is often very difficult to find. Owing
to this transparency, to the extreme delicacy of the embryo, and to the
character of the white, the removal of an early embryo from the egg of
the alligator is a difficult operation and is accomplished only after
some practice.

THE DEVELOPMENT OF THE EMBRYO

As the writer has pointed out elsewhere (59), the embryo of the
alligator is often of considerable size when the egg is laid. This
makes the obtaining of the earliest stages of development a difficult
matter; so that the writer, as has already been said, like S. F. Clarke
(17), made three trips to the South in quest of the desired material.
Voeltzkow (78) experienced the same difficulty in his work on the
crocodile, and made several trips to Africa before he succeeded in
obtaining all the desired stages of development.

To obtain the earliest stages, I watched the newly made nests until the
eggs were laid, and in this way a number of eggs were obtained within
a very few hours after they had been deposited, and all of these eggs
contained embryos of a more or less advanced stage of development.
Gravid females were then killed, and the eggs removed from the
oviducts. These eggs, although removed from a “cold-blooded” animal,
generally contained embryos of some size, and only one lot of eggs
thus obtained contained undeveloped embryos, which embryos refused to
develop further in spite of the most careful treatment. Voeltzkow (78)
found, in the same way, that the earlier stages of the crocodile were
extremely difficult to handle; so that, in order to obtain the earlier
stages, he was reduced to the rather cruel expedient of tying a gravid
female and periodically removing the eggs from the oviducts through a
slit cut in the body wall.

The older embryos are hardy and bear transportation well, so that it is
comparatively easy to obtain the later stages of development.

For the stages up to the formation of the first four or five somites, I
am indebted, as I have already said, to Professor Clarke, and, since I
have had opportunity to examine only the sections and not the surface
views of these stages, I shall quote directly Clarke’s paper in the
_Journal of Morphology_ (17) in description of these surface views.

STAGE I

FIGURES 2-2_f_ (PLATES VI., VII.)

The youngest embryo that we have for description is shown in Figures 2
and 2_a_. Of Figure 2 Clarke says:

“The limiting line between the opaque and pellucid areas is clearly
marked, and within the latter is a shield-shaped area connected by
the narrower region of the primitive streak with the area opaca. The
blastopore is already formed near the posterior end of the shield.

“A ventral view of another embryo of the same age (Fig. 2_a_), seen
from the ventral side, shows that the blastopore extends quite through
the blastoderm, in an oblique direction downwards and forwards, from
the dorsal to the ventral side. The thickened area of the primitive
streak is here very prominent. There is, too, the beginning of a curved
depression at the anterior end of the shield, the first formation of
the head-fold.”

Transverse sections of this stage are shown in Figures 2_b_-2_f_.

Figure 2_b_, through the anterior region of the blastoderm, shows a
sharply defined ectoderm (_ec_) which is composed of three or four
layers of cells in the median region, while it gradually thins
out laterally. Closely underlying this ectoderm is a thin sheet of
irregular cells, the entoderm (_en_).

Figure 2_c_ is about one fifth of the length of the blastoderm
posterior to the preceding and represents approximately the same
conditions, except that there is an irregular thickening of the
entoderm in the median region (_en_). This thickening apparently marks
the anterior limit of the mesoderm, to be discussed shortly.

Figure 2_d_ represents the condition of the blastoderm throughout
about one third of its length, posterior to the preceding section.
The somewhat regular folds in the ectoderm (_ec_) are probably not
medullary folds, but are such artificial folds as might easily be
produced in handling the delicate blastoderm. The thickening of the
entoderm, noticed in the preceding figure, is here more sharply
defined, and as we pass toward the blastopore becomes separated
somewhat from the entoderm proper as a middle layer or mesoderm (Fig.
2_e_, _mes_). It would thus seem, from a study of these sections, that
most of the mesoderm is derived from the entoderm. In fact, all of the
mesoderm in front of the blastopore seems to have this origin, for it
is not until the anterior edge of the blastopore is reached that there
is any connection between the ectoderm and entoderm (Fig. 2_e_).

Figure 2_e_ is a section through the region just mentioned, where,
medially, the ectoderm, mesoderm, and entoderm form a continuous mass
of cells. Laterally the mesoderm (_mes_) is a distinct layer of cells
of a fairly characteristic mesodermal type. The notochord is not yet
discernible, though a slight condensation of cells in the middle line
may indicate its position.

Figure 2_f_ is one of the four sections that were cut through the
blastopore (_blp_), which is a hole of considerable size that opens,
as the figure shows, entirely through the blastoderm. Along the walls
of the blastopore the ectoderm and entoderm are, of course, continuous
with each other and form a sharply defined boundary to the opening. As
we pass laterally from the blastopore the cells become less compact,
and are continued on each side as the mesodermal layer (_mes_). In this
series the sections posterior to the blastopore were somewhat torn, and
so were not drawn; but they probably did not differ materially from
those of the corresponding region of the immediately following stages,
which are shown in Figures 3_m_ and 6_i_ and will be described in their
proper order.

STAGE II

FIGURES 3-3_o_ (PLATE VII., VIII., IX.)

The next stage to be described is shown in surface views in Figures 3
and 3_a_. Of this stage Clarke says:

“The head-fold rapidly increases in depth and prominence, as shown in
Figure 3, which is a ventral view a few hours later [than the preceding
stage]. The time cannot be given exactly, as it is found that eggs of
the same nest are not equally advanced when laid, and differ in their
rate of development. The lighter curve in front of the head-fold is the
beginning of the anterior fold of the amnion. The notochord has been
rapidly forming, and now shows very distinctly on the ventral side,
when viewed by transmitted light. A dorsal view of the same embryo
(Fig. 3_a_) shows that the medullary or neural groove is appearing, and
that it ends abruptly anteriorly near the large transverse head-fold.
Posteriorly it terminates at the thickened area in front of the
blastopore, which still remains open.”

Figures 3_b_-_m_ are drawn from transsections of an embryo of about
this stage of development. For a short distance in front of the
beginning of the head-fold, there is a mass of cells of considerable
thickness between the ectoderm and entoderm. In Figure 3_b_ these cells
appear as an irregular thickening of the entoderm, while in Figure 3_c_
they form a continuous mass, uniting the upper and lower germ layers.
This condition is seen, though in a much less striking degree, in the
following stage of development. As to its significance the writer is
not prepared to decide.

Figure 3_d_ passes through the head-fold, which in this embryo
was probably not so far developed as it was in the embryo shown in
Figures 3 and 3_a_. Not having seen the embryo, however, before it was
sectioned, the writer cannot be certain of this point. The ectoderm and
entoderm are here of nearly the same thickness.

Figure 3_e_ is a short distance posterior to the preceding. It shows a
marked thickening of the ectoderm in the medial region (_ec_), which is
continuous posteriorly with the anterior ends of the medullary folds
that are just beginning to differentiate (Figs. 3_f_-_h_).

Figure 3_g_ passes through the anterior end of the medullary plate
or folds (_mf_), whichever they may be called. The ectoderm of the
folds is thickened and is considerably elevated above the rest of the
blastoderm. There is scarcely any sign, in this region, of a medullary
groove. The entoderm (_en_) is considerably thickened in the medial
region, this thickening being continuous posteriorly, as in the
preceding stage, with the mesoderm.

In Figure 3_h_, cut in a plane at some distance posterior to the
preceding, the medullary groove (_mg_) is well marked; its bordering
folds gradually thin out laterally to the thickness of the ordinary
ectoderm. The medial thickening of the entoderm is very marked, but it
has not in this region separated into a distinct mesoblastic layer.

Immediately under the medullary groove is a dense mass of cells
(_nt_), apparently the anterior end of the notochord in process of
formation.

Figure 3_i_, still farther toward the blastopore, shows the medullary
groove wider and shallower than in the more anterior sections. The
mesoderm (_mes_) is here a layer laterally distinct from the entoderm.
In the middle line it is still continuous with the entoderm, and at
this place it is the more dense mass of cells that may be recognized as
the notochord (_nt_). It is evidently difficult to decide whether this
group of cells (_nt_), which will later become a distinct body, the
notochord, is derived directly from the entoderm or from the mesoderm,
which is itself a derivative of the entoderm. There is here absolutely
no line of demarcation between the cells of the notochord and those of
the mesoderm and entoderm.

In Figure 3_j_ the ectoderm (_ec_) is nearly flat, scarcely a sign of
the medullary groove appearing. The mesoderm (_mes_) is here a distinct
layer, entirely separate from both notochord (_nt_) and entoderm
(_en_). The notochord is a clearly defined mass of cells, distinct,
as has been said, from the mesoderm, but still closely united with
the underlying entoderm, which is much thinner than the ectoderm.
This condition of the notochord, which is found throughout about one
third of the length of the embryo, would give the impression that the
notochord is of a distinctly entodermal origin.

In Figure 3_k_ there is no sign of the medullary groove, though the
ectoderm (_ec_) is still much thickened in the middle line. The section
passes, posterior to the notochord, through the anterior edge of the
ventral opening of the blastopore (_blp_). The mesoderm (_mes_) is here
again continuous with the entoderm, around the edge of the blastopore,
but is distinct from the ectoderm.

Figure 3_l_ represents the third section posterior to the preceding.
The blastopore, which passes upward and backward through the
blastoderm, is seen as an enclosed slit (_blp_). It is surrounded
by a distinct layer of compactly arranged cells continuous with the
thickened ectoderm (_ec_) above, with the thin entoderm (_en_) below,
and laterally with the gradually thinning and scattering mesoderm
(_mes_).

Figure 3_m_ is the next section posterior to the one just described.
It passes through the dorsal opening of the blastopore (_blp_), which
appears as a deep, narrow cleft with thick ectodermal borders. The
three germ layers are still continuous with each other, though the
connection of the entoderm with the other two is slight. The sections
posterior to this one will be described in the next stage, where they
have essentially the same structure and are better preserved.

Figures 3_n_ and 3_o_ are sagittal sections of an embryo of about the
stage under discussion. In both figures the head-fold is seen as a deep
loop of ectoderm and entoderm, while the head-fold of the amnion is
seen at _a_.

The beginning of the foregut is seen in Figure 3_n_ (_fg_), which is
the more nearly median of the two sections, Figure 3_o_ being a short
distance to the side of the middle line.

In Figure 3_o_ the thin entoderm (_en_) is separated from the much
thicker ectoderm (_ec_) by a considerable layer of rather loose
mesoderm (_mes_). In Figure 3_n_, which is almost exactly median in
position, there is, of course, no mesoderm to be seen in front of
the blastopore, and the entoderm shows a considerable increase in
thickness, due to the formation of the notochord (_nt_). The blastopore
(_blp_) is the most striking feature of the figure, and is remarkable
for its great width in an antero-posterior direction. Its anterior and
posterior borders are outlined by sharply defined layers of ectoderm
and entoderm. Posterior to the blastopore the lower side of the
ectoderm is continuous with a considerable mass of cells, the primitive
streak (_ps_).

STAGE III

FIGURES 4, 4_a_, 5, 5_a_ AND 6-6_i_ (PLATES X., XI.)

“Figures 4 and 4_a_ are of an embryo removed, on June 18th, from an egg
which had been taken out of an alligator two days before. Figure 4, a
dorsal view, is of special interest in that it shows a secondary fold
taking place in the head-fold. This grows posteriorly along the median
dorsal line, forming a V-shaped process with the apex pointing backward
toward the blastopore. There is quite a deep groove between the arms of
the V. The head-fold on the ventral side, as seen in Figure 4_a_, made
from the same embryo as Figure 4, grows most rapidly on the mid-line,
and also becomes thicker at that place. The medullary folds now begin
to form on either side of the medullary groove, ending posteriorly on
either side of the blastopore and anteriorly on either side of the
point of the V-shaped process in the middle of the head-fold. This is
seen in Figure 5, which is a dorsal view of an embryo from an egg three
days after it was taken out of an alligator. A ventral view of the same
embryo (Fig. 5_a_) represents the thickened process on the mid-line at
its greatest development. For some reason the notochord did not show in
this embryo, possibly owing to particles of the yolk material adhering
about the mid-line.

“In an embryo a day or two older, the V-shaped fold of the head-fold
is seen to have broken through at the apex, and each of the arms thus
separated from one another unites with the medullary fold of its
respective side. This can be seen in Figure 6, which is a dorsal view
of part of an embryo a day or two older than the one represented by
Figures 5 and 5_a_.

“This is so unexpected a method of formation for the anterior part
of the medullary folds that I have made use of both Figures 4 and 5.
They were made from very perfect specimens, and the sections of both of
them, and of the specimen from which Figure 6 was drawn, proves that
the structure is what it is indicated to be in surface appearance. That
is, the transverse sections posterior to the V, in the embryos shown in
Figures 4 and 5, show the medullary groove and the medullary folds; the
several sections passing through the apex of the V show neither groove
nor folds, but only a median thickening; and in front of the point or
apex of the V the successive sections discover a gradually widening
groove between the arms, which is also much deeper than the shallow
groove found posterior to the V. While I have not seen, and from the
nature of the conditions one cannot see, the change actually proceeding
from the form of Figure 5 to that of Figure 6, still the explanation
given appears to be the only one possible” (17).

A somewhat extended series of transverse sections of an embryo of about
this age is represented in figures 6_a_-_i_.

Figure 6_a_ is a section through the head-fold; it passes through the
extreme anterior end of the secondary folds (_sf_) that were described,
in surface view, above (Figs. 5 and 6). The section was not quite at
right angles to the long axis of the embryo, so that the fold on the
right was cut farther toward its anterior end than was the fold on
the left. The pushing under of the head causes a forward projection
of the secondary folds, so that the fold to the right appears as a
rounded mass of cells with a small cavity near its center. On the
left the plane of the section passes through the posterior limit of
the head-fold, and shows the cells of the secondary fold continuous
with the dorsal side of the ectoderm (_ec_). As pointed out above by
Clarke, the secondary folds are here some distance apart, and gradually
approach each other as we proceed toward the posterior. The entoderm
(_en_) is here flat and takes no part in the secondary folds.

In Figure 6_b_, a short distance back of the one just described, the
secondary folds (_sf_) are much larger and are closer together. On the
right the section passes through the extreme limit of the head-fold, so
that the secondary fold of that side is still a closed circle, with a
few scattered cells enclosed. On the left the section is posterior to
the head-fold; on this side the secondary fold is seen as a high arch
of ectoderm, with a thick mass of entoderm beneath it.

Figure 6_c_ represents a section which passes back of the head-fold on
both sides. The secondary folds (_sf_) are seen as a pair of ectodermal
arches continuous with each other in the middle line of the embryo.
The ectoderm of the folds is much thickened and gradually becomes
thinner distally. On the right the entoderm shows the same thickening
(_en_) that was shown on the left side of the preceding figure. This
thickened appearance of the entoderm is due to the fact that the
section passes through the anterior limit of a tall fold of that layer,
which underlies the similar fold of the ectoderm that has already been
described. This secondary fold of the entoderm is seen on the left side
of the section. It may be traced through several sections, but soon
flattens out posteriorly.

Figure 6_d_ is a short distance posterior to the preceding. The
secondary folds are here much less pronouncedly arched and the deep
groove between them is reduced to a line (_l_). The entoderm (_en_) is
no longer markedly arched and is closely adherent, along the median
plane, to the ectoderm, where there is seen the thickening (_th_) that
has been mentioned by Clarke (see above). Springing from the entoderm
on each side of this thickening is a small mass of mesoderm (_mes_).

The section immediately posterior to the one just described is
represented in Figure 6_e_. The line (_l_) which separated the two
secondary folds in the preceding section is no longer present, so
that the ectoderm (_ec_) is continuous from side to side, with only
a shallow depression (_mg_), which may be considered as the extreme
anterior end of the medullary groove. The median thickening (_th_) is
cut near its posterior limit and still shows a close fusion of the
germ layers. There is no line of demarcation between the gradually
flattening secondary folds of the anterior end of the embryo and
the just forming medullary folds of the posterior end, so that it is
impossible to say whether the thickening of ectoblast in this figure
should be called secondary folds or medullary folds. As a matter of
fact, the secondary folds become, of course, the anterior ends of the
medullary folds. The mesoblast (_mes_) is here of considerable extent,
and its entodermal origin is beyond doubt, though not well shown in the
figure.

Figure 6_f_ is about one sixth of the length of the embryo posterior to
the preceding. The medullary thickening of the ectoderm (_ec_) is still
marked and the shallow medullary groove (_mg_) is fairly distinct. The
entoderm (_en_) is medially continuous with both mesoderm (_mes_) and
notochord (_nt_), though these two tissues are otherwise distinct from
each other.

Figure 6_g_ is nearly one third the length of the embryo posterior to
the preceding and passes through the posterior third of the embryo.
The medullary thickening of the ectoderm (_ec_) is marked, but shows
no sign of a medullary groove; in fact, the median line is the most
elevated region of the ectoderm. The notochord (_nt_) is larger in
cross-section than in the more anterior regions. It is still continuous
with the entoderm (_en_) and is fairly closely attached to, though
apparently not continuous with, the mesoderm (_mes_) on each side.

Figure 6_h_ passes through the blastopore (_blp_). The appearance of
the section is almost identical with that of Figure 2_f_, already
described.

Figure 6_i_ is five sections posterior to the preceding and has about
the same structure as the corresponding sections in the preceding two
stages, where this region of the embryo was injured, and hence not
drawn. Continuous with the posterior border of the blastopore (seen in
the preceding figure) is the deep furrow, the primitive groove (_pg_).
The ectoblast (_ec_) bordering this groove is much thickened and may be
called the primitive streak. The lower side of this primitive streak
is continuous with the mesoblast (_mes_), while the entoderm (_en_)
is here entirely distinct from the mesoderm. It is evident that the
mesoderm posterior to the blastopore is proliferated from the lower
side of the ectoblast and not from the upper side of the entoblast, as
is the case anterior to the blastopore. The primitive groove gradually
becomes more and more shallow, as it is followed toward the posterior,
until it is no longer discernible; back of this point the primitive
streak may be traced for a considerable distance, becoming thinner and
thinner until it too disappears, and there remains only the slightly
thickened ectoblast underlaid by the thin and irregular layers of
mesoblast and entoblast. The primitive streak may be traced for a
distance equal to about one third the distance between the head-fold
and the blastopore.

STAGE IV

FIGURES 7_a_-7_h_ (PLATES XI., XII.)

No surface view of this stage was seen by the writer, and hence is not
figured. The figures were drawn from one of the series of sections
obtained through the courtesy of Prof. S. F. Clarke. This series was
marked “3 Urwirbeln,” so that the embryo was apparently slightly
younger than the youngest stage obtained by myself and represented in
Figures 8 and 8_a_.

Figure 7_a_ represents a section that passed through the head-fold of
the amnion (_a_) just in front of the head-fold of the embryo; the
amniotic cavity here appears as a large empty space.

Figure 7_b_ is several sections posterior to the preceding; it passes
through the head-fold of the embryo, but is just back of the head-fold
of the amnion. The deep depression of the ectoderm (_ec_) and entoderm
(_en_) caused by the head-fold is plainly seen. In this depression lie
the ends of the medullary folds, distinct from each other both dorsally
and ventrally. Each medullary fold is made up of two parts--a medial,
more dense nervous layer (_nl_), and a distal, less dense epidermal
layer (_ep_). The section corresponding to this one will be more fully
described in connection with the following stage.

Figure 7_c_ is some distance posterior to the preceding, though
the exact distance could not be determined because of a break in
the series at this point. The section passes through the posterior
limit of the head-fold. The medullary groove (_mg_) is very deep and
comparatively wide; around its sides the germ layers are so closely
associated that they may scarcely be distinguished. Ventral to the
medullary groove the foregut (_fg_) is seen as a crescentic slit.

Figure 7_d_ is about a dozen sections posterior to the one just
described and is about one seventh the length of the embryo from the
anterior end. The embryo is much more compressed, in a dorso-ventral
direction, and the medullary groove (_mg_) is correspondingly more
shallow. Where the ectoderm (_ec_) curves over to form the medullary
folds it becomes much more compact and somewhat thicker. The notochord
(_nt_) is large and distinct, but is still fused with the entoderm
(_en_). The mesoderm (_mes_) forms a well-defined layer, entirely
distinct from both the notochord and the entoderm. From this region,
as we pass caudad, the size of the embryo in cross-section gradually
decreases and the medullary groove becomes more shallow.

Figure 7_e_ is about one third of the length of the embryo from the
posterior end, and is only a few sections from the caudal end of the
medullary groove. The ectoderm (_ec_) is much thinner than in the
preceding figure and the medullary groove (_mg_) is much more shallow.
The notochord (_nt_) is of about the same diameter as before, but
is here quite distinct from the entoderm (_en_) as well as from the
mesoderm (_mes_).

Figure 7_f_ is seven sections posterior to Figure 7_e_. The medullary
groove has disappeared and the medullary folds have flattened to form
what might be called a medullary plate (at the end of the reference
line _ec_), which continues to the anterior border of the blastopore.
The notochord (_nt_) is larger in cross-section than in the more
anterior regions; it is still distinct from the entoderm.

Figure 7_g_ passes through the blastopore and shows essentially the
same structure as was described in connection with the corresponding
section of Stage I (Fig. 2_f_).

Figure 7_h_ represents the region of the primitive groove (_pg_) and
primitive streak (_ps_). The section shows the typical structure for
this region--a thick mass of cells is proliferating from the ventral
side of the ectoderm (_ec_) and is spreading laterally to form a
distinct mesoderm (_mes_). The entoderm (_en_) is entirely distinct
from the other layers.

STAGE V

FIGURES 8-8_j_ (PLATES XII., XIII., XIV.)

On opening the egg this embryo (Figs. 8 and 8_a_) was found to lie
on the end of the yolk, near the center of the irregular, lighter
area which was mentioned in connection with the description of the
egg. The length of the embryo proper is 3 mm. This was the youngest
stage found in 1905, and approximates quite closely the condition of
the chick embryo after 24 hours’ incubation. The dorsal aspect of
this embryo, viewed by transmitted light, is shown in Figure 8. The
medullary folds (_mf_) have bent over until they are in contact, though
apparently not fused for a short distance near their anterior ends. As
will be described in connection with the sections of this stage, the
medullary folds are actually fused for a short distance at this time,
though in surface views they appear to be separated from each other.
In the Madagascar crocodile (78) the first point of fusion of the
medullary folds is in the middle region of the embryo, or perhaps even
nearer the posterior than the anterior end of the medullary groove.
Throughout the greater part of their length the medullary folds are
still widely separated; posteriorly they are merged with the sides of
the very distinct primitive streak (_ps_), which seems, owing to its
opacity, to extend as a sharp point toward the head. Extending for the
greater part of the length of the primitive streak is the primitive
groove (_pg_), which, when the embryo is viewed by transmitted light,
is a very striking feature at this stage of development and resembles,
in a marked way, the same structure in the embryo chick. Clarke (17)
figures the blastopore at this stage as a small opening in front of the
primitive streak, but does not mention any such condition as above
described at any stage of development. Five pairs of somites (_s_)
have been formed and may be seen, though but faintly outlined, in
both dorsal and ventral views of the embryo; they lie about half-way
between the extreme ends of the embryo. The head-fold (_h_, Fig. 8_a_)
shows plainly in a ventral view as a darker, more opaque anterior
region, extending for about one fourth the length of the embryo. The
still unfused region of the medullary folds may be seen also in the
ventral view at _mg_. The head-fold of the amnion (_a_) forms a very
thin, transparent hood over the extreme anterior end of the embryo.
The tail fold of the amnion has not made its appearance, and in fact
is not apparent at any stage in the development. This is true also of
the Madagascar crocodile. The notochord (_nt_) may be seen in a ventral
view as a faint, linear opacity extending along the middle line from
the head-fold to the primitive streak.

Two sagittal sections of this stage are shown in Figures 8_b_ and 8_c_.
The embryo from which the sections were made was apparently somewhat
crooked, so that it was not possible to get perfect longitudinal
sections. For example, in Figure 8_b_ the plane of the section is
almost exactly median in the extreme posterior and middle regions,
but is on one side of the middle line elsewhere. This explains the
enormous thickening of the ectoblast in the region of the head,
where the section passes through one of the medullary folds (_mf_) at
its thickest part; and also explains the fact that the ectoblast is
thinner in the middle region (_ec_), where the section passes through
the medullary groove, than it is farther toward the blastopore where
the section cuts the edge of the medullary folds. The outlines of the
middle and extreme posterior regions of the ectoblast are much more
irregular and ragged than is shown in the figure. The plane of the
section passes through the notochord (_nt_) in the posterior region,
but not in the anterior end of the embryo, where a layer of mesoblast
(_mes_) is seen. The great size of the blastopore (_blp_) is well
shown, as is the beginning of the foregut (_fg_). Comparison of this
figure with the more anterior transverse sections and with the dorsal
surface view of this stage will make the rather unusual conditions
comprehensible.

Figure 8_c_ is cut to one side of the median plane, distal to the
medullary folds. Being outside of the medullary folds, the ectoderm
(_ec_) is thinner and less dense than in Figure 8_b_; anteriorly it is
pushed down and back as the head-fold, and posteriorly it becomes thin
where it forms the dorsal boundary of the primitive streak (_ps_).

The foregut (_fg_), as would be expected, is not so deep as in the
median section (8_b_). The most striking feature of the section is the
presence of five mesoblastic somites (_s_). Each somite, especially
the second, third, and fourth, is made up of a mass of mesoblast whose
cells are compactly arranged peripherally, but are scattered in the
center, where a small myocœl may be seen.

A series of transverse sections of the embryo shown in Figures 8 and
8_a_ is represented in Figures 8_d_-_j_.

Figure 8_d_ is through the anterior end of the embryo; the posterior
edge of the amnion is cut only on one side (_a_). The medullary folds
(_mf_) are shown as two distinct masses of tissue, separated by a
considerable space from each other, both dorsally and ventrally;
they are underlaid by the ectoderm of the head-fold, through which
the section passes. A mass of yolk (_y_) is shown at one side of the
section.

Figure 8_e_ represents a section a short distance posterior to the one
just described, and passes through the short region where the dorsal
edges of the medullary folds have fused with each other. The ventral
side of the medullary groove (_mg_) is, as in the preceding section,
still unclosed. An epidermal layer of ectoblast (_ep_) is now distinct
from the nervous layer (_nl_).

Figure 8_f_ is through a region still farther toward the posterior end.
Here the medullary groove is _again_ open above, and is still open
below. A well-marked space is seen between the epidermal (_ep_) and
nervous (_nl_) layers of the ectoderm, but no mesoblast is yet to be
seen.

Figure 8_g_ passes through the middle part of the head-fold, and shows
that the medullary folds in this region are fused below, but are widely
separated above, where their margins are markedly bent away from the
mid-line. Between the epidermal and nervous layers of the ectoderm
a considerable mass of mesoderm cells (_mes_) is seen. The curious
appearance of the preceding four figures, as well as the first three
figures of the next stage, was at first quite puzzling, until a model
of the embryo was made from a series of sections. It was then plain,
as would have been the case before, except for the unusual depth
dorso-ventrally of the head of the embryo, why the medullary canal
should at the extreme anterior end be open both dorsally and ventrally,
while a few sections caudad it is open only ventrally, and still
farther toward the tail it is again open both above and below. These
conditions are produced by the bending under of the anterior region of
the medullary folds, probably by the formation of the head-fold. It is
apparently a process distinct from the ordinary cranial flexure, which
occurs later. The fusion of the medullary folds to form a canal begins,
as has been already mentioned, near the anterior end, whence it extends
both forward and backward. Hence, if the anterior ends of the medullary
folds be bent downward and backward, their unfused dorsal edges will
come to face ventrally instead of dorsally and sections through the
anterior part of this bent-under region will show the medullary canal
open both above and below, as in Figure 8_d_, while sections farther
caudad pass through the short region where the folds are fused, and we
have the appearance represented in Figure 8_e_. In Figure 8_f_ is shown
a section passing posterior to the short, fused region of the folds,
and we again have the medullary canal open both above and below. Figure
8_g_ represents a section through the tip of the bent-under region of
the medullary folds, which are here fused below and open above.

Figure 8_h_ passes through the posterior part of the head-fold, between
the limits of the fold of the ectoderm and that of the entoderm. The
medullary groove (_mg_) is here very wide and comparatively shallow;
its walls are continued laterally as the gradually thinning ectoderm
(_ec_). The enteron (_ent_) is completely enclosed, and forms a large,
somewhat compressed, thick-walled cavity. Between the dorsal wall
of the enteron and the lower side of the medullary canal lies the
notochord (_nt_), a small, cylindrical rod of closely packed cells
derived, in this region at least, from the entoderm. In the posterior
region of the embryo it is not possible to determine with certainty
the origin of the notochord, owing to the close fusion of all three
germ layers. Between the wall of the enteron and the lower side of
the ectoderm is a considerable mass of mesoderm (_mes_), which here
consists of more scattered and angular cells than in the preceding
section.

Figure 8_i_ shows the appearance of a section through the mesoblastic
somites, in one of which a small myocœl (_myc_) is seen. As is seen by
the size of the figure, which is drawn under the same magnification as
were all the sections of the series, the embryo in this region is much
smaller in section than it is toward either end, especially toward the
anterior end. The medullary groove (_mg_) is still more shallow than
in the more anterior sections, and the ectoderm (_ec_), with which
its folds are continuous laterally, is here nearly horizontal. The
mesoblast (_mes_) is of a more compact nature than in the preceding
section and shows little or no sign of cleavage, although a distinct
myocœl may be seen and cleavage is well marked in sections between this
one and the preceding.

The notochord (_nt_) has about the same appearance as in the preceding
section, but is more distinctly separated from the surrounding cells.

Figure 8_j_ is through the posterior end of the embryo; it shows the
relation of parts in the region of the primitive streak. Although
not visible in surface views, and hence not represented in Figure 8,
the medullary groove is continued without any line of demarcation
into the primitive groove, and the medullary folds into the edges of
the primitive streak, so that it is impossible to set any definite
boundaries between these structures unless the dorsal opening of the
blastopore be taken as the point of division. The medullary groove
(_mg_), if it be here so called, is proportionately more shallow than
in the preceding figure and is actually much wider. The section passes
behind the posterior end of the notochord, so that structure is not
seen. Though not so well indicated as might be desired in the figure,
the three germ layers are here indistinguishable in the middle line,
and in the center of this mass of cells the blastopore (_blp_) or
neurenteric canal may be seen as a small vertical slit. As will be more
fully described in the following stage, this canal opens dorsally a
few sections posterior to the one under discussion and ventrally a few
sections farther toward the head.

In all the sections of this stage the ectoderm and entoderm are fairly
thick in the region of the embryo proper, but become thinner until
reduced to a mere membrane as we pass to more distal regions. Both
layers are composed of loosely arranged cells, with scattered nuclei.
Where the ectoderm becomes thickened to form the medullary folds, the
cells are much more compactly arranged; hence this region stands out in
strong contrast to the rest of the ectoderm.

STAGE VI

FIGURES 9_a_-9_m_ (PLATES XIV., XV.)

The embryo represented by this series of transverse sections is
intermediate in development between those represented in surface views
by Figures 8 and 10. The amnion and head-fold are nearly the same as
in Figure 8; the medullary folds are intermediate in development, the
anterior end not showing so marked an enlargement as shown in Figure
10, _v′_. There are six or seven faintly distinguishable somites.

Figure 9_a_ represents a section through the anterior part of the
head-fold; it shows one unusual condition: the head lies entirely
beneath the surface of the yolk. This condition is quite confusing when
the section is studied for the first time. The pushing of the head
under the yolk is shown at its commencement in Figure 11. The process
continues until nearly the entire anterior half of the embryo is
covered; but when the embryo attains a considerable size it is seen to
lie entirely above the yolk, as in the chick. According to Voeltzkow’s
figures (78) this same condition is found in the crocodile, and Balfour
also mentions it in connection with the lizard. The fusion of the
medullary folds has made considerable progress, so that the entire
anterior end of the canal is enclosed, except in the region where the
folds are bent down and back, as in the preceding stage; here the folds
are still distinct from each other, leaving the medullary canal open
on the ventral side, as shown in Figures 9_a_ and 9_b_. In the section
under discussion the ectoderm (_ec_) is a very thin membrane on top of
a considerable mass of yolk, while no entoderm can be distinguished.
The amnion (_a_) completely surrounds the embryo as an irregular
membrane of some thickness in which no arrangement into layers can
be seen. The epidermal ectoderm (_ep_) is composed of the usual
loosely arranged cells, so that it is clearly distinguishable from the
compactly arranged cells of the nervous layer (_nl_), from which it is
separated by only a line.

In Figure 9_b_, which shows a section a short distance posterior to the
preceding, the medullary canal (_mc_) is somewhat deeper and is still
open ventrally. There is a distinct space between the nervous (_nl_)
and epidermal (_ep_) layers of the ectoderm, in which space a few
mesoblast cells (_mes_) may be seen. The section is cut just posterior
to the edge of the amnion, so that there is now neither amnion nor yolk
above the embryo.

Figure 9_c_ is about ten sections posterior to Figure 9_b_. The
section passes through the anterior wall of the bent-under part of the
medullary canal (_mc′_), so that the actual canal is shown only on the
dorsal side (_mc_), where it is completely closed and begins to assume
the shape of the typical embryonic spinal cord. The space between the
superficial (_ep_) and nervous (_nl_) layers of the ectoderm is quite
extensive and is largely filled by a fairly compact mass of mesoderm
(_mes_).

Figure 9_d_, although only five sections posterior to the preceding,
shows a marked change in structure. The medullary canal (_mc_) is here
of the typical outline for embryos of this age. A large, compact mass
of cells (_ent_) appears at first glance to be the same that was noted
in the preceding stage at the tip end of the turned-under medullary
canal; it is, however, the extreme anterior wall of the enteron, which
is in close contact with the above-mentioned tip of the medullary
canal. Between this anterior wall of the enteron, of which wall it is
really a part, and the medullary canal is the notochord (_nt_). The
space surrounding the notochord and enteron is filled with a fairly
compact mass of typical, stellate mesoblast cells. The depression of
the ectoderm (_ec_) and entoderm (_en_) of the blastoderm caused by the
formation of the head-fold is here less marked, and the dorsal side of
the embryo in this region is slightly elevated above the level of the
blastoderm.

Figure 9_e_ represents a section passing through the posterior edge
of the head-fold. The epidermal ectoderm is here continuous with the
thin layer of superficial ectoderm (_ec_) of the blastoderm, while
the entoderm (_en_) of the blastoderm is still continuous beneath the
embryo. The thick ectoderm of the embryo is sharply differentiated from
the thin layer of ectoderm that extends laterally over the yolk. The
pharynx (_ent_) is a large cavity whose wall is thick except at the
dorsal side, where it is thin and somewhat depressed, apparently to
make room between it and the medullary canal for the notochord (_nt_).

Figure 9_f_ is about twenty sections posterior to the preceding
section, and passes through the point of separation of the folds of
the entoderm (_en_). From this point the entoderm gradually flattens
out, leaving the enteron unenclosed. The medullary canal (_mc_) and
notochord (_nt_) are about as in the preceding section, but the
ectoderm (_ep_) is somewhat thinner and more flattened. The mesoderm
(_mes_) on the right side exhibits a distinct cleavage, the resulting
body cavity (_bc_) being a large, triangular space.

Figure 9_g_, the twenty-fifth section posterior to that represented in
Figure 9_f_, shows a marked change in the form of the embryo. While
of about the same lateral dimensions, the dorso-ventral diameter of
the embryo in this region is less than one half what it was in the
head region. The epidermal ectoderm (_ep_) is now nearly horizontal
in position and is not so abruptly separated laterally from the thin
lateral sheets of ectoblast. The medullary groove (_mg_) is here a
very narrow vertical slit. At this stage the fusion of the medullary
folds has taken place over the anterior third of the embryo. For a
short distance, represented in about thirty-five sections, the canal
is open as in the figure under discussion; for the next one hundred
sections (about one third the length of the embryo) in the region of
the mesoblastic somites the canal is again closed, while throughout the
last one third of its length the canal is widely open dorsally. The
enteron is here entirely open ventrally, the entoderm being almost
flat and horizontal. The notochord (_nt_) is distinctly outlined and is
somewhat flattened in a dorso-ventral direction. The body cavity (_bc_)
is well marked, but is separated by a considerable mass of uncleft
mesoblast from the notochord and the walls of the medullary groove.

A space of about one hundred sections, or one third the length of the
embryo, intervenes between Figures 9_g_ and 9_i_. This is the region of
the mesoblastic somites, and in this region, as has been above stated,
the medullary canal is completely enclosed. It is evident then that the
entire anterior two thirds of the medullary canal is enclosed except
for the short region represented in Figure 8_g_. Whether or not this
short open region between the two longer enclosed regions is a normal
condition the material at hand does not show.

Figure 9_h_ represents a typical section in the region of the
mesoblastic somites just described. It shows the enclosed medullary
canal (_mc_), the body cavity (_bc_) on the right, and a mesoblastic
somite with its small cavity (_myc_) on the left. The entire section
is smaller than the sections anterior or posterior to this region, and
seems to be compressed in a dorso-ventral direction, this compression
being especially marked in the case of the notochord.

Figure 9_i_ is through a region nearly one hundred sections posterior
to the preceding, and cuts the embryo, therefore, through the
posterior one fourth of its length. The chief difference between this
and the preceding section is in the medullary canal, which is here open
and is in the form of a wide groove with an irregular, rounded bottom
and vertical sides. The size of the section is considerably greater
than in the preceding, the increase being especially noticeable in the
notochord (_nt_), which is cut near its posterior end. There is little
or no sign of mesoblastic cleavage.

Figure 9_j_ is about twenty sections posterior to Figure 9_i_. The
medullary groove (_mg_) is considerably larger than in the more
anterior regions, and its folds are somewhat inclined toward each
other, though still wide apart. The notochord and entoderm are fused
to form a large, compact mass of tissue close under the ventral wall
of the medullary groove. On the ventral side of this mass of cells a
groove (_blp_) marks the anterior and ventral opening of the blastopore
shown in the next figure. The mesoblast shows no sign of cleavage.

Figure 9_k_ shows the medullary groove (_mg_) in about the same
position as in the preceding section. The blastopore (_blp_) is here
seen as a small cavity in the center of the large mass of cells that
was noted in the last figure. The entoderm (_en_) is continuous from
side to side, but is not so sharply differentiated from the other germ
layers as is represented in the figure.

Figure 9_l_ is four sections back of the preceding; the wide, dorsal
opening (_blp_) of the blastopore or neurenteric canal into the
medullary groove (_mg_) is shown. The blastopore or neurenteric canal,
then, is still at this stage a passage that leads entirely through the
embryo, the medullary canal being in this region unenclosed above.
Ventrally it is seen as a narrow opening through the entoderm; it then
passes upward and backward, behind the end of the notochord, as a
small but very distinct canal, which may be traced through about ten
sections. The enclosed portion of the canal lies, as has been stated
(Figure 9_k_, _blp_), in the center of the mass of cells that is fused
with or is a part of the floor of the medullary groove.

The above-described neurenteric canal is essentially like that
described by Balfour in the _Lacertilia_. He does not say, however,
and it is not possible to tell from his figures, whether there is a
long, gradually diminishing groove posterior to the dorsal opening of
the canal, as in the alligator. He says that the medullary folds fuse
posteriorly until the medullary canal is enclosed over the opening of
the neurenteric canal; also that “the neurenteric canal persists but a
very short time after the complete closure of the medullary canal.”

In Figure 9_m_, for about thirty sections (one tenth the entire length
of the embryo), behind the section represented in the last figure,
there is a very gradual change in the embryo, converting the deep
groove, _mg_ in Figure 9_l_, into the shallow slit, _pg_, Figure 9_m_.

There is no line of demarcation between the typical medullary groove
region of Figure 9_l_ and the equally typical primitive groove region
represented in Figure 9_m_. As was noted in the preceding stage, the
medullary folds are quite continuous with the folds of the primitive
streak, and the medullary groove with the primitive groove; so that,
unless we take the dorsal opening of the neurenteric canal as the point
of separation, there is no line of division between these structures.
The entoderm (_en_) and the lateral regions of the ectoderm (_ec_) and
mesoderm (_mes_) in Figure 9_m_ are about as they were in Figure 9_l_,
but in the middle line is seen a compact mass of cells forming the
primitive streak (_ps_), with the shallow primitive groove (_pg_) on
the dorsal side. The cells on each side of the primitive groove and for
a short distance below it are compact, as is shown in the figure, but
as we pass ventrally and laterally they become looser and more angular
to form the lateral sheets of mesoblast (_mes_), very much as is the
case in the chick and other forms. For a few sections posterior to the
one shown in Figure 9_m_ the primitive streak may be seen, then it
disappears, and only the ectoderm and entoderm remain as thin sheets of
tissue above the yolk.

STAGE VII

FIGURES 10 AND 10_a_ (PLATES XV., XVI.)

Although of practically the same size as the preceding, this stage has
advanced sufficiently in development to warrant a description.

The medullary folds are apparently still slightly open for the greater
part of their length, though they are evidently fused together in the
head region, except at the extreme end. Transverse sections, however,
of Figure 12, in which the medullary folds, from the dorsal aspect,
seemed open (_mg_) as in Figure 10, have shown that these folds are
fused throughout their length.

The first cerebral vesicle (_v′_) is clearly indicated as an
enlargement of the anterior end of the nervous system, and a slight
enlargement (_v″_) posterior to the first probably represents the
second cerebral vesicle.

There are now eight pairs of somites (_s_).

The head-fold (_h_) now shows in both dorsal and ventral views,
appearing in the former, when viewed by transmitted light, as a
lighter, circular area on either side of the body, just posterior to
the hinder edge of the amnion.

The head-fold of the amnion (_a_) has extended about twice as far
backward as it did in the preceding stage.

Owing to the opacity caused by the medullary folds being in contact
along the middle line, the notochord is no longer visible in surface
views.

The head at this stage begins to push down into the yolk in a strange
way that will be described later.

STAGE VIII

FIGURES 11-11_k_ (PLATES XVI., XVII., XVIII.)

This stage is about one fourth longer than the preceding. The medullary
canal is enclosed throughout its entire length, though it appears in
surface view (Fig. 11) to be open in the posterior half (_mc_) of the
embryo. An enlargement of this apparently open region at the extreme
posterior end (_pg_) is probably caused by the remains of the primitive
groove or the neurenteric canal, and a slight opacity at the same point
may be caused by the primitive streak. The anterior end of the neural
tube is bent in a ventral direction (_v_), as in the preceding stage.
The somites (_s_) now number fifteen pairs; they are somewhat irregular
in size and shape.

The head-fold is not so striking a feature as in the preceding stage.
The head-fold of the amnion (_a_) now covers nearly two thirds of the
embryo. The heart (_ht_) is seen as a dark, rounded object projecting
to the right side of the neural canal, just anterior to the first
somite. The vitelline blood-vessels are just beginning to form, but are
not shown in the figure.

The depression of the anterior region that was noted in the preceding
stage has advanced so far that a considerable part of the embryo now
projects forward under the blastoderm. In some cases it is almost
concealed in a dorsal view; in other cases it may easily be seen
through the transparent membranes, especially after clearing.

In opening eggs of this stage one is at first apt to underestimate the
size of the embryos, since the anterior part of the embryos cannot be
seen until after they are removed from the yolk and are viewed from the
ventral side.

The embryo from which the series of transverse sections of this stage
was made, while of the same state of development as that shown in
Figure 11, was more fully covered by the blastoderm than is shown in
the surface view in question.

Figure 11_a_ passes through the tip of the head. Dorsal to the embryo
is the ectoderm and a thick mass of yolk (_y_). The amnion (_a_) is
seen as an irregular membrane which entirely surrounds the head.
The medullary canal (_mc_) is entirely closed except at the extreme
anterior end, which is bent downward so that the opening is on the
ventral side. The nervous (_nl_) and epidermal (_ep_) layers of the
ectoderm are in contact throughout, but are clearly distinguishable
because of the difference in the compactness of their cells.

In Figure 11_b_ is represented a section, behind the preceding, which
passes through the posterior tip of the turned-under anterior end
(_mc′_). Here the medullary canal is closed both above (_mc_) and
below (_mc′_). The amnion (_a_) has about the same appearance as in the
more anterior section, but there is here a considerable space, filled
with mesoblast (_mes_), between the nervous (_nl_) and epidermal (_ep_)
layers of ectoderm.

Figure 11_c_ is twenty sections, about one tenth the length of the
embryo, posterior to the one last described. The large mass of
overhanging yolk (_y_) is still present, as is also the amnion (_a_),
though the latter no longer passes entirely around the embryo; only the
true amnion could be made out. The thickened walls of the medullary
canal have reduced that cavity to a narrow, Y-shaped slit (_mc_).
The notochord (_nt_) is very slender in this region, compared to its
diameter farther toward the posterior end. The enteron (_ent_) is a
large cavity, whose wall is made up of loosely arranged cells except
around a median, ventral depression where the cells are more compact.
This depression may be traced through ten or fifteen sections and may
represent the beginning of the thyroid gland, though this point was
not worked out with certainty. Surrounding the notochord and enteron
is a loose mass of typical, stellate mesoblast cells (_mes_), which
are cleft on either side to form the anterior limit of the body cavity
(_bc_). Between the body cavity below and the enteron above, on each
side, is a small blood-vessel (_bv_) which when followed caudad is
found to open ventrally and medially into the anterior end of the heart.

Figure 11_d_ is about a dozen sections posterior to the preceding. The
appearance of the overhanging yolk (_y_), of the amnion (_a_), and of
the notochord (_nt_) is about as in the more anterior section. The
medullary canal (_mc_) is a straight, vertical slit, and the depression
in the floor of the pharynx (_ent_) is much more shallow. The body
cavity (_bc_) is much larger and extends across the mid-ventral line
beneath the heart (_ht_), which is cut through its middle region. The
heart may be traced through about twenty sections (one tenth the length
of the embryo); its mesoblastic wall (_mes′_) is thin and irregular,
and is lined by a distinct endothelium (_en′_) whose exact origin has
not yet been worked out.

Figure 11_e_ is just back of the heart, and shows in its place the
two vitelline veins (_vv_). The depression in the floor of the
enteron (_ent_) is entirely distinct from the one that has been
mentioned above, and is simply the posterior limit of the head-fold
of the entoderm; the fifth section posterior to this shows where this
depression opens ventrally to the yolk sac. The other structures shown
in the figure are not markedly different from what was seen in Figure
11_d_.

Figure 11_f_ is about one tenth the length of the embryo posterior to
Figure 11_e_. The chief differences here noticed are in the enteric and
cœlomic cavities. The former is no longer enclosed, a dorsal fold in
the entoderm being all that remains of the cavity that was seen in the
more anterior figures, while the latter is here reduced to a narrow
cleft between the somatic and splanchnic mesoblast. A thickening of
the mesoblast on either side of the notochord, especially on the left,
represents a mesoblastic somite. The medullary canal (_mc_) is more
open than in the more anterior sections.

For about one third of the length of the embryo posterior to Figure
11_f_ there is a gradual flattening, in a dorso-ventral direction,
with loss of the amnion, until the condition represented in Figure
11_g_ is reached. The most striking feature of this region is the great
thickness of the ectoderm (_ec_), which is still made up of scattered,
irregular cells. In the middle line, directly over the medullary canal
(here a nearly cylindrical tube), is a sort of break in the ectoderm,
as though there had not been a complete fusion of the epidermal layer
when the nervous layer came together on the closure of the medullary
groove. This break in the ectoderm may be followed back to the region
of the primitive streak, and will be mentioned again. As has been
noted, the medullary canal (_mc_) is nearly circular in cross-section,
and is closely underlaid by the notochord (_nt_), which is several
times the diameter that it was in more anterior sections. The mesoblast
(_mes_) is a comparatively thin layer, intermediate in thickness
between the ectoderm and entoderm. It shows laterally a slight
separation to form the body cavity.

Figure 11_h_ is about ten sections posterior to Figure 11_g_, and
differs from it chiefly in that the notochord (_nt_) is continuous with
the lower side of the medullary canal (_mc_), though still distinct
from the underlying entoderm (_en_).

Figure 11_i_, four sections farther from the head, shows the same
greatly thickened ectoderm (_ec_) with the same break (_ec′_) in the
middle line. The section is posterior to the notochord and passes
through the anterior edge of the blastopore or, as it may now perhaps
better be called, the neurenteric canal. The cells of the medullary
wall are continuous with those of the entoderm. The mesoderm (_mes_) is
still distinct from the other germ layers.

Figure 11_j_ is the next section posterior to the one just described
and differs from it only in showing the actual opening of the
neurenteric canal (_nc_) into the medullary canal (_mc_). The medullary
canal extends, with gradually diminishing caliber, for about fifteen
sections posterior to the point at which the neurenteric canal empties
into it. The mesoblast (_mes_) is so closely attached to the lower wall
of the neurenteric canal that it seems to be actually continuous with
it.

For a considerable distance posterior to the end of the medullary
canal we find the structure similar to that shown in Figure 11_k_,
which is about the twentieth section posterior to Figure 11_j_. The
break (_ec′_) in the ectoderm is here seen as a compact group of cells
which at first glance seem to be continuous with a rounded mass of
cells below (_ps_). Examination under greater magnification, however,
shows that the two groups of cells are distinct. As the sections are
followed back of this region, the upper mass of cells (_ec′_) gradually
disappears, and after its disappearance the lower mass (_ps_), which is
already continuous with the mesoderm (_mes_) on either side, becomes
continuous with the under side of the ectoderm. The mass of cells
(_ps_) is apparently the primitive streak, though it is distinct from
the ectoderm for a considerable distance posterior to the neurenteric
canal. Just what may be the meaning of the thickened ridge of ectoderm
(_ec_) it is difficult to determine.

STAGE IX

FIGURES 12-12_g_ (PLATES XVIII., XIX.)

The entire length of the embryo proper is 6.5 mm. from the extreme
posterior end to the region of the midbrain (_v²_), which now, on
account of the cranial flexure, forms the most anterior part of the
body. Besides being slightly longer than the preceding stage, the
embryo has increased in thickness, especially in the anterior region,
where the enlargement of the cerebral cavity is considerable.

Body torsion has begun (Fig. 12), so that the anterior third of the
embryo now lies on its right side, while the rest of the body is still
dorsal side up. The direction of body torsion does not seem to be as
definite as it is in the chick, some alligator embryos turning to the
right side, others to the left. Clarke has illustrated this fact in his
alligator figures. He says (17) that embryos lie “more frequently on
the left, but often on the right side.”

The head is distinctly retort-shaped, and at the side of the forebrain
(_v′_) a small crescentic thickening is the optic vesicle (_e_). The
auditory vesicle, though of considerable size, does not show in this
surface view. The head-fold (_h_) extends for about one third the
length of the entire embryo, though its exact limit is difficult to
determine in surface view. There is no sign of a tail-fold.

About seventeen pairs of somites are present.

The amnion extends over the anterior two thirds of the embryo.

The above-mentioned increase in the diameter of this embryo over that
of the preceding is evident when the first two transverse sections of
this stage are compared with the corresponding sections of the earlier
stage; in the middle and posterior regions there is not very much
difference in size.

Figure 12_a_ passes through the region of the forebrain. This end
of the embryo lies on its side, as was noted above and as may be
recognized from the relative positions of the head and the overlying
yolk (_y_). The great size of this and the following figure is due
partly to the increase in size mentioned above and partly to the
fact that the sections pass through the region of cranial flexure.
The present figure (12_a_) represents the brain cavity as large and
dumbbell-shaped, with comparatively thick walls of compactly arranged
cells. The ventral end of this cavity (_fb_) is cut anterior to the
region of the optic vesicles, while the dorsal end (_mb_) may perhaps
be called the midbrain. In the sections that follow this one the two
cavities are distinct from each other. The medullary canal, as was
stated above, is now completely enclosed, except for the ventral
opening of the neurenteric canal, to be presently noticed. Surrounding
the brain is a considerable mass of mesoblast (_mes_). It is composed
of the typical stellate cells. The ectoderm (_ec_) is made up of
the same irregularly and loosely arranged cells that have been seen
in earlier stages; it is of unequal thickness in different regions,
the thicker parts being at the sides. The amnion (_a_) has the usual
appearance, and in this region of course completely surrounds the
embryo.

Figure 12_b_ is ten sections posterior to the section just
described. The width of the embryo is greater in this region, but
the dorso-ventral diameter is about the same as in the more anterior
section.

The overlying yolk and blastoderm are not shown in any figure of
the series except the first. In this figure the forebrain (_fb_) and
midbrain (_mb_) are widely separated instead of being connected, as
in the preceding figure, where the section passed through the actual
bend of the cranial flexure. The anterior and ventral part of the
cranial cavity, the forebrain (_fb_), is nearly circular in outline.
It exhibits on one side a well-marked optic vesicle (_ov_), which
is sufficiently advanced in development to show a rudimentary optic
stalk. The outer wall of the optic vesicle is in close contact with
the superficial ectoderm, which shows as yet no sign of the formation
of a lens vesicle. The plane of the section being probably not quite
at right angles to the long axis of the embryo, the optic vesicle of
one side only was cut. The wall of this part of the forebrain is of
about the same thickness and appearance as in the preceding stage.
The other cerebral cavity (_mb_) of this section is probably the
hinder part of the midbrain, though it may be the anterior part of the
hindbrain; there is no sharp line of demarcation between these regions
of the brain. This cavity (_mb_) is much smaller in section than the
forebrain; its walls are of about the same thickness.

Ventral to the midbrain is the anterior end of the notochord (_nt_),
surrounded by the mesoblast. At various places throughout the mesoblast
irregular open spaces may be seen; these are blood-vessels. The
ectoderm (_ec_) and amnion (_a_) have about the same appearance as in
the preceding figure, though the former seems somewhat thinner.

Figure 12_c_ is just back of the bent-under forebrain represented
in the preceding figure and in front of the main body of the heart.
The plane of the section not being at right angles to the long axis
of the body (as was mentioned above), the figure is not bilaterally
symmetrical. The neural canal, since the section passes through the
auditory vesicles, may here be called the hindbrain (_hb_). It has an
almond-shaped cavity, surrounded by a wall of medium thickness. In
close contact with the wall of the hindbrain, on each side, is the
inner side of the auditory vesicle (_o_), which is seen as a deep,
wide-mouthed pit in the superficial ectoderm. On the right side of
the section the auditory pit is cut through its middle region; it is
simply a thickened and condensed area of the ectoderm which has been
invaginated in the usual way. Directly beneath the hindbrain is the
notochord (_nt_), on each side of which, in the mesoblast, is the
dorsal aorta (_ao_), or rather the continuation of the aorta into
the head. Beneath these structures and extending from one side of
the section to the other is the pharynx (_ph_); its lining wall is
fused on each side with the ectoderm, but there is no actual opening
to the exterior. These points of contact (_g_) between entoderm and
ectoderm are of course the gill clefts; they are not yet visible from
the outside. The roof of the pharynx is flat and comparatively thin,
while the floor is thickened and depressed to form a deep, wide pit,
traceable through six or eight sections. This pit may be the thyroid
gland already noticed in the preceding stage. Below the main cavity
of the pharynx and close to each side of the thyroid rudiment just
mentioned is a large blood-vessel (_tr_). These two vessels when traced
posteriorly are found to be continuous with the anterior end of the
heart, and hence may be called the truncus. They were noticed in Figure
11_c_, _bv_. The ectoderm surrounding the lower side of the embryo was
so thin and indistinct that it could scarcely be distinguished from
the mesoderm of that region. The amnion (_a_) is still a continuous
envelope entirely surrounding the embryo.

Figure 12_d_, about twenty sections posterior to Figure 12_c_, is
in the posterior heart region. The spinal cord (_sc_), as might be
expected, is smaller than in the more anterior region, but is otherwise
not markedly different from what was there seen. The notochord (_nt_)
also has the same appearance as before. The enteron (_ent_) shows
of course in this region no gill clefts; it is a small, irregular
cavity with thicker walls than in the figure just described. The
ventro-lateral depression is entirely distinct from the depression
that was called the thyroid rudiment in the preceding figure. Dorsal
to the enteron are the two dorsal aortæ (_ao_), now smaller and more
ventral to the notochord than in the preceding figure. Ventral to the
enteron is the large heart (_ht_), projecting below the body cavity,
which is no longer enclosed. The mesodermic wall (_mes′_) of the heart
is still comparatively thin and is separated by a considerable space
from the membranous endocardium (_en′_). The extent and shape of the
heart are shown in the surface view of this stage. On the right side
of the section the body cavity extends to a point nearly opposite the
middle of the spinal cord, considerably dorsal to the notochord, while
on the left side the dorsal limit of the body cavity is scarcely level
with the lower side of the notochord. Between the dorsal end of the
body cavity and the side of the spinal cord, on the left, is a dense
mass of mesoblast (_s_), one of the mesoblastic somites. A few sections
either anterior or posterior to the one under discussion will show the
condition of the two sides reversed--that is, the body cavity will
extend to the greater distance on the left and will be interrupted by
a mesoblastic somite on the right. It is evident, then, that the upper
angle of the body cavity is extended dorsally as a series of narrow
pouches between the somites. The mesoblast that lines the body cavity,
the splanchnopleure (_sm_) and somatopleure (_so_), is somewhat denser
than the general mass of mesoblast, so that these layers are quite
distinct, the former (_sm_) extending around the enteron (_ent_) and
heart (_ht_), and the latter (_so_) being carried dorsalward as the
mesoblastic part of the amnion (_a_). The amnion may be traced through
about 130 of the 200 sections into which this embryo was cut.

Figure 12_e_ is nearly one fourth the length of the embryo posterior to
Figure 12_d_; it is approximately in the middle region. The diameter
of the embryo has been gradually decreasing until now it is very much
less than in the head region. The section being behind the head-fold
the entoderm (_en_) is nearly flat and the enteron is quite unenclosed.
The canal of the spinal cord (_sc_) is smaller in proportion to the
thickness of its walls, and the notochord (_nt_) is somewhat larger
than in the preceding sections. In proportion to its extent, the
ectoderm is very thick. Under the notochord the dorsal aortæ (_ao_) are
seen as two large, round openings in the mesoblast. On the left side
the section passes through the center of a somite and shows a small,
round myocœl (_myc_). The mesoblastic layer of the amnion (_so_) is
distinct throughout from the ectoblastic layer (_a_).

The most important structures to be here noted are the first rudiments
of the Wolffian ducts (_wd_). They are seen in the present section
as lateral ridges of mesoblast projecting outward and upward toward
the ectoblast, which suddenly becomes thin as it passes over them.
These ridges or cords of mesoblast are as yet quite solid. They arise
suddenly at about the eightieth section of the series of two hundred
and may be traced through about forty sections, or one fifth of the
length of the embryo. Their exact length is difficult to determine
because, while their anterior ends are blunt and sharply defined, they
taper so gradually posteriorly that it is hard to tell just where they
end. They apparently originate anteriorly and gradually extend toward
the tail. In a slightly younger embryo the rudimentary Wolffian duct
could be seen as a still smaller rod of cells extending posteriorly for
a few sections, from the seventy-fifth section of a series of about two
hundred. In the particular series under discussion the left rudimentary
Wolffian duct was about one fifth longer than the right one.

Figure 12_f_ is just posterior to the head-fold of the amnion, passing,
in fact, on the left side through the extreme edge of its lateral fold,
which is shown as a upward bend in the ectoblast and somatopleure.

The ectoblast (_ec_) shows the same remarkable thickening that was
noted in the corresponding region of the preceding stage. The spinal
cord (_sc_), notochord (_nt_), aortæ (_ao_), and entoderm (_en_) need
no special mention. The mesoderm seems to be separated by unusually
wide spaces from both ectoderm and entoderm, and is made up of rather
closely packed cells except around the aortæ, where there seems
scarcely enough tissue to hold these vessels in place. The body cavity
(_bc_) is large, and a small myocœl (_myc_) is seen on the left.

Figure 12_g_ is through the neurenteric canal (_nc_), a distinct
opening through the floor of the spinal canal. The section is of course
just back of the posterior end of the notochord. The entoderm (_en_)
along the margin of the neurenteric canal is naturally continuous with
the wall of the spinal cord (_sc_). The ectoderm (_ec_) is thicker than
ever, except in the median plane, where it passes over the spinal cord.
The mesoblast is more abundant than in the preceding figure, and shows
on the left what appears to be a distinct myocœl (_myc_), though in
surface view the mesoblastic somites do not extend this far toward the
tail.

STAGE X

FIGURES 13-13_g_ (PLATES XIX., XX., XXI.)

This embryo (Fig. 13) is about 5 mm. in length, and hence is slightly
smaller than the preceding stage, though somewhat more advanced in
development. The medullary canal is still _apparently_ unclosed for
a short distance at the extreme posterior end; this appearance is
probably due to the neurenteric canal (_nc_) and to the thinness of
the roof of the medullary canal rather than to any lack of fusion of
the medullary folds. The optic vesicle is more distinct than in the
preceding stage; a somewhat similar, though smaller, opacity (_o_)
marks the position of the ear. There are now about twenty pairs of
somites, though it is difficult to determine their exact number on
account of the torsion of the body. The amnion is at about the same
stage of development as in Stage IX. The heart (_ht_) is a large double
mass, whose outlines may be dimly seen when the embryo is viewed by
transmitted light. The vitelline vessels (_vv_) are still but faintly
outlined in the vascular area; the veins and arteries cannot yet be
distinguished from each other. The gill clefts, though not visible
externally in the embryo drawn, may be seen in sections of this stage
as evaginations of the wall of the pharynx.

The transverse sections of this stage are slightly more advanced in
development than was the embryo that has just been described in surface
view. Only those sections have been figured which show a decided
advance in the development of some special structures over their
condition in the preceding stage. The sections of the preceding stages
were drawn under a magnification of eighty-seven diameters; those of
this and the following stage were drawn under a magnification of only
forty-one diameters. All of the figures have been reduced one half in
reproduction.

Figure 13_a_ is the most anterior section of this series to be
described. On account of the cranial flexure, which causes the long
axis of the forebrain to lie at right angles to that of the spinal
cord, this section cuts the head region longitudinally. The ectoderm
(_ec_) is of varying thickness, the thickest areas being on each
side of the forebrain; it is more compact than in the earlier stages,
and, owing to the low magnification under which it is drawn, it is
represented here by a single heavy line. Under this magnification only
the nuclei of the mesoderm cells (_mes_) can be seen, so that this
tissue is best represented by dots, more closely set in some places
than in others. The forebrain is an elongated cavity (_fb_) with thick,
dense walls. Attached to each side of the forebrain is an optic vesicle
(_ov_), which is considerably larger than in the preceding stage. The
connection between the cavity of the forebrain and that of the optic
vesicle is not seen in this section; it is a wide passage that may be
seen in several sections posterior to the one under discussion. The
beginning of the invagination of the optic vesicle to form the optic
cup may be seen on both sides, but more plainly on the right. On the
right side also is noticed a marked thickening of the ectoderm, which
is invaginated to form a small pit, the lens vesicle (_lv_); on the
left side the section is just behind the lens vesicle. Above the optic
stalk on each side, in the angle between the optic vesicle and the
side of the forebrain, is a small blood-vessel (_bv_). Several other
blood-vessels may be seen at various places in the mesoblast, four of
them near the pharynx being especially noticeable. The hindbrain (_hb_)
is wider than, but not so deep as, the forebrain; its walls are very
thick laterally, but are thin on the dorsal and ventral sides. The
dorsal wall is reduced to a mere membrane, which, with the overlying
ectoderm, has been pushed into the brain cavity, as is generally the
case with such embryos. Close to the ventral wall of the hindbrain the
notochord (_nt_) is seen. The character of the notochord has already
begun to change; the cells are becoming rounded and vacuolated, with
but few visible nuclei except around the periphery of the notochord.
Near the center of the section, close to the ventral end of the
forebrain, is the pharynx (_ph_), cut near its anterior limit; it is
here a small, irregularly rectangular cavity with a comparatively thin
wall. On the left side of the pharynx the first gill cleft (_g_) is
indicated as a narrow diverticulum reaching toward the ectoderm. A few
sections posterior to this one the first gill cleft is widely open to
the exterior. As has been said, in the surface view of this stage above
described none of the gill clefts showed; so that in this respect at
least the sectioned embryo was more nearly of the state of development
of the embryo represented in Figure 14, to be described later.

Figure 13_b_, about forty sections posterior to Figure 13_a_, passes
through the hindbrain in the region of the ears. Being back of the
region affected by cranial flexure, this section is of course of much
less area than the preceding. The ectoderm shows no unusual features;
it is of uniform thickness except where it becomes continuous with
the entoderm around the mandibular folds (_md_); there it is somewhat
thickened. The most striking feature of the section is the presence
of two large auditory vesicles (_o_). The section being not quite at
right angles to this part of the embryo, the vesicles are not cut in
exactly the same plane; the one on the left is cut through its opening
to the exterior, while the one on the right appears as a completely
enclosed cavity. In a section a short distance posterior to this one
the appearance of the vesicles would be the reverse of what it is here.
As may be seen in the figure, the vesicles are large, thick-walled
cavities lying close to the lateral walls of the hindbrain. The
hindbrain itself has the usual triangular cross-section, with thick
lateral walls and a thin, wrinkled dorsal wall. Close to the ventral
side of the hindbrain lies the notochord (_nt_), on each side of
which, in the angle between the brain and the auditory vesicles, is
a small blood-vessel (_bv_). Ventral to these structures and close
to the dorsal wall of the pharynx (_ph_) are the two large dorsal
aortæ (_ao_). The ventral side of the section passes through the open
anterior end of the pharynx (_ph_). On the left is seen the widely open
hyomandibular cleft (_g′_), between the main body of the section and
the mandibular arch (_md_). On the right side the plane of the section
was such that the hyomandibular cleft was not cut through its external
opening. In each mandibular fold a large aortic arch (_ar_) is seen,
and also a slight condensation of mesoblast, the latter probably being
the forerunner of cartilage.

Figure 13_c_ passes through the anterior part of the heart about
seventy-five sections posterior to Figure 13_b_. The embryo in this
region is narrow but deep (dorso-ventrally), the depth being largely
due to the size of the heart. The ectoderm (_ec_) is considerably
thickened on each side of the pharynx (_ph_); this thickened area
may be traced for some distance both anteriorly and posteriorly from
this point; its significance could not be determined. The spinal cord
(_sc_) and notochord (_nt_) need no special description; the former is
smaller and the latter larger than in the more anterior sections. The
two large blood-vessels (_ac_) near the spinal cord and notochord are
probably the anterior cardinal veins. The aortæ are cut by the plane
of this section just anterior to their point of fusion into a single
vessel. A few blood corpuscles are seen in the right aorta. The enteron
(_ent_), cut posterior to the region of the gill clefts, is a large
elliptical cavity, with its long axis in a transverse position. Its
entodermal wall is comparatively thin and smooth, with the cell nuclei
arranged chiefly on the outer side, _i. e._, away from the cavity of
the enteron. The body cavity (_bc_) is here still unenclosed, and its
walls, the somatic stalk, are cut off close to the body of the embryo.
The heart (_ht_), the most conspicuous feature of this section, is
nearly as large in cross-section as all the rest of the embryo. As seen
in such a section it is entirely detached from the body of the embryo,
and in this particular case has about the shape of the human stomach.
The mesoblastic portion of its wall (_mes′_) is of very irregular
thickness; it forms a dense layer entirely around the outside, except
for the pointed dorsal region, and is especially thick along the
ventral margin, where it is thrown into well marked folds, the heavy
muscle columns. Lining the cavity of the heart is the membranous
endothelium (_en′_), and between this and the dense outer wall just
described is a loose reticular tissue with but few nuclei.

As the series is followed toward the tail the sections diminish in size
until, at a point about one third the embryo length from the posterior
end, they are of scarcely one fourth the area of the sections through
the region of the hindbrain.

Figure 13_d_ is about one hundred and twenty-five sections posterior
to Figure 13_c_. Although not so small as the sections that follow
it, this section is considerably smaller in area than the one last
described. The amnion (_a_), which was not represented in the last
three figures, is very evident here. The spinal cord (_sc_) is
considerably smaller here than in the preceding figure, while the
notochord (_nt_) is not only relatively but actually larger than in
the more anterior regions. Beneath the notochord is the aorta (_ao_),
now a single large vessel. The mesoblast on each side of the body is
here differentiated into a distinct muscle plate (_mp_). These muscle
plates have very much the appearance of the thickened ectoderm seen in
the younger stages of development. At about its middle region (_i. e._,
at the end of the reference line _ec_) each muscle plate is separated
from the overlying ectoderm by an empty space; this space is still more
marked in some other series. Ventral to the aorta, and supported by a
well marked though still thick mesentery (_ms_), is the intestine. It
is a small, nearly cylindrical tube with thick walls; the splanchnic
mesoblast which surrounds it is more dense than the general mass of
mesoblast; it was somewhat torn in the section and is so represented in
the figure. The urinary organs have made considerable progress since
the last stage. In the figure under discussion they are seen as a group
of tubules on either side of the aorta. The tubule most distant from
the middle line, on each side, is the Wolffian duct (_wd_). It extends
through the posterior two thirds of the embryo and varies in diameter
at different points; it is usually lined with a single layer of cubical
cells which contain large nuclei. The Wolffian bodies (_wt_) are a
mass of slightly convoluted tubules that may be traced throughout the
greater part of the region through which the Wolffian duct extends.
These tubules also vary somewhat in diameter, but they are usually
of greater caliber than the duct. No actual nephrostomes are to be
seen, though the occasional fusion of a tubule with the peritoneal
epithelium, as is seen on the left side of the present figure, may
represent such an opening.

Figure 13_e_ is about one hundred and forty sections posterior to the
section just described. The embryo is here very slender, so that the
contrast between this and the first figure (13_a_) of this stage is
remarkable. Except in size, this section does not differ greatly from
the preceding. The spinal cord, notochord, etc., are smaller than
before, but are of about the same relative size. The mesentery (_ms_)
in the section drawn was torn across, so that the intestine is not
represented. Medial to the Wolffian duct is a tubule (_wt_), which
seems to be the same as those which were called Wolffian tubules in the
preceding stage, but which may be the beginning of the ureter.

Figure 13_f_, about two hundred and fifty sections posterior to the
last, passes through the extreme posterior end of the embryo. The
section is nearly circular in outline and is somewhat larger than
the preceding. The amnion (_a_) completely encircles the embryo. The
ectoderm (_ec_) is of fairly even thickness, and the mesoblast which it
encloses is of the usual character. The spinal cord (_sc_) is nearly
circular in outline, as is its central canal. The digestive tract
(_ent_) is larger in section than it was in more anterior regions;
it is nearly circular in cross-section and its walls are made up of
several layers of cells, so that it resembles to a considerable degree
the spinal cord of the same region. In the narrow space between the
spinal cord and the hindgut is seen the notochord (_nt_), somewhat
flattened and relatively and actually smaller than in the preceding
figure. A few scattered blood-vessels may be seen in the mesoblast at
various places.

A sagittal section of an embryo of this stage, drawn under the same
magnification as were the transverse sections, is shown in Figure
13_g_. The embryo being bent laterally could not be cut by any one
plane throughout its entire length, so that only the anterior end is
represented in the figure. The amnion (_a_) may be clearly seen except
at certain places where it is closely adherent to the superficial
ectoderm. Under the low magnification used the superficial ectoderm
cannot be distinguished from the ectoderm of the nervous system.
The plane of the section being in the anterior end almost exactly
median, this part of the central nervous system is seen as the usual
retort-shaped cavity, while in the region back of the brain, where
the neural canal is narrow, the section passes through the wall of
the spinal cord (_sc_) and does not show the neural canal at all.
The wall of the forebrain (_fb_) is quite thick, especially at the
extreme anterior end; the wall of the midbrain (_mb_), where the marked
cranial flexure takes place, is somewhat thinner, and it gradually
becomes still thinner as it is followed posteriorly over the hindbrain
(_hb_). Between the floors of the fore- and hindbrains, in the acute
angle caused by the cranial flexure, is the anterior end of the
notochord (_nt_), the only part of that structure that lies in the
plane of the section. Ventral and posterior to the notochord is a large
cavity, the pharynx (_ph_), whose entoblastic lining can scarcely be
distinguished under this magnification from the surrounding tissues.
The stomodeal opening being as yet unformed, the pharynx is closed
anteriorly; posteriorly also, owing to the plane of the section, the
pharynx appears to be closed, since its connection with the yolk stalk
is not shown. In the floor of the pharynx, almost under the reference
line _ph_, a slight depression marks the position of the first gill
cleft. In the mesoblast ventral to the pharynx and near the gill cleft
just mentioned, a couple of irregular openings represent the anterior
end of the bulbus arteriosus, posterior and ventral to which is the
heart (_ht_), a large, irregular cavity. The dorsal aorta (_ao_) may
be seen as an elongated opening in the mesoblast, extending in this
section from the middle region of the pharynx to the posterior end of
the figure where it is somewhat torn. Two of the eighteen or twenty
pairs of mesoblastic somites possessed by this embryo are shown at the
posterior end of the figure (_s_), where the plane of the section was
far enough from the median line to cut them.

STAGE XI

FIGURE 14 (PLATE XXI.)

Only the anterior region of this embryo is shown in the figure, which
is a ventro-lateral view. While there is some change in the general
shape and in parts of the head, the reason for figuring this stage is
to show the first gill cleft (_g′_), which lies at an acute angle to
the long axis of the neck behind the eye (_e_). The cleft is narrow
but sharp and distinct in outline; it shows neither in this nor in the
following stages the branched, Y-shaped outline mentioned by Clarke.

STAGE XII

FIGURES 15-15_f_ (PLATES XXI., XXII.)

In this stage, also, only the anterior region of the embryo is figured
in surface view. The shape of the head is about the same as in the
preceding stage, but it is drawn in exact profile. Three gill clefts
(_g¹⁻³_) are now present, and are wide and distinct. The first cleft,
as in the preceding stage, lies at an acute angle to the long axis of
the pharynx and nearly at right angles to the second cleft. The third
cleft sends a wide branch (_g⁴_) toward the posterior, as has been
described by Clarke, from which, or in connection with which according
to Clarke, the fourth cleft will develop. All three clefts may be
distinctly seen to open entirely through the pharyngeal wall. The
outlines of the visceral folds, especially of the mandibular, begin to
be apparent. The nasal pit (_n_) now shows as a round depression in
front of the more definitely outlined eye (_e_). The auditory vesicle
(_o_) is so deep beneath the surface that it may be seen only by
transmitted light.

Figures 15_a_-_e_ represent transverse sections of an embryo of about
this general state of development, except that the gill clefts are not
so definitely open as in the surface view.

Figure 15_a_, the most anterior section of the series, passes through
the forebrain (_fb_) in the region of the eyes, and through the
hindbrain (_hb_) anterior to the auditory vesicles. The forebrain
is here a large cavity with a dense wall of a comparatively even
thickness. Owing probably to the section not being exactly in the
transverse plane, the eyes are cut in different regions, that on the
left (_ov_) being cut through its stalk, while that on the right
(_oc_) is cut near its middle region and hence does not show any
connection with the forebrain. The almost complete obliteration of the
cavity of the optic vesicle to form the optic cup by the invagination
of the outer wall of the vesicle is shown on the right side of the
section (_oc_). The lens vesicle (_lv_) is completely cut off from the
superficial ectoderm (_ec_), which is comparatively thin. The hindbrain
(_hb_) has the usual shape for that structure. Its ventral wall is
dense and thick, while its roof is reduced to the usual thin, wrinkled
membrane. Close to the floor of the hindbrain lies the notochord
(_nt_), which is large and is distinctly vacuolated. To the right of
the hindbrain a large mass of darkly stained cells (_cn_) is one of the
cranial nerves, which is connected with the hindbrain a few sections
anterior to the one under consideration. The pharynx (_ph_), which is
cut near its extreme anterior end, is represented by three irregular
cavities near the base of the forebrain. Scattered throughout the
mesoblast, which makes up the greater part of the section, are numerous
blood-vessels (_bv_).

Figure 15_b_ is twenty sections posterior to Figure 15_a_ and passes
through the tip of the bent-under forebrain (_fb_). On the left the
section is anterior to the optic vesicle and barely touches the side of
the optic stalk, which is seen as a small lump on the ventro-lateral
wall of the brain. On the right the connection of the optic vesicle
(_ov_) with the forebrain is shown. Dorsal to the optic vesicle just
mentioned is a markedly thickened and slightly invaginated region of
the ectoderm (_n_); this is the nasal pit; on the left side of the
figure the thickening is shown, but the section did not pass through
the invagination. The hindbrain (_hb_) is somewhat narrower than in
the preceding figure, but is otherwise about the same; the origin of
a cranial nerve is seen on its left side (_cn_). The notochord (_nt_)
has the same appearance as in the preceding section. A number of
blood-vessels may be seen, the pair lying nearest the notochord being
the aortæ (_ao_), while the two other pairs, on either side of the
fore- and hindbrains, are the anterior cardinals (_ac_). The first
aortic arches are shown at _ar_. On the left the section passes through
the exterior opening of the first gill cleft (_g′_), so that the
mandibular fold (_md_) on that side is a distinct circular structure,
made of a dense mass of mesoderm surrounded by a rather thick ectoderm.
The mesoderm of this fold is especially dense near the center, probably
the beginning of the visceral bar. Near the center is also seen the
aortic arch that has already been mentioned. On the right the section
does not pass through the external opening of the first gill cleft
(_g′_) so that the tissue of the mandibular fold is continuous with
the rest of the head. It is of course the slight obliquity of the
section that causes the pharynx (_ph_) to be completely enclosed on
the right, while on the left it is open to the exterior both through
the gill cleft and between the mandibular fold and the tip of the
head. The superficial ectoderm shown here as a heavy black line varies
considerably in thickness, being thickest in the region of the nasal
pit already mentioned and thinnest over the roof of the hindbrain. The
amnion (_a_) in this, as in the other sections of the series, has the
appearance of a thin, very irregular line.

Figure 15_c_ is posterior to the region affected by cranial flexure
and so shows only one region of the embryo, that of the hindbrain
(_hb_), which is here of essentially the same structure as above
described. On each side of the hindbrain is a large auditory vesicle
(_o_); that on the left is cut through its center and shows the
beginning of differentiation, its lower end being thick-walled and
rounded, while its upper end is more pointed and has a thin, somewhat
wrinkled wall. The notochord (_nt_) is slightly larger than in the more
anterior sections. Numerous blood-vessels (_bv_, _ar_) are seen in the
mesoblast. The pharynx (_ph_) is here open ventrally and also through
the gill cleft of the left side; on the right side the plane of the
section did not pass through the external opening of the cleft. The
mesoblast of the visceral folds is much more dense than that of the
dorsal region of the section.

Figure 15_d_, as is evident, is a section through the region of the
heart, which appears as three irregular cavities (_ht_) with fairly
thick mesoblastic walls (_mes′_) lined with endothelium (_en′_). The
body wall, though consisting of but little besides the ectoderm (_ec_),
completely surrounds the heart, and the pericardial or body cavity
thus formed extends dorsally as a narrow space on either side of the
foregut, giving the appearance of a rudimentary mesentery, though no
especial development of such a structure would naturally be expected in
this region of the embryo. The foregut (_ent_) is a moderately large
cavity lined with a very distinct entoderm of even thickness. Dorsal
to the foregut are three large blood-vessels, a median, and now single,
dorsal aorta (_ao_), and a pair of cardinal veins (_cv_). The notochord
(nt) is small and is flattened against the ventral side of the spinal
cord (_sc_), which latter structure needs no special mention. The
muscle plates (_mp_) are considerably elongated, so that they now
extend ventrally to a point slightly below the upper angles of the body
cavity.

Figure 15_e_ is through the middle region of the embryo, and, owing to
the curvature of the body, is not an exact dorso-ventral section; this
accounts, in part at least, for the unusual diameter in a dorso-ventral
direction of the aorta (_ao_), which is very large in proportion to
the other structures. The posterior cardinal vein is shown on the
left, but not on the right. The relative sizes of the spinal cord
(_sc_) and notochord (_nt_) are very different from what was seen in
the preceding figure. In this section the spinal cord is considerably
smaller than in the preceding, while the notochord is very much larger;
in fact the notochord here seems abnormally large when compared to
corresponding sections of other series. It is true, however, that while
the spinal cord has been diminishing in diameter the notochord has
been increasing. The spinal cord, notochord, and dorsal aorta are all
so large that they are flattened against each other, the pushing in of
the ventral side of the spinal cord being even more marked than is
shown in the figure. On either side of the spinal cord a large spinal
ganglion (_sg_) is seen, closely wedged in between the spinal cord and
the adjacent muscle plate (_mp_). As in the preceding stage, there is a
marked space between the muscle plate and the adjacent ectoderm (_ec_).
The somatic mesoblast at the upper angle of the unenclosed body cavity
is thickened on each side and somewhat bulged out by the Wolffian
body to form what might be termed a Wolffian ridge (_wr_). In the
mid-ventral line is the considerably developed mesentery (_ms_), from
which the intestine has been torn. The Wolffian bodies now consist, on
each side, of a group of five or six tubules (_wt_) of various sizes,
near which in a more ventro-lateral position, close to the upper angle
of the body cavity, is the more distinct Wolffian duct (_wd_). The
allantois is fairly large by this time, and may be seen in the most
posterior sections as an irregular, thick-walled outgrowth from the
hindgut.

A horizontal section through the anterior end of an embryo of this
age is shown in Figure 15_f_. While enclosed of course in the same
membranous amnion (_a_), the pharyngeal region of the section is
separated by a considerable space from the more anterior region where
the section passes through the forebrain (_fb_) and eyes. The spinal
cord (_sc_), notochord (_nt_), muscle plates (_mp_), aortæ (_ao_),
and anterior cardinal veins (_ac_) need no special description. The
appearance of the pharynx (_ph_), with its gill clefts and folds, is
quite similar to that of the corresponding structures in the chick.
None of the four clefts (_g¹⁻⁴_) show, in the plane at which the
section was cut, any connection with the exterior; in fact the fourth
cleft (_g⁴_) would scarcely be recognized as a cleft if seen in this
section alone. One or two of the more anterior clefts are open to the
exterior. Three pairs of aortic arches are seen, and each visceral fold
has a central condensation of mesoblast.

STAGE XIII

FIGURES 16-16_g_ (PLATES XXII., XXIII.)

The embryo (Fig. 16) now lies on one side, body torsion being complete.
The curvature of the body is so marked that the exact length is
difficult to determine. The eye (_e_) and ear (_o_) have about the
same superficial appearance as in the preceding stage. The nose is
not shown in this figure. About thirty somites are present; the exact
number cannot be determined in surface view. The amnion is complete,
though not shown in the figure, and the tail (_t_) is well formed. The
umbilical stalk was torn in the removal of the embryo, so that it is
not shown in the figure. The dim outline of the now convoluted heart
may be seen if the “cleared” embryo be viewed by transmitted light;
it is not shown in the figure. The allantois (_al_) is a rounded sac
of considerable size just anterior to the tail. Four gill clefts
(_g¹⁻⁴_) are now present; the most posterior one is more faint than is
represented in the figure, and it could not be definitely determined
from a surface view whether or not it opened to the exterior. The
mandibular fold (_md_) is now fairly well outlined, but there is as yet
no sign of the maxillary process.

Figure 16_a_ is the most anterior of a series of transverse sections
made of an embryo of the approximate age of the surface view just
described; it passes through the tip of the forebrain (_fb_) and shows
the nasal pit (_n_) of the right side. The great thickening of ectoderm
in the region of the nasal invagination is represented by a solid
line. Owing to the obliquity of the section, the left nasal pit was
not cut. The mesoblast is quite dense and contains two or three small
blood-vessels near the roof of the brain. The plane of this section,
owing to the cranial and body flexure, cut the embryo also in the
region of the pharynx; this part of the section was, as a matter of
convenience, omitted from the drawing.

Figure 16_b_ is in reality more anterior in position, considering
the entire embryo, than the preceding; but the region of the embryo
represented is more posterior, so that it is described at this point.
The greatly elongated outline of the brain is due to its being cut
through the region of flexure, so that the forebrain (_fb_), or,
perhaps, midbrain, is shown at one end, and the hindbrain (_hb_) at
the other. The walls of these cavities are somewhat wrinkled and
irregular and their constituent cells are beginning to show slight
differentiation, though this is not shown in the figure. On the left
side are seen a couple of darkly stained masses; one is the origin of
a cranial nerve (_cn_); and the other is one of the auditory vesicles
(_o_), which is still more irregular in outline than it was in the
preceding stage. The only blood-vessels to be seen are a few very small
ones that lie close to the wall of the brain. The ectoderm is quite
thin at all points.

Figure 16_c_, the largest section of this series, passes through the
forebrain in the region of the eyes and through the gill clefts. The
forebrain (_fb_) exhibits on the left a marked thickening of its wall
(_ch_), the edge of the cerebral hemisphere of that side, which is just
beginning to develop; on its right side the lower part of the forebrain
is connected by a well marked optic stalk (_os_) with the optic cup
(_oc_), in whose opening lies the lens vesicle (_lv_), now reduced to a
crescentic slit by the thickening of its posterior wall. The mesoblast
is more dense in those parts of the section adjacent to the pharynx
than in the more distant regions, and the ectoderm thickens in a marked
way as it approaches the borders of the pharynx and gill clefts. Only
a few small blood-vessels (_bv_) are to be seen in the region of the
forebrain.

Parts of three pairs of clefts (_g_) are shown in the figure: one
pair opens widely on either side, so that there is a large area of
the section that is distinct from the two still larger portions and
contains a small, thick-walled cavity (_g_) on the right side; this
cavity is a gill cleft that is cut through neither its outer nor its
pharyngeal opening.

No structures other than this small portion of a gill cleft and a few
blood-vessels are to be seen in this middle region of the section. In
the more posterior part of the section, in which the notochord (_nt_)
is located, a pair of curved clefts may be seen, opening entirely
through the wall on the left, but closed on the right (_g_). One
distinct pair of aortic arches is shown (_ar_), and also the dorsal
aortæ (_ao_), which are of very unequal size. The spinal cord (_sc_)
and muscle plates need no special description.

Figure 16_d_ is in the region of the heart (_ht_) and lungs (_lu_). The
former is an irregular cavity whose walls, especially on the ventral
side (_mes′_), are becoming very thick and much folded. Although thin,
the body wall completely surrounds the heart, as would be expected,
since this was true of the preceding stage. The lung rudiments (_lu_)
and the foregut from which they have arisen have the same appearance
as in the chick; they consist of three small, thick-walled tubes so
arranged as to form a nearly equilateral triangle. They are surrounded
by a swollen, rounded mass of mesoblast which almost completely fills
the surrounding portion of the body cavity (_bc_). The pleural sides
of these crescentic portions of the body (or pleural) cavity--that
is, the boundary of the mass of mesoblast just mentioned--are lined
with a thickened layer of cells, shown by the solid black lines in the
figure. The lung rudiments may be traced through about fifty sections
of this series, or about one twelfth of the entire series. At the
dorsal angle of the part of the body cavity (_bc_) just described, near
the dorsal aorta (_ao_), are two dark, granular masses (_ge_), which,
under a higher magnification than is here used, are seen to consist of
a small group of blood-vessels filled with corpuscles; although several
sections in front of the anterior limits of the kidneys these are
evidently glomeruli. They may be traced, though diminishing in size,
far toward the tail, in close connection with the Wolffian bodies. At
intervals they are connected by narrow channels with the dorsal aorta;
no such connection was present in the section drawn. The notochord
(_nt_), spinal cord (_sc_), muscle plates (_mp_), and spinal ganglia
(_sg_) need no special mention. The mesoblast is beginning to condense
in the neighborhood of the notochord, and the ectoderm is slightly
thickened laterally and dorsally.

Figure 16_e_ is in the region of the liver and the Wolffian bodies;
it also shows the tip of the ventricular end of the heart. The liver
(_li_) is a large irregular mass, of a blotchy appearance under this
magnification, lying between the heart (_vn_) and the intestine (_i_).
Under greater magnification it is seen to be made up of indefinite
strings of cells; and its still wide opening into the intestine may be
seen in more posterior sections. The intestine (_i_), which in this
section might be called the stomach, is a fairly large cavity with
the usual thick entodermic walls; it is supported by a comparatively
narrow mesentery. The body cavity on the side next this mesentery has
the same thick lining that was noted in the region of the lungs. The
convolutions of the thick peritoneal lining may easily be mistaken in
places for parts of the enteron. The Wolffian bodies may be seen as
two groups of tubules (_wt_) in their usual location. The heart is
cut through the ventricle (_vn_), as has been said. The section being
at right angles to the long axes of the villi-like growths of the
myocardium, the depressions between these mesoblastic cords are seen as
a number of small irregular areas, each one lined with its endocardium.
The incompleteness of the body wall below the heart is apparently due
to an artificial break and not to a lack of fusion. The only point that
need be mentioned in connection with the structures of the dorsal part
of the section is that the distinctness of the myocœl (_myc_) on the
right side is somewhat exaggerated.

Figure 16_f_ is in the middle region of the embryo where both
splanchnopleure and somatopleure are unfused. Owing chiefly to the
unclosed condition of the midgut (_i_) and to the increase in length
of the mesentery (_ms_), the section is quite deep dorso-ventrally.
The continuation of the amnion (_a_) with the somatopleure is of course
here evident.

The most striking feature of the section is the marked projection of
the Wolffian ridges, though no local enlargements of these ridges
indicate the rudiments of the limbs. A large mass of Wolffian tubules
(_wt_) is seen projecting into the upper part of the body cavity on
each side; close to each of these masses is the posterior cardinal
vein (_pc_), and between them is the large aorta (_ao_). The other
structures are about as in the preceding section.

Figure 16_g_ represents a sagittal section of the anterior half of
the body of an embryo of this or possibly a slightly younger stage of
development. The three regions of the brain are clearly indicated, as
well as the cavity of the spinal cord (_sc_). The roof of the hindbrain
has been made too thick in the figure; it should be represented by a
mere line. A little mesoblast is to be seen at places between the roof
of the brain and the superficial ectoderm. A slight invagination of the
epithelium (_p_), between the floor of the brain and the anterior end
of the notochord, probably represents the beginning of the hypophysis.
No indication of the paraphysis is yet to be seen. Extending from the
region of the hypophysis to the posterior end of the section is the
notochord (_nt_); it is much vacuolated and gradually increases in
thickness toward the posterior, though its outline is quite irregular;
except at the extreme anterior end and at one or two other places,
it lies in close contact with the floor of the neural tube. Directly
under the notochord lies, in the posterior half of the figure, the
large dorsal aorta (_ao_). The pharynx (_ph_), opening between the end
of the forebrain and the thick mandibular fold (across which opening
the amnion, _a_, of course extends), is a funnel-shaped space which
passes out of the plane of the section toward the posterior end of the
figure. Its thick endodermal lining extends to the mandibular fold
on the ventral side, while on the dorsal side it gradually thins out
and becomes continuous with the thin ectoderm that extends over the
forebrain. Just back of the mandibular fold is the bulbus (_b_), and
back of that is the edge of the ventricle (_vn_). Posterior and dorsal
to the ventricle the liver (_li_) is seen as an irregular mass of
cells, and dorsal to the liver one of the Wolffian bodies (_wt_) is cut
through its extreme edge.

STAGE XIV

FIGURES 17-17_g_ (PLATES XXIII., XXIV.)

Body flexure has increased until now the forebrain and tail are almost
in contact (Fig. 17). The eye has developed somewhat; the ear vesicle,
which is not shown in the figure, is small and seems to lie nearer the
ventral side; the nasal pit is much larger and is crescentic in shape.
The hyomandibular cleft (_g′_) still persists as a small crescentic
slit, while the next three clefts are now represented merely by
superficial grooves separated by distinct ridges, the visceral folds.
No indication of a fifth cleft is seen. The maxillary process (_mx_)
grows ventralward under the forebrain and is already longer than the
mandibular arch (_md_).

The chief advance in development over the preceding stage, besides
the formation of the maxillary process, is in the appearance of the
appendages (_aa_ and _pa_); they have the characteristic shape of the
rudimentary vertebrate appendage, though the anterior pair seem to
point in an unusual direction at this stage and to be slightly more
developed than the posterior. The curious, anteriorly directed heart
(_ht_) is, perhaps, somewhat abnormal. The umbilical stalk (_u_) is
comparatively narrow and, like the allantois, was cut off close to the
body.

Transverse sections of an embryo of this stage are represented in
Figures 17_a_-_g_, drawn under a lower magnification than were any of
the preceding figures.

Figure 17_a_ is in the region of the pharynx, and passes through the
forebrain (_fb_) and posterior part of the hindbrain (_hb_). In the
thick walls of both of these structures histological differentiation
has begun, so that even under low power an inner granular and an outer
clear zone may be distinguished. Under greater magnification the
presence of short fibers may be made out among the cells. The cerebral
hemispheres (_ch_) are well-marked structures, their asymmetry being
of course due to the obliquity of the section. Only one eye is cut by
the plane of the section, and this one shows no connection with the
forebrain. The outer wall of the optic cup (_oc_) is so thin that under
this magnification it can scarcely be seen as a dark line surrounding
the retinal wall. The lens (_ln_) is now a solid mass, of the usual
type for vertebrate embryos, its front or outer wall being a scarcely
discernible line. The hindbrain (_hb_) has the usual form for that
region and does not differ particularly from what was noted in earlier
stages except in the histological differentiation that has already
been mentioned. As with the eye, it is only on the right side that the
auditory vesicle (_o_) is shown. It shows some differentiation, but not
so much as would be seen were it cut in another region. In the center
of the section the pharynx (_ph_) forms an irregular cavity connected
with the exterior on the left by a gill cleft (_g_) and by another slit
which is simply the anterior margin of the stomodæum. On the right
neither of these openings is in the plane of the figure, though the
gill cleft (hyomandibular), which lies close to the auditory vesicle,
is almost an open passage. A few small blood-vessels are scattered
through the section; one of these (_bv_), lying between the notochord
(_nt_) and the floor of the brain, is noticeable from its being very
closely packed with corpuscles, so that at first glance, under low
magnification, it looks more like a nerve than a blood-vessel.

Figure 17_b_ is also through the pharyngeal region, a short distance
behind the preceding section. The growth of the cerebral hemispheres
(_ch_) is better shown than in the preceding figure, as is also the
general form of the optic cup (_oc_). On the left the nasal cavity
(_n_) is seen as an elongated slit with thick walls; it is cut near,
but not through, its opening to the exterior. The same gill cleft
(_g_) that was seen in the preceding figure is seen here as a narrow,
transverse cleft, open at both ends. Between the notochord (_nt_) and
the spinal cord (_sc_) is the same, though now double, blood-filled
vessel (_bv_) that was seen in the preceding section. The other
blood-vessels are larger here than in the more anterior region. There
is a faint condensation of mesoblast in the neighborhood of the
notochord, and a more marked condensation (_mp_) farther toward each
side is the curiously shaped muscle plate.

Figure 17_c_ is through the heart region, and that organ is cut through
the opening from the lower or ventricular into the upper or auricular
chamber. The thickening of the wall of the ventricle, which was noticed
in the preceding stage, has increased to such an extent that there
is now a marked difference in the thickness of the ventricular and
auricular walls. As in the preceding stage, the body wall is torn,
probably in handling, so that it appears to be incomplete around the
ventral side of the heart. Dorsal to the heart two small circular
holes (_ent_) with thick walls are the œsophagus and trachea, cut
anterior to the point of bifurcation of the latter into the bronchial
or lung rudiments. On either side of these structures is an elongated
blood-vessel (_ac_), the anterior cardinal vein, its elongation being
due to the fact that it is cut at the place where it turns downward to
empty into the heart. Dorsal to the œsophagus are the aortæ (_ao_),
which are here cut just at the point where the two vessels unite to
form one; the next section, posterior to the one under discussion,
shows an unpaired aorta. The notochord (_nt_) and spinal cord (_sc_)
need no description, except to note that the latter shows active
histological differentiation, numerous mitotic figures being seen under
higher magnification, especially in the cells that line the spinal
canal. On the right of the cord the edge of a spinal ganglion (_sg_) is
seen, in connection with which in other sections are seen the clearly
defined nerve roots. The condensation of mesoblast around the notochord
is quite evident, and in close contact with this medial condensation
are two very characteristic, S-shaped muscle plates (_mp_), which
extend from the level of the dorsal side of the spinal cord to the
upper limits of the cardinal veins. In some sections the muscle plates
even yet show slight remains of the myocœl at the dorsal end.

Figure 17_d_ is in the region of the posterior end of the heart (_ht_),
which is cut through the tip of the ventricle, and the anterior end
of the liver (_li_), which has the appearance of a mass of darkly
stained cords or strands of cells surrounding a large blood-vessel
(_mv_). This blood-vessel may be called the _meatus venosus_, though
it is not separated by any line of demarcation from the auricle. A few
sections anterior to this region the _meatus venosus_ opens dorsally
into a large vessel on each side (_dc_), which at first glance seems a
part of the body cavity, but which is in reality the _ductus Cuvieri_,
formed by the union of the anterior and posterior cardinal veins. An
irregular, crescentic cleft (_bc_), lying medial and parallel to each
of the Cuvierian vessels, is the body cavity. In the upper angle of
this cavity is a granular mass, the glomerulus, that of the left side
being accompanied by the extreme anterior end of the Wolffian duct.
In the rounded mass of mesoblast, between the cleft-like regions of
the body cavity, the lung rudiments (_lu_), and the œsophagus (_oe_)
are seen as three small, circular openings; that of the œsophagus is
somewhat smaller than the other two. The notochord (_nt_), spinal cord
(_sc_), and muscle plates (_mp_) have almost the same appearance as in
the preceding section. A spinal ganglion (_sg_) is seen on each side of
the spinal cord; the one on the left shows a well-defined spinal nerve
(_sn_), which may be traced ventrally as far as the end of the muscle
plate, along whose medial side it courses. The ventral nerve root is
plainly seen; the dorsal root, in this section, less plainly. The
amnion (_a_) and abdominal wall are, as in the preceding figure, torn
in the region of the ventricle.

Figure 17_e_ is a short distance posterior to the figure just
described. The liver is cut through its middle region and forms a
large, darkly staining, reticular mass on the left side of the figure.
The digestive tract is seen at two places to the right of the liver;
the smaller and more ventral of these openings (_i_) may be called
the intestine, while the larger is evidently the stomach (_i′_). The
body wall is here unfused and becomes suddenly thinner as it passes
upward into the amnion (_a_). The Wolffian tubules (_wt_) form a very
conspicuous mass on either side of the mesentery, in close connection
with the posterior cardinal veins (_pc_). In the mesoblast between the
dorsal aorta (_ao_) and the notochord are two small, irregular, darkly
stained masses (_sy_). These are shown in the preceding two figures,
but were not mentioned in the description. They may be traced through
a great part of the length of the embryo back of the head region; at
intervals corresponding in length to the distance between the spinal
ganglia they are enlarged, while between these enlargements they are
very small in cross-section. At certain points a small blood-vessel is
given off by the dorsal aorta to the immediate neighborhood of each of
these small areas. Although they show no connection with the central
nervous system, these structures appear to be the rudiments of the
sympathetic nerves.

Figure 17_f_ is in the region just in front of the hind legs. The
abdominal walls are here unfused, and into the unenclosed body cavity
projects the intestine (_i_), supported by a narrow mesentery and
surrounded by a comparatively thick mass of mesoblast. The Wolffian
body and duct form a mass of considerable size on each side of the
mesentery. The Wolffian body is cut near its posterior end and consists
of smaller tubules than in the more anterior regions. The Wolffian
ducts (_wd_), on the other hand, are very large and are much more
clearly distinguishable from the Wolffian tubules than in the preceding
sections. The Wolffian ridges (_wr_) are very marked projections on the
sides of the body, and in a region farther caudad become especially
developed as the posterior appendages, to be described in connection
with the following section. Both spinal ganglia are shown in this
figure (_sg_), and in connection with the left ganglion the spinal
nerve (_sn_), extending ventrally as far as the level of the Wolffian
duct. The sympathetic nerve rudiments do not extend so far caudad as
the plane of this section. The dorsal end of each muscle plate (_mp_)
is seen, in this and other sections, to be slightly enlarged to form
a round knob; this knob contains a distinct cavity (not shown in the
figure), the myocœl.

In Figure 17_g_, owing to the curvature of the body, the plane of the
section passes through the body at three places: through the region of
the heart and lungs (Fig. 17_d_), through the region of the posterior
appendages, and through the tail. In fact, the plane of the section
represented by each of the preceding figures cut the embryo in more
than one region, but for the sake of simplicity only one region was
represented in each figure. In the figure under discussion only the leg
and tail regions have been drawn, though the latter region (_t_), being
cut through one of its curves, is seen as an elongated body with a
section of the spinal cord, notochord, etc., at each end. Both regions
shown in the figure are enclosed in the same fold (_a_) of the amnion.
Of the structures in the dorsal side of the larger or more anterior
part of this figure nothing need be said. The most striking feature
of the section is the presence of the large posterior leg rudiments
(_pa_). As was noted in the preceding figure, they are, as usual,
merely local enlargements or projections of the mesoblast (covered, of
course, with ectoblast) of the Wolffian ridge. They are, as shown in
this section and in the surface view of this stage (Fig. 17), bluntly
pointed projections from the sides of the body. The anterior appendage
seems to be slightly more developed than the posterior, as was noted
in describing the surface view of the embryo. The digestive tract is
cut through its extreme posterior end, in the region that may be termed
the cloaca (_cl_), for into it at this point the Wolffian ducts open
(_wdo_). As the narrow cloacal chamber is followed toward the tail,
it becomes still smaller in diameter, and the ventral depression or
cleft seen in this figure gradually becomes deeper until its walls
are continuous with the ectoderm that covers the ventral projection
of mesoderm between the hind legs; no actual opening to the exterior
is present, however. There is a space of about twenty-five or thirty
sections (in a series of eight hundred) between the posterior ends of
the Wolffian bodies and the cloacal openings of the Wolffian ducts. The
body cavity (_bc_) and the posterior cardinal veins (_pc_) are very
small in this region, as might be expected.

STAGE XV

FIGURE 18 (PLATE XXIV.)

Only the head of this embryo is represented, as the general state of
development is about the same as in the preceding stage.

The chief object in making the figure is to show the five gill clefts
(_g¹⁻⁵_). The fifth cleft, though small and probably not open to the
exterior, is quite distinct in this embryo. The writer would feel more
doubt of its being a true, though rudimentary, gill cleft had not
Clarke (17) found a fifth pair of clefts in his material. The nasal pit
has advanced in development somewhat and shows the beginning of the
groove that connects it with the mouth. In this figure the crescentic
hyomandibular cleft shows its connection with the groove between the
mandibular and the hyoid folds.

STAGE XVI

FIGURE 19 (PLATE XXIV.)

This embryo is only slightly more developed than the preceding. Body
flexure is so great that the forebrain and tail nearly touch. Only
the anterior three gill clefts are visible. The maxillary process
(_mx_) is longer and more narrow; the mandibular fold has not changed
appreciably. The nasal pit (_n_) is now connected by a distinct
groove with the stomodæum. The appendages have increased in size, the
posterior (_pa_) being the longer. The anterior appendage (_aa_) is
distinctly broadened to form the manus, while no sign of the pes is to
be seen at the extremity of the posterior appendage. The heart (_ht_)
is still very prominent. The stalk of the umbilicus (_u_), which is
quite narrow, projects from the ventral wall in the region between the
heart and the hind legs. The tail is of considerable length and is
closely coiled.

STAGE XVII

FIGURES 20-20_j_ (PLATES XXV., XXVI.)

The superficial changes noted in this stage chiefly concern the head,
which has increased considerably in length (Fig. 20). The curvature
of the body is slightly more marked, and the tail is more tightly
coiled at the end. There are still signs of three gill clefts. The
maxillary process (_mx_) is long and narrow, while the mandibular arch
(_md_) is still short and broad. The fronto-nasal region has greatly
increased and has the aquiline profile noted by Clarke. The nasal
groove has disappeared, and there remains the small opening (_n_) at
the side of the fronto-nasal region, near the end of the still separate
maxillary process. The umbilicus is in about the same condition as in
the preceding stage, but the heart is less prominent. The outline of
the manus (_ma_) is more definite, and the extremity of the posterior
appendage is distinctly flattened out to form the rudimentary pes
(_pe_). The position of the elbow-joint in the anterior appendage is
seen at the end of the reference line _aa_.

Typical transverse sections of this stage are shown in Figures
20_a_-_j_.

Figure 20_a_ is a section through the middle region of the head,
cutting the hindbrain on one side and the forebrain on the other. The
walls of the brain show rather more histological differentiation than
was seen in the preceding sections, though this cannot be shown under
the low magnification used. The hindbrain (_hb_), which is cut near its
anterior border, exhibits the usual membranous dorsal and thick ventral
walls. The forebrain is here seen as three distinct cavities--a median
third ventricle (_tv_), with a thick ventral wall, and a thin dorsal
wall extended to form a large paraphysis (_epi_), and two lateral
ventricles (_ch_), the cavities of the cerebral hemispheres, whose
walls are quite thick except on the side next the third ventricle.
The sections of this series being slightly oblique, the eye is here
cut on the right side only, where it is seen as a large, semicircular
cavity (_e_) with thick, dense walls. The mesoblast, in which several
blood-vessels (_bv_) are seen, exhibits three distinct areas--a median,
lighter zone, with a more dense area on either side. The significance
of this variation in the density of the mesoblast is not apparent.

Figure 20_b_ is only a few sections posterior to the section just
described. It is drawn chiefly to show the appearance of the forebrain,
the other structures being about as in the preceding figure, except
that both eyes (_e_) are here represented. The section passes through
the wide opening between the third (_tv_) and the lateral ventricles
(_ch_) and cuts the anterior edge of the pineal body[8] (_epi_). The
paraphysis is very large and is directed backward instead of forward,
as is usually the case among the lower vertebrates (if the alligator
may be so classed). It is shown in Figure 17_a_ of a preceding stage
and will be again shown in a sagittal section to be described later.
The same areas of more dense and less dense mesoblast noted in the
preceding figure are seen here.

[8] Subsequent investigation showed that the structure here described
as the pineal body is, in reality, the paraphysis; the pineal body is
absent in _A. mississippiensis_.

Figure 20_c_, though still in the head region, shows several features
that were not seen in the preceding figures. On the left of the
hindbrain (_hb_) the auditory vesicle (_o_), which is now considerably
more advanced than in earlier figures, is seen as a larger,
flask-shaped cavity and a smaller, round one. Between the larger cavity
and the hindbrain is the root of a cranial nerve (_cn_), apparently the
eighth, since in another section it comes in close contact with the
wall of the larger part of the auditory vesicle just mentioned. On the
right side, ventral to the hindbrain, another and much larger nerve
(_cn_) is seen. Nearly in the center of the figure is seen a small,
irregular, thick-walled cavity (_p_); this is the pituitary body,
and its connection with the roof of the pharynx may easily be made
out in another section. The mesoblast in this region of the sections
contains numerous large and small blood-vessels and exhibits certain
denser areas which probably represent the beginnings of the cranial
cartilages. No sign of the forebrain is seen (the plane of the section
passing in front of that region), except that the tip of the wall of
one of the cerebral hemispheres (_ch_) is cut. The left nasal chamber
(_n_) is shown: it will be noted again in the following section. The
eye on the right side shows no remarkable features; its lens (_ln_) is
large and lies well back of the lips of the optic cup, which may now be
called the iris (_ir_). A thin layer of mesoblast has pushed in between
the lens and the superficial ectoderm to form the cornea, and the outer
wall of the optic cup is now distinctly pigmented. The inner wall of
the optic cup is beginning to differentiate into the retinal elements.
The eye on the left side is cut farther from its central region and
has a very different appearance from the eye just described. This
unusual appearance is due to the fact that the section passed through
the choroid fissure, which is very large and seems to be formed by the
pushing in of the walls of the cup and not by a mere cleft in these
walls. This fissure is hardly noticeable in the stage preceding the
present, and in a stage slightly older it has disappeared; so that it
would seem to be a very transient structure. It apparently is formed at
about the time that the optic stalk, as such, disappears. It is in the
region of the choroid fissure, if not through it, that the optic nerve
(_on_) enters the eye. Through the fissure also enters a vascular tuft
of mesoblast (_pt_) which may be seen projecting into the optic cup
after the disappearance of the fissure. This loop of blood-vessels is
doubtless the pecten.

Figure 20_d_ represents a section through the hindbrain (_hb_), pharynx
(_ph_), and tip of the snout. On either side of the hindbrain are
a convoluted auditory vesicle (_o_), and several blood-vessels and
nerves, while ventral to it is seen the anterior end of the notochord
(_nt_), around which the mesoblast is somewhat more dense than
elsewhere. The pharynx (_ph_) sends out toward the surface a narrow
gill cleft (_g′_) in the neighborhood of each auditory vesicle. These
clefts connect with the exterior by very narrow slits, not seen in
the plane of this section. The opposite end of the pharynx, as seen
in this figure, opens on the left (_pn_) into the nasal chamber. The
nasal cavity on the right is cut in such a plane that it shows neither
its external nor its pharyngeal opening. The nasal passages are here
fairly long and nearly straight chambers; their lining epithelium is
quite thick in the middle region, but becomes thinner where it merges
into the epithelium of the pharynx at one end, and into the superficial
epithelium at the other end. The unusual appearance of the eye (_e_),
on the right side of the figure, is due to the fact that the plane of
the section cut tangentially through the extreme edge of the eye in the
region of the choroid fissure.

Figure 20_e_ is only a short distance posterior to the preceding. On
the left side the pharynx (_ph_) is connected with the exterior through
the stomodæum, and on the right the hyomandibular cleft (_g′_) is cut
almost through its opening to the exterior. The auditory vesicle (_o_)
on the right is cut near its middle region, while that on the left
is barely touched by the plane of the section. The notochord (_nt_),
with its condensed area of mesoblast, is somewhat larger than in the
preceding section. The nasal canal on the right (_n_) is cut through
neither anterior nor posterior opening, while on the left side the
canal shows the anterior opening (_an_).

Figure 20_f_, which is in the region of the posterior part of the
pharynx and the anterior part of the heart, shows some rather unusual
conditions.

The spinal cord (_sc_) and notochord (_nt_), with the faintly
outlined condensations of mesoblast in their region, need no special
description. The pharynx (_ph_) is here reduced to an irregular,
transversely elongated cavity, the lateral angles of which are
connected on each side with the exterior through a tortuous and almost
closed gill cleft (_g_), which must be followed through many sections
before its inner and outer openings may be determined. Dorsal to the
pharynx numerous blood-vessels (_bv_), both large and small, may be
seen, while ventral to it is noticed a faint condensation of mesoblast
(_la_), in the form of an inverted T, the anlage of the laryngeal
structures. The ventral portion of the figure is made up of a nearly
circular thin-walled cavity, the pericardium (_pr_). Most of the
pericardial cavity is occupied in this section by the thick-walled
ventricle (_vn_), above which is the bulbus (_b_) and the tip of the
auricle (_au_). The bulbus is nearly circular in outline, though its
cavity is very irregular. A few sections anterior to this, the opening
of the bulbus into the ventricle is seen.

In Figure 20_g_ the section represented is only a short distance
posterior to the one represented by Figure 20_f_. The mesoblastic
structures in the neighborhood of the spinal cord (_sc_) and notochord
(_nt_) will be described in connection with the next figure, where they
are more clearly defined. The œsophagus (_oe_)--or posterior end of the
pharynx, whichever it may be called--is here a crescentic slit, with
its convex side upward; ventrally it opens by a narrow glottis into the
trachea (_ta_). The trachea is surrounded by the same condensed area of
mesoblast (_la_) that was mentioned in connection with the preceding
figure, but the condensation is here more marked. From the bulbus
(_b_) an aortic arch (_ar_) extends upward for a short distance on the
right side, while to the left of the œsophagus an aortic arch (_ar_)
is cut through the upper part of its course. Ventral to the bulbus the
ventricle (_vn_) and two auricles (_au_) are seen surrounded by the
pericardial wall.

Figure 20_h_ is in the region of the liver (_li_), which has about the
same position in relation to the auricles (_au_) that was occupied by
the ventricle in the last figure. The auricles are connected with each
other by a wide passage. The trachea (_ta_) and the œsophagus (_oe_)
are entirely distinct from each other; the former is a small, nearly
circular hole, while the lumen of the latter is obliterated and its
walls form a solid, bow-shaped mass of cells. Since there is a narrow
space between this mass of cells and the surrounding mesoblast, it
might be thought that the lumen of the œsophagus had been closed by
the simple shrinkage of its walls; higher magnification, however,
fails to show any sign of a collapsed lumen. It is doubtless the
problematic and temporary closure of the œsophagus that is noticed in
other forms. On each side of the œsophagus, in close relation with the
anterior cardinal vein (_ac_), is noticed a nerve (_cn_) cut through a
ganglionic enlargement. When traced forward these nerves are seen to
arise from the region of the medulla, and when followed caudad they
are found to be distributed chiefly to the tissues surrounding the
newly formed bronchi; they are doubtless the tenth cranial nerves. On
the right side of the figure the close connection of this nerve with
the near-by gill cleft is seen. Above the paired aortæ (_ao_) the
sympathetic nerves (_sy_) will be noticed. The mesoblast surrounding
the spinal cord (_sc_) and notochord (_nt_) is distinctly condensed
(more so than the figure shows) to form what may be called the centrum
(_c_) and neural arch (_na_) of the vertebræ. The arch, owing to the
slight obliquity of the section, shows here only on one side. The
spinal cord is not yet completely enclosed by the neural arches. The
muscle plates (_mp_) are in close connection with the rudiments of the
vertebræ just mentioned. The spinal cord (_sc_) is here differentiated
into three areas--a dense, deeply stained area immediately around the
neurocœl; a less dense area of cells surrounding the inner area and
extending ventralward as a rounded projection on each side; and an
outer layer, with few or on nuclei, surrounding the inner two layers
except on the dorsal side.

In Figure 20_i_ the size and complexity of the figure are due, it will
be easily understood, to the fact that the plane of the section passed
through the curve of the body, thus practically cutting the embryo in
two regions--an anterior, where the lungs (_lu_) and liver (_li_) are
seen, and a posterior, where the Wolffian bodies (_wt_) are present.
The spinal cord and the surrounding structures have almost the same
characteristics at both ends of the figure, except that the primitive
spinal column is rather more distinct in the posterior end of the
section. The posterior cardinal veins (_pc_), Wolffian ducts (_wd_),
and Wolffian bodies (_wt_) are also prominent structures of this end
of the figure, the last being made up of a great number of tubules.
The extreme anterior ends of the Wolffian bodies are seen in the other
half of the section in the upper angles of the body cavity, dorsal
to the lung rudiments (_lu_). Filling most of the body cavity (_bc_)
and making up the greater part of the middle of the figure are the
liver (_li_), now a very large organ; the stomach (_i′_), also quite
large; the pancreas (_pan_), a small body lying near the stomach; and
the lungs (_lu_), which here consist of several thick-walled tubes,
surrounded by lobes of mesoblast. The other features of the figure need
no special mention.

Figure 20_j_ is through the base of the posterior appendages (_pa_),
in which the cartilages are already being outlined by condensations
of mesoblast. The intestine (_i_) is cut in two regions--at a more
anterior point, where it is seen as a small, circular hole surrounded
by mesoblast and hung by a narrow mesentery, and through the cloacal
region, the larger and more ventral cavity, into which the Wolffian
ducts (_wd_) open a short distance caudad to this section. The
blood-vessels present a rather curious appearance. A short distance
anterior to this point the aorta has divided into three, or it might
be said that it has given off two, large branches. These two branches,
one on either side near the posterior cardinal vein, pass toward the
ventral side of the embryo on each side of the cloaca and end at
about the region represented by the present figure. The small portion
of the aorta that remains after the giving off of the two branches
just described continues, as the caudal artery (_ca_), into the tail;
it is a small vessel just under the notochord, and gives off small,
paired branches at regular intervals toward the vertebral region. The
posterior cardinal veins (_pc_), posterior to the openings of the
Wolffian ducts into the cloaca, unite to form a large caudal vein lying
just ventral to the caudal artery.

STAGE XVIII

FIGURE 21 (PLATE XXVII.)

This embryo, as may be seen, for example, by the form of the
appendages, is slightly further developed than the one represented in
Figure 20. The figure is from a photograph of a living embryo as it
lay in the egg, a portion of the shell and shell membranes having been
removed. The embryo, which lies on its left side, is rather faintly
outlined because of the overlying allantois. The allantois has been
increasing rapidly in size, and is here so large that it extends
beneath the cut edges of the shell at all points except in the region
in front of the head of the embryo, where its border may be seen. Its
blood-vessels, especially the one that crosses the head just back of
the eye, are clearly shown in the figure, and in the living specimen,
when filled with the bright red blood, they form a most beautiful
demonstration. As in the chick, the allantois lies close beneath the
shell membranes and is easily torn in removing them.

STAGE XIX

FIGURE 22 (PLATE XXVII.)

Figure 22 is a photograph of a somewhat older embryo, removed from the
egg and freed of the fetal membranes. The appendages show the position
of both elbow and knee joints, and in the paddle-shaped manus and pes
the digits may be faintly seen. The tail is very long and is spirally
coiled, the outer spiral being in contact with the frontal region of
the head. The jaws are completely formed, the upper projecting far
beyond the lower. The elliptical outline of the eyes is noticeable, but
the lids are still too little developed to be seen in this figure. The
surface of the embryo is still smooth and white.

STAGE XX

FIGURES 23-23_b_ (PLATE XXVII.)

In this surface view (Fig. 23) several changes are seen, though no
very great advance in development has taken place. The outlines of
the digits (five in the manus and four in the pes) are now well de
fined; they even project slightly beyond the general outline of the
paddle-shaped part. The tail has begun to straighten out, and it now
extends across the front of the face. The lower jaw has increased in
length, but is still shorter than the upper. The eyelids, especially
the upper, are beginning to be discernible in surface view. Though
still without pigment, the surface of the body is beginning to show by
faint transverse lines the development of scales; these lines are most
evident in this figure in the middle region of the tail, just before it
crosses the nose.

A sagittal section of the entire embryo (except the tail) of this
age is shown in Figure 23_a_. In the head region the section is
nearly median, while the posterior part of the body is cut slightly
to one side of the middle line. At the tip of the now well-developed
snout is seen one of the nostrils (_an_), cut through the edge; its
connection with the complicated nasal chamber (_n_) is not here seen,
nor is the connection of the nasal chamber with the posterior nares
(_pn_). The pharynx (_ph_), is anteriorly connected with the exterior
through the mouth (_m_) and the nares, while posteriorly it opens into
the œsophagus (_oe_); the trachea (_ta_), though distinct from the
œsophagus, does not yet open into the pharynx. In the lower jaw two
masses of cartilage are seen, one near the symphysis (_mk_) and one
near the wall of the trachea, doubtless the rudiment of the hyoid. The
deep groove back of the Meckel’s cartilage (_mk_) marks the tip of the
developing tongue, which here forms the thick mass on the floor of
the mouth cavity. Dorsal to the pharynx a mass of cartilage (_se_) is
developing in the sphenethmoid region. This being a median section, the
ventricles of the fore- (_fb_), mid- (_mb_), and hindbrain (_hb_) are
seen as large cavities, while the cerebral hemispheres (_ch_) appear
nearly solid, only a small portion of a lateral ventricle showing. The
paraphysis (_epi_) is cut a little to one side of the middle and so
does not show its connection with the brain. At the base of the brain
the infundibulum (_in_) is seen as an elongated cavity whose ventral
wall is in close contact with a group of small, darkly staining alveoli
(_p_), the pituitary body. Extending posteriorly from the pituitary
body is a gradually thickening mass of cartilage (_bp_), which
surrounds the anterior end of the notochord (_nt_) and may be called
the basilar plate. In its anterior region, where the section is nearly
median, the spinal column shows its canal, with the enclosed spinal
cord, while toward the posterior end of the figure the vertebræ are cut
to one side of the middle line, and hence show the neural arches (_na_)
with the alternating spinal ganglia (_sg_). Near the posterior end of
the figure the pelvic girdle (_pl_) is seen. The largest organ of the
embryo, as seen in this section, is the heart, of which the ventricle
(_vn_) seems to be closely surrounded, both in front and behind, by the
auricles (_au_). The liver (_li_) is the large, reticular mass back of
the heart. Dorsal and anterior to the liver is the lung (_lu_), now of
considerable size and development. The enteron is cut in several places
(_oe_, _i_) and its walls are beginning to show some differentiation,
though this cannot be seen under the magnification here used. One of
the Wolffian bodies is seen as a huge mass of tubules (_wt_) extending
from the pelvic region, where the mass is greatest, to the region of
the lungs. The Wolffian tubules stain darkly and the whole structure
forms a very striking feature of the section. Dorsal to the posterior
end of the Wolffian body is a small, oval mass of very fine tubules
(_k_), which do not stain so darkly as do the Wolffian tubules;
this mass is apparently the beginning of the permanent kidney, the
metanephros. Its tubules, though their origin has not been determined,
seem to be entirely distinct from the tubules of the Wolffian body.

A single vertical section through the anterior part of the head of an
embryo of this age has been represented in Figure 23_b_. On the right
side the plane of the section cut through the lens of the eye (_ln_);
on the left side the section was anterior to the lens. The upper (_ul_)
and lower (_ll_) eyelids are more evident here than in the surface
view. Owing to the hardness of the lens, its supporting structures
were torn away in sectioning. The vitreous humor is not represented
in the figure. The superior (_ur_) and inferior (_lr_) recti muscles
are well shown on the right side; they are attached to the median
part of a Y-shaped mass of cartilage (_se_), which may be termed
the sphenethmoidal cartilage. Between the branches of this Y-shaped
cartilage the anterior ends of the cerebral hemispheres (_ch_)--better
called, perhaps, the olfactory lobes--are seen. Between the lower end
of the sphenethmoidal cartilage and a dorsally evaginated part of
the pharynx are two small openings (_pn_); when traced forward these
tubes are found to open into the convoluted nasal chamber, while a
short distance posterior to the plane of this figure they unite with
each other and open almost immediately into the pharynx. The rather
complicated structures of the nasal passages of the alligator have been
described by the writer in another paper (57). In the lower jaw the
cartilage (_mk_) is seen on either side and several bands of muscle
are developing in the mesoblast. Two deep grooves give form to what
may be called the rudimentary tongue (_tn_). In both jaws one or two
tooth rudiments (_to_) may be distinguished as small invaginations of
ectoderm.

STAGE XXI

FIGURE 24 (PLATE XXVII.)

In this stage the curvature of the body and tail is less marked than
was seen in the last surface view. The body has increased greatly in
size, so that the size of the head is relatively not so great. The
size of the eye in relation to that of the head is much diminished
also. The five anterior and four posterior digits are well formed, and
their claws are of considerable size, though of course not present on
all the digits. The outlines of scales may be traced from the tip of
the tail to the skull; they are especially prominent along the dorsal
profile. The skin is just beginning to show traces of pigment, which
is, however, not shown in the photograph. The umbilical stalk is seen
projecting with a loop of the intestine from the abdominal wall; this
is shown more clearly in the next stage. The embryo now begins to
exhibit some of the external characteristics of the adult alligator.

STAGE XXII

FIGURE 25 (PLATE XXVIII.)

This embryo needs no particular description. It has reached in its
external appearance practically the adult condition, although there is
still considerable yolk (not shown in the figure) to be absorbed, and
the embryo would not have hatched for many days. Pigmentation, begun in
the last stage, is now complete. The umbilical stalk is clearly seen
projecting from a large opening in the body wall. The long loop of the
intestine that extends down into the yolk sac is here evident, and it
is hard to understand how it can all be drawn up into the body cavity
when the umbilical stalk is withdrawn. No sharp shell-tooth at the tip
of the snout, such as is described by Voeltzkow (78) in the crocodile,
is here seen.

STAGE XXIII

FIGURE 26 (PLATE XXVIII.)

This figure shows the relative sizes of the just-hatched alligator and
the egg from which it came. It also shows the position of the young
alligator in the egg, half of the shell having been removed for that
purpose. The blotchy appearance of the unopened egg is due chiefly to
stains produced by the decayed vegetation of the nest. At hatching the
young alligator is about 20 cm. long, nearly three times the length of
the egg; but the tail is so compressed that, though it makes up about
half of the length of the animal, it takes up very little room in the
egg.

SUMMARY

Owing to the fact that the embryo may undergo considerable development
before the egg is laid, and also to the unusual difficulty of removing
the very young embryos, the earlier stages of development are very
difficult to obtain.

The mesoderm seems to be derived chiefly by proliferation from the
entoderm, in which way all of that anterior to the blastopore arises.
Posterior to the blastopore the mesoderm is proliferated from the lower
side of the ectoderm in the usual way. No distinction can be made
between the mesoderm derived from the ectoderm and that derived from
the entoderm.

The ectoderm shows during the earlier stages a very great increase in
thickness along the median longitudinal axis of the embryo.

The notochord is apparently of entodermal origin, though in the
posterior regions, where the germ layers are continuous with each
other, it is difficult to decide with certainty.

The medullary folds have a curious origin, difficult to explain without
the use of figures. They are continuous posteriorly with the primitive
streak, so that it is impossible to tell where the medullary groove
ends and the primitive groove begins, unless the dorsal opening of the
blastopore be taken as the dividing point.

The amnion develops rapidly, and entirely from the anterior end.

The blastopore or neurenteric canal is a very distinct feature of all
the earlier stages up to about the time of closure of the medullary
canal.

Preceding the ordinary cranial flexure there is a sort of temporary
bending of the head region, due apparently to the formation of the
head-fold.

During the earlier stages of development the anterior end of the embryo
is pushed under the surface of the blastoderm, and is hence not seen
from above.

Body torsion is not so definite in direction as in the chick, some
embryos lying on the right side, others on the left.

Of the gill clefts, three clearly open to the exterior and probably a
fourth also. A probable fifth cleft was seen in sections and in one
surface view.

The first trace of the urinary system is seen as a dorsally projecting,
solid ridge of mesoblast in the middle region of the embryo, which
ridge soon becomes hollowed out to form the Wolffian duct.

The origin of the hypophysis and paraphysis is clearly seen; the latter
projects backward.

No connection can be seen between the first rudiments of the
sympathetic nerves and the central nervous system.

The lumen of the œsophagus is for a time obliterated as in other forms.

The choroid fissure is a very transitory but well-marked feature of the
eye.

LETTERING FOR ALL FIGURES ON PLATES VI.-XXVIII.

_a_, head-fold of amnion.
_aa_, anterior appendage.
_ac_, anterior cardinal vein.
_al_, allantois.
_an_, anterior nares.
_ao_, aorta.
_aop_, area opaca.
_ap_, area pellucida.
_ar_, aortic arch.
_au_, auricle.
_b_, bulbus arteriosus.
_bc_, body cavity.
_blp_, blastopore.
_bp_, basilar plate.
_bv_, blood-vessel.
_c_, centrum of vertebra.
_ca_, caudal artery.
_ch_, cerebral hemisphere.
_cl_, cloaca.
_cn_, cranial nerve.
_cp_, posterior choroid plexus.
_cv_, cardinal veins.
_dc_, ductus Cuvieri.
_e_, eye.
_ec_, ectoderm.
_ec′_, thickening of ectoderm,
_en_, entoderm.
_en′_, endocardium.
_ent_, enteron.
_ep_, epidermal layer of ectoderm.
_epi_, paraphysis.
_es_, embryonic shield.
_f_, fronto-nasal process.
_fb_, forebrain.
_fg_, foregut.
_g¹⁻⁵_, gill clefts.
_gf¹⁻⁶_, gill folds.
_gl_, glomerulus.
_h_, head-fold.
_hb_, hindbrain.
_ht_, heart.
_i_, intestine.
_i′_, stomach.
_in_, infundibulum.
_ir_, iris.
_it_, iter.
_k_, kidney (metanephros).
_l_, remains of groove between secondary folds.
_la_, larynx (cartilages of).
_li_, liver.
_ll_, lower lid of eye.
_ln_, lens.
_lr_, inferior rectus muscle of eye.
_lu_, lungs.
_lv_, lens vesicle.
_m_, mouth.
_ma_, manus.
_mb_, midbrain.
_mc_, medullary canal.
_me′_, tip end of medullary canal.
_md_, mandibular fold.
_mes_, mesoderm.
_mes′_, myocardium.
_mf_, medullary fold.
_mg_, medullary groove.
_mk_, Meckel’s cartilage.
_mp_, muscle plate.
_ms_, mesentery.
_mv_, meatus venosus.
_mx_, maxillary fold.
_myc_, myocœl.
_n_, nasal invagination or cavity.
_na_, neural arch of vertebra.
_nc_, neurenteric canal.
_nl_, nervous layer of ectoderm.
_nt_, notochord.
_o_, ear vesicle.
_oc_, optic cup.
_oe_, œsophagus.
_on_, optic nerve.
_os_, optic stalk.
_ov_, optic vesicle.
_p_, pituitary body.
_pa_, posterior appendage.
_pan_, pancreas.
_pc_, posterior cardinal vein.
_pe_, pes.
_pg_, primitive groove.
_ph_, pharynx.
_pl_, pelvis.
_pn_, posterior nares.
_pr_, pericardial cavity.
_ps_, primitive streak.
_pt_, pecten.
_rt_, retina.
_s_, somites.
_sc_, spinal cord.
_se_, sphenethmoid cartilage.
_sf_, secondary fold.
_sg_, spinal ganglion.
_sm_, splanchnic mesoblast.
_sn_, spinal nerve.
_so_, somatic mesoblast.
_st_, stomodæum.
_sy_, sympathetic nervous system.
_t_, tail.
_ta_, trachea.
_tg_, thyroid gland.
_th_, thickening and posterior limit of _sf_.
_tn_, tongue.
_to_, tooth anlage.
_tr_, truncus arteriosus.
_tv_, third ventricle of brain.
_tv′_, third ventricle of brain.
_u_, umbilical stalk.
_ul_, upper lid of eye.
_ur_, superior rectus muscle of eye.
_v′-″-‴_, first, second, and third cerebral vesicles.
_va_, vascular area.
_vm_, vitelline membrane.
_vn_, ventricle of heart.
_vv_, vitelline blood-vessels.
_wd_, Wolffian duct.
_wdo_, opening of Wolffian duct.
_wr_, Wolffian ridge.
_wt_, Wolffian tubules.
_y_, yolk.

EXPLANATION OF FIGURES 1-26 ON PLATES VI.-XXVIII.

All of the figures, with the exception of the photographs and those
copied by permission from S. F. Clarke, were drawn under a _camera
lucida_.

The magnification of each figure, except those from Clarke, is
indicated below.

The photographs were made by the author, and were enlarged for
reproduction by the photographic department of the Smithsonian
Institution. The other surface views were made, under the author’s
direction, by Miss C. M. Reese.

With the exception of Stage III., all of the figures of any one stage
are given the same number, followed where necessary by a distinguishing
letter, so that it is possible to tell at a glance which section and
surface views belong together. The transverse sections are all cut in
series from anterior to posterior.

FIGURE 1. Surface view of egg. × ²⁄₃.

1_a_. Egg with part of the shell removed to show the chalky band in
the shell membrane. × ²⁄₃.

FIGURES 2 and 2_a_. Dorsal and ventral views respectively of the
blastoderm before the formation of the notochord, medullary folds,
etc. After Clarke.

2_b_-2_f_. Transverse sections of an embryo of the age represented in
Figures 2 and 2_a_. × 43.

3 and 3_a_. Ventral and dorsal views respectively of an embryo a few
days older than that represented in Figures 2 and 2_a_. After Clarke.

3_b_-3_m_. Transverse sections of an embryo of the age shown in
Figures 3 and 3_a_. × 43.

FIGURES 3_n_ and 3_o_. Two sagittal sections of an embryo of the same
stage as Figures 3 and 3_a_. × 43.

4 and 4_a_. Dorsal and ventral views respectively of a slightly older
embryo than the one shown in Figures 3 and 3_a_. Figure 4_a_ shows
only the head region. After Clarke.

5 and 5_a_. Dorsal and ventral views respectively of an embryo of
almost the same age as the preceding, to show the further development
of the medullary folds. After Clarke.

FIGURE 6. Dorsal view of an embryo only a day or two older than the
preceding. After Clarke.

FIGURES 6_a_-6_i_. A series of transverse sections of this stage. ×
43.

FIGURES 7_a_-7_h_. A series of transverse sections of an embryo
slightly older than the one shown in Figures 4-6. × 43. (No surface
view of this stage is figured.)

8 and 8_a_. Dorsal and ventral views respectively of an embryo with
five pairs of mesoblastic somites. × 20. (Drawn by transmitted
light.)

8_b_ and 8_c_. Two sagittal sections of an embryo of this stage. × 43.

FIGURES 8_d_-8_j_. A series of transverse sections of the embryo
represented in Figures 8 and 8_a_. × 43.

9_a_-9_m_. A series of transverse sections of an embryo somewhat more
advanced in development than the one represented in the last series.
× 43.

FIGURES 10 and 10_a_. Dorsal and ventral views respectively of an
embryo with eight pairs of mesoblastic somites. × 20. (Drawn chiefly
by transmitted light.)

FIGURE 11. Dorsal view of an embryo with fourteen pairs of
mesoblastic somites. The area pellucida and the developing vascular
area are shown, the latter having a mottled appearance. The pushing
of the head under the blastoderm is also shown. × 20. (Drawn chiefly
by transmitted light.)

FIGURES 11_a_-11_k_. A series of transverse sections of an embryo of
this stage. × 43.

FIGURE 12. Dorsal view of an embryo with about seventeen pairs of
mesoblastic somites. Part of the area pellucida is represented. (Both
transmitted and reflected light were used in making the drawing.) ×
13.

FIGURES 12_a_-12_g_. A series of transverse sections of an embryo of
this stage. × 43.

FIGURE 13. Surface view of an embryo with about twenty pairs of
mesoblastic somites. × (about) 15. (Drawn with both reflected and
transmitted light.)

FIGURES 13_a_-13_f_. A series of transverse sections of an embryo
slightly more developed than the one shown in Figure 13. × 20.

FIGURE 13_g_. A sagittal section of an embryo of about the age of the
one represented in Figure 13. × 20.

14. Head of an embryo with one pair of gill clefts; ventro-lateral
view. × 13.

15. Profile view of the head of an embryo with three pairs of gill
clefts. × 13.

FIGURES 15_a_-15_e_. A series of transverse sections of an embryo of
about the age of the one represented in Figure 15. × 20.

FIGURE 15_f_. A horizontal section through the anterior region of an
embryo of the age of that shown in Figure 15. × 20.

16. Surface view in profile of an embryo with four pairs of gill
clefts. × (about) 12.

FIGURES 16_a_-16_f_. A series of transverse sections of an embryo of
the approximate age of the one represented in Figure 16. × 20.

FIGURE 16_g_. A sagittal section of an embryo of the age (possibly
slightly younger) of the one represented in Figure 16. × 20.

17. Surface view in profile of an embryo at the time of origin of the
limbs. × (about) 5.

FIGURES 17_a_-17_g_. A series of transverse sections of an embryo of
the age of the one represented in Figure 17. × 7.

FIGURE 18. Surface view in profile of the head of an embryo slightly
larger than, though of about the same state of development as, the
one represented in Figure 17. Reproduced here chiefly to show the
gill clefts. × (about) 3.

19. Surface view of an embryo somewhat more developed than the one
just described. × (about) 3.

FIGURE 20. Surface view of an embryo older than the one represented
in Figure 19; with well-developed manus and pes. × (about) 5.

FIGURES 20_a_-20_j_. A series of transverse sections of an embryo of
the age of the one represented in Figure 20. × 7.

FIGURE 21. A photograph of a living embryo in the egg, showing the
allantois, yolk mass, etc. The embryo is somewhat more developed than
the one shown in Figure 20. × ²⁄₃.

22. A photograph of a still larger embryo, removed from the shell and
freed from the fetal membranes. × (about) 1.

23. A photograph of a still more advanced embryo, in which the digits
are quite evident and the scales are beginning to show. × (about) 1.

23_a_. A sagittal section of an embryo of the age of the one
represented in Figure 23; the tail has not been shown in this figure.
× (about) 3.

23_b_. A vertical section through the head of an embryo of about the
size (perhaps slightly smaller) of the one shown in Figure 23. ×
(about) 3.

24. A photograph of an older embryo in which the pigmentation of the
scales is evident, though not shown in the figure. × (about) 1.

25. A photograph of an embryo in which the pigmentation and the
development of the body form are practically complete. The
allantois, unabsorbed yolk, etc., have been removed. × (about) ³⁄₄.

26. A photograph of a just-hatched alligator, of an alligator egg,
and of a young alligator in the egg just before hatching. × (about)
³⁄₇.

[Illustration: PLATE VI. _1_, _1a_, THE EGG; _2_, _2a_, STAGE I.]

[Illustration: PLATE VII. _2b_-_2f_, STAGE I. _3_, _3b_, STAGE II.]

[Illustration: PLATE VIII. _3a_-_3g_, STAGE II.]

[Illustration: PLATE IX. _3h_-_3o_, STAGE II.]

[Illustration: PLATE X. STAGE III.]

[Illustration: PLATE XI. _6_-_6i_, STAGE III; _7a_, STAGE IV.]

[Illustration: PLATE XII. _7b_-_7h_, STAGE IV; _8_, _8a_, STAGE V.]

[Illustration: PLATE XIII. STAGE V.]

[Illustration: PLATE XIV. _8i_-_8j_, STAGE V; _9a_-_9g_, STAGE VI.]

[Illustration: PLATE XV. _9h_-_9m_, STAGE VI; _10_, STAGE VII.]

[Illustration: PLATE XVI. _10a_, STAGE VII; _11a_-_11d_, STAGE VIII.]

[Illustration: PLATE XVII. STAGE VIII.]

[Illustration: PLATE XVIII. _11i_-_11k_, STAGE VIII; _12_-_12b_, STAGE
IX.]

[Illustration: PLATE XIX. _12c_-_12g_, STAGE IX; _13a_-_13c_, STAGE
X.]

[Illustration: PLATE XX. STAGE X.]

[Illustration: PLATE XXI. _13g_, STAGE X; _14_, STAGE XI; _15_-_15c_,
STAGE XII.]

[Illustration: PLATE XXII. _15d_-_15f_, STAGE XII; _16_-_16d_, STAGE
XIII.]

[Illustration: PLATE XXIII. _16c_-_16g_, STAGE XIII; _17_, STAGE XIV.]

[Illustration: PLATE XXIV. _17a_-_17g_, STAGE XIV; _18_, STAGE XV;
_19_, STAGE XVI.]

[Illustration: PLATE XXV. STAGE XVII.]

[Illustration: PLATE XXVI. STAGE XVII.]

[Illustration: PLATE XXVII. STAGE XVIII; _22_, STAGE XIX; _23_-_23b_,
STAGE XX; _24_, STAGE XXI.]

[Illustration: PLATE XXVIII. _25_, STAGE XXII, ALLIGATOR EMBRYO; _26_,
STAGE XXIII, ALLIGATOR JUST HATCHED AND RELATIVE SIZE OF EGG.]

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INDEX

Abdominal aorta and branches, 212

Abdominal ribs, 80

Acetabulum, 85

Air chamber of egg, 228

Albumen of egg, 229, 230

Allantois, 299, 300, 328

Alligator, 6
abundance of, 8
American, 3
and cane rat, 28
and muskrat, 28
attack from, 14
bellowing of, 18
catching of, 34
cave of, 12
Chinese, 38
Cynocephalus, 110
daylight hunting of, 33
dealers in, 34
derivation of name of, 40
differs from crocodile, 7
digging from cave, 33
distribution of, 10
economic importance of, 26
eggs used as food, 35
feeding of, 12
fire hunting of, 32
habitat of, 8
hatching of, for sale, 35
hibernation of, 12, 13
hides, annual output of, 28
for card cases, etc., 30
chief centers for, 27
damaged in removal, 31
Floridian, 28, 29
highest priced, 32
length and width of, 30
from Louisiana, 28, 29, 30
methods of cutting, 31
Mexican, 29, 30
removal of, 31
salting of, 31
shipment of, 31
from South and Central America, 27, 28
from Southern States, 28
varieties of, 29
value of different sizes of, 28, 29
value to hunter of, 28
hole, 11
hunting, 32
Joe, 9
killing of, 33
for sport, 27
laws for protection of, 28
leather, first used, 26
for shoes, 27
imitation, 32
present use of, 32
meat, preparation of, 35
smoking of, 36
use as food, 35
mississippiensis, 3, 7
raw hides, selling of, 32
sale of live, 34
sinensis, 3, 16
the stuffing of, 34
swimming of, 13, 14
tanned hides, sale of, 32
teeth, sale of, 34
value of, 34
trail, 11, 13
unknown to ancients, 40
use of tail, 14
value of live, 34

Alligatoridæ, 1

Amnion, 236, 247, 251, 259, 266, 267, 268, 269, 270, 274, 275, 278,
290, 334

Ampullæ, 149

Ancestry, 4

Annulus tympanicus, 149

Aortic arches, 203, 296, 299

Appendages, development of, 308, 315, 317, 327, 328

Appendicular skeleton, 81

Area opaca, 233

Area pellucida, 233

Arkansas Alligator Farm, 201

Arterial system, 212
first reference to, 44

Arteries, of anterior region, 215, 216
brachial, 218
caudal, 216
cervical, 220
cloacal, 316
cœliac, 212
collateralis colli, 217, 219
common carotid, 219, 221
crural, 216
dorsal aorta, 212
fibular, 214
first hæmorrhoidal, 216
gastric, 212
gastro-hepatico-intestinal, 212
iliac, 214
inferior dental, 223
injection of, 201
internal carotid, 70, 221
internal mammary, 217
ischiadicæ, 214
lingual, 220
lumbar, 213
mandibular, 220
mesenteric, 213
œsophageal, 217
pancreo-intestinal, 212
pelvic, 214, 216
pleural, 217
of posterior region, 213
primary carotid, 203, 218, 220
pulmonary, 203
radial, 218
rectal, 216
right subclavian, 203, 216
sciatic, 214
second hæmorrhoidal, 216
spleno-intestinal, 212
subclavian, left, 219
subscapular, 217
superior dental, 223
thoracic, 218
thyroid, 217
tibial, 214
ulnar, 218
urogenital, 214
vertebral, 217

Arytenoid cartilage, 147

Atlantosaurus, 4

Atlas, 53

Auditory capsule, 72

Auditory vesicle, 274, 277, 286, 294, 297, 302, 309, 320, 322

Auricles, 204

Axis, 54

Bartram’s account, 8

Basilar plate, 331

Belly skin, 31

Belodon, 5

Bile duct, 154

Bird and crocodile, 40, 41

Blastopore, 233, 234, 235, 236, 240, 246, 249, 250, 252, 257, 263,
264, 272

Body cavity, development, 279, 281, 287

Body flexure, 307, 317, 318

Bones, alisphenoid, 68
angular, 76
of anterior limb, 82
articular, 75
basilingual plate, 76
basioccipital, 67, 68, 72
basisphenoid, 67
calcaneum, 88
centrale, 84
clavicle, 82
coracoid, 81, 82
coronoid, 75
dentary, 74
epiotic, 73
epipubis, 86
episternum, 82
exoccipital, 70
fibula, 87
fibulare, see calcaneum
of foot, 88
frontal, 60
humerus, 82
hyoid, 76
ilium, 84
integumental, 50
interclavicle, 81
ischium, 85
jugal, 62, 67
lachrymal, 62
malar, 62
maxilla, 61, 64
mesethmoid, 72
metacarpals, 84
nasal, 60
opisthotic, 73
palatine, 65
parietal, 59
of pelvic girdle, 84
pisiform, 83
of posterior limb, 84
postfrontal, 59
prefrontal, 60
premaxilla, 61, 63
pro-otic, 73
pterygoid, 66, 70
pubis, 86
quadrate, 62, 69
quadratojugal, 64, 67
radius, 83
scapula, 81
splenial, 75
squamosal, 59
supra-angular, 75
supraorbital, 62
suprascapula, 81
tarsalia, 88
tibia, 87
tibiale-centrale, 88
transpalatine, 66
ulna, 83
ulnare, 80
vomer, 72

Brain, 132

Breeding habits of alligator, 18

Bronchial rings, 199

Buttons, 27

Caiman, 36
of Amazon, 37
banded, 3
black, 3, 36
latirostris, 3
niger, 3, 7
palpebrosus, 3
round-nosed, 3
sclerops, 3
species of, 37
spectacled, 3, 37
teeth of, 36
trigonotus, 3
ventral armor of, 36

Capitulum of rib, 78

Carpus, 83

Cauda equina, 131

Cement, 65

Centrum, development of, 325

Cerebellum, 132, 133

Cerebral hemispheres, 132, 133, 134
development of, 302, 309, 310, 332

Cerebral peduncles, 133, 134

Cerebral vesicles, 266, 273, 319

Cervical cord, 132

Chalky band of egg, 229

Chewing muscles, 90

Chinese alligator, 3

Chorda, tympani, 137

Choroid, 147

Choroid fissure, 321, 322, 337

Clarke, S. F., 226, 227, 228, 230, 231, 232, 233, 236, 243, 247, 250,
274, 293, 317, 318

Classification, 1

Claws, 46, 84
development of, 333

Cleavage of mesoblast, 263

Clitoris, 196

Cloaca, 155
embryonic, 316, 327

Cloacal glands, 156

Cocoa, Fla., 27

Columella, 74, 149

Conjunctiva, 136

Conjunctival gland, 146

Conus arteriosus, 203

Copulation of crocodile, 195

Copulatory organs, 194

Cornea, 146
development of, 321

Corn marks, 29, 30

Cornua of hyoid, 76

Corpora cavernosa, 194

Cranial cartilages, 320

Cranial flexure, 273, 276, 283, 291

Cranial nerves, 132, 135, 302, 320, 325

Cranium, 58

Cricoid cartilage, 197

Crocodile--Crocodilus, 6
African, 39
caves of, 41
distribution of, 40
egg laying of, 41
held sacred, 40
in Madagascar, 40
mentioned by Herodotus, 40
Voeltzkow’s account, 40
American, 2, 37
colors of, 39
distribution of, 38
Ditmars’ experience, 38
range of, 37
cataphractus, 2
Cuban, 2, 39
derivation of name of, 40
Guatemala, 2
intermedius, 2, 39
johnstoni, 2
Madagascar, 2
eggs of, 228
hatching of, 41
nest of, 41
man-eating, 40, 43
marsh, or mugger, 42
moreletti, 2
Nile, 21, 39
niloticus, 2, 39
Orinoco, 2, 39
palustris, 3, 42
migration of, 42
porosus, 2
rhombiferus, 2, 39
robustus, 2
rough-backed, 3
salt-water, 2, 42
in captivity, 43
habitat, 43
size of, 42
skeleton of, 51
sharp-nosed, 2
Siamese, 2
swamp, 3

Crocodilia, 1

Crocodilidæ, 1, 2, 3

Deaths by crocodiles in Africa, 40
in India, 43

Deltoid ridge, 83

Dentine, 65

Dermal skeleton, 47

Diaphragm, 115

Digestive system, 152

Digestive tract, fixation of, 159
histology of, 189
outline of, 158

Digits, development of, 329, 332

Dinosauria, 4

Dorsal aorta, development of, 277, 278, 327

Dorsal fissure, 131, 132, 133

Dorsal shield, 47

Drum, 148, 149

Ductus Cuvieri, 312

Ear, 147, 148

Ectoderm, 233

Eggs, 227, 231
artificial incubation of, 24
incubation of, 22
number of, per nest, 21, 22, 23
shape of, 228
shell of, 228
size of, 25, 227, 228
taken from oviduct, 24
variation in size of, 26
weight of, 25

Elizabeth Thompson Science Fund, 226

_El lagarto_, 40

Embryo, development of, 231
earliest stages of, 232, 233
position of, 230
removal of, from egg, 231
stages:
I., 233;
II., 235;
III., 240;
IV., 247;
V., 249;
VI., 257;
VII., 266;
VIII., 267;
IX., 273;
X., 282;
XI., 293;
XII., 293;
XIII., 300;
XIV., 307;
XV., 316;
XVI., 317;
XVII., 318;
XVIII., 328;
XIX., 328;
XX., 329;
XXI, 333;
XXII., 334;
XXIII., 334

Embryology, summary of, 335

Enamel, 65

Endoskeleton, 50

Enteron, development of, 261, 262, 269, 271, 278, 287

Entoderm, 234

Epidermal skeleton, 46

Epiglottis, 197

Episternum, 81

Epitrichial cells, 48

Eustachian tube, 72, 149

Everglades, 10, 220

External auditory meatus, 70, 73, 148

External mandibular foramen, 75

Extracolumellar cartilage, 74

Eye, 144
glands of, 144

Eyeball, 146

Eyelids, 144
development of, 329, 332

Feeding of alligators, 15

Fenestra ovalis, 73

Fissura ventralis, 131

Food of alligators, 15

Foramen ovale of skull, 68

Forebrain, 274, 276, 284, 291, 294, 302, 308, 319

Foregut, 240, 248, 252, 297

Foreskin, 195

Fort Pierce, Fla., 27

Fourth ventricle, 132

Fronto-nasal region, 318

Fundic region of stomach, 152

Gastroliths, 44, 45, 153

Gavial, 6
food of, 44
Indian, distribution of, 43
eggs and nest of, 44
meaning of, 44

Gavialidæ, 1, 3

Gavialis gangeticus, 2
character of, 43, 44
size of, 43

Genital ducts, 156

Geographical distribution of Crocodilia, 6

Gescmackwärzchen, 165

Gharial, 44

Gill clefts, 277, 283, 285, 293, 294, 299, 301, 302, 303, 316, 317,
318, 323, 336

Gizzard, 153

Glans penis, 195

Glenoid cavity, 81

Glomeruli, 304

Glottis, development of, 324

Growth of alligators, 16

Hallux, 88

Harderian gland, 136, 145

Head-fold, 233, 236, 237

Heart, 202, 204
development of, 267, 270, 279, 283, 287, 297, 303, 310

Hindbrain, 277, 284, 294, 308, 319

Histology of enteron, 157
of integument, 48

Horn alligator, 31

Horny layer, 48

Hyoid, 151
development of, 330

Hyomandibular cleft, 286, 307, 309, 322

Hypophysis, 53, 133
development of, 306, 320, 331, 337

Incubation, period of, 25

Infundibulum, 133, 134
development of, 330

Integument, histology of, 48

Internal auditory meatus, 73

Internal mandibular foramen, 76

Intestine, 154
development of, 289

Iris, 146
development of, 321

Jacksonville, Fla., 34

Kidneys, 192

Kissimmee, Fla., 27

Labyrinth, 148

Lachrymal canal, 145

Lachrymal gland, 144, 145

Lake Kissimmee, 10

Lake Worth, 37

Large intestine, epithelium of, 187
histology of, 186
_see_ Rectum

Larynx, 197
development of, 323

Lateral disks of stomach, 153

Lateral ventricle, developing, 319

Laying season of alligator, 18

Lens, 147

Lens vesicle, 276, 284, 294, 302, 309, 321, 332

“Leviathan” of Book of Job, 40

Liver, 154
development of, 304, 307, 312, 326

Lower jaw, 74

Lungs, 199, 200
capillaries of, 200
development of, 303, 304, 312, 331

Mandible, 74

Mandibular fold, 296, 301, 308, 317, 318

Manus, 84, 317, 318, 329

Mating season of alligator, 19

Maxillary process, 301, 308, 317, 318

Meatus venosus, 312

Meckel’s cartilage, 330

Medulla, 132

Medullary canal, 254, 255, 258, 259, 260, 262, 267, 268, 269, 270,
271, 272, 275, 282, 291

Medullary folds, 250, 251, 253, 254, 256, 258, 265, 266, 291
origin of, 336

Medullary groove, 236, 237, 238, 241, 242, 244, 245, 248, 250, 253,
255, 256, 263, 265, 313, 334, 336, 337

Medullary plate, 249

Melbourne, Fla., 27

Mesentery, development of, 299

Mesoderm, 234

Metanephros, 332

Miami, Fla., 27

Midbrain, 275, 276

Middle ear, 149

Mouth, 150

Muscles:
abdominal, 112
ambiens, 118
anconæus, 104
atlanti-mastoideus, 96
capiti-sternalis, 96
caput coraco-scapulare, 105
caput humerale mediale, 106
caput humerale posticum, 105
caput humeri laterale, 105
caput scapulare laterale, 104
carpo-metacarpalis, 111
carpo-metacarpalis V., 111
carpo-phalangei, 109, 110
carpo-phalangeus, 111
carpo-phalangeus primus digiti V., 111
caudali-ilio-femoralis, 120
caudi-femoralis, 121
cerato-hyoideus, 93
cervicalis adscendens, 96
collo-capitis, 94
collo-occipitis, 96
collo-scapularis superficialis, 97
collo-squamosus, 95
collo-thoraci-suprascapularis profundus, 98
coraco-antebrachialis, 101
coraco-brachialis, 101
coraco-ceratoideus, 92
costo-coracoideus, 93, 99
costo-scapularis, 94
costo-vertebralis lateralis, 94
costo-vertebralis medialis, 94
deltoideus scapularis inferior, 103
diaphragmatic, 115
dorsalis scapulæ, 103
of dorsal neck region, 94
dorso-humeralis, 102
dorso-scapularis, 97
episterno-ceratoideus, 92
epistropheo-vertebralis, 95
extensor hallucis proprius, 129
extensor ilio-tibialis, 118
extensor longus digitorum, 125
of eyeball, 146
femoro-tibialis, 119
flexor digitorum brevis, 128
flexor longus digitorum, 127
flexor tibialis externus, 122
flexor tibialis internus, 122
of forearm, 107
gastrocnemius, 126
humero-antebrachialis inferior, 102
humero-carpi-radialis, 108
humero-carpi-ulnaris, 108
humero-metacarpalis, 108
humero-radialis, 106
humero-radialis brevis, 108
humero-radialis internus, 107
humero-radialis lateralis, 109
humero-radialis longus, 107
humero-radialis medialis, 109
humero-ulno-phalangei, 110
ilio-femoralis, 120
ilio-fibularis, 119
ilio-ischio-caudalis, 130
intercostales, 115
intermaxillaris, 91
interosseus cruris, 128
ischio-femoralis, 123
latus colli, 92
maxillo-coracoideus, 93
maxillo-hyoideus, 93
metacarpo-phalangeus, 111
metacarpo-phalangeus I., digiti V., 112
obliquus abdominis externus, 112
obliquus abdominis internus, 113
occipito-cervicalis medialis, 94
occipito-epistropheus, 96
occipito-maxillaris, 91
pectoralis, 100
pectoralis minor, 100
peroneus anterior, 125
pisiformi phalangeus primus digiti V., 111
of posterior appendages, 118
pterygo-maxillaris, 91
pubi-ischio-femoralis externus, 123
pubi-ischio-femoralis internus, 124.
pubi-ischio-femoralis posterior, 124
quadratus lumborum, 115
rectus abdominis, 113
rectus internus, 115
rectus lateralis, 114
rectus ventralis, 113
retractor oculi, 137
rhomboideus, 99
of scapula, 96
scapulo-humeralis profundus, 103
sphincter colli, 91
squamoso-cervicalis medialis, 95
sterno-atlanticus, 97
subscapularis, 104
supracoracoideus, 100
of tail, 129
temporalo-maxillaris, 90
teres major, 103
tibialis anticus, 125
tibialis posticus, 128
transversus abdominis, 113
ulno-carpi-radialis, 109
ulno-radialis, 107
of ventral side of neck, 91

Muscle plates, 289, 297, 311, 325

Musk glands, 156

Myocardium, development of, 305

Myocœl, 256, 280, 281, 282, 311

Nasal passages, 151

Nasal pit, 294, 307, 310, 317, 322, 330

Nephrostome, 290

Nerves
abducens, 133, 134, 135, 137
acoustic, 133, 137
alveolar branch, inferior, 136
axillaris, 141
brachialis longus inferior, 141
brachialis longus superior (radialis), 141
brachial plexus, distribution of, 140
coraco-brachialis, 141
crural and ischiadic plexuses, 142, 143
cutaneus brachii et antebrachialis medialis, 141
cutaneus pectoralis, 141
dorsalis scapulæ (posterior), 141
facial, 133, 137
frontal branch, 136
glossopharyngeal, 134, 137
hypoglossal, 68, 134, 138
latissimi dorsi, 141
nasal branch, 136
oculomotor, 68, 133, 134, 135
olfactory, 135
optic, 68, 134, 135
pectoralis, 141
pneumogastric, _see_ vagus
postsacral, 142, 143
presacral, 142, 143
sacral, 143
scapulo-humeralis profundus, 141
spinal (1-4), 138, 139, 140
subscapularis, 141
supracoracoideus, 140
teres major, 141
thoraci inferiores, 140
trigeminal, 68, 133, 135, 136, 137
trigeminal, inferior maxillary branch, 136
trigeminal, ophthalmic branch, 136
trigeminal, superior maxillary branch, 136
trochlear, 133
vagus, 68, 134, 137

Nervous epithelium of ear, 149

Nervous layer of ectoderm, 259, 268

Nervous system, 131

Nest of alligator, compactness of, 21
construction of, 21
form of, 21
location of, 20
size of, 21
temperature in, 24

Neural arches, development of, 325, 331

Neural groove, _see_ Medullary groove

Neurenteric canal, 264, 267, 272, 275, 282, 336

New York Zoölogical Park, crocodilians in, 39

Nictitating membrane, 144

Notochord, 236, 238, 245, 248, 249, 251, 255, 256, 260, 263, 266, 269,
270, 285, 295, 306, 335

Nuchal shield, 47

Obex, 133

Oblique muscles, 146

Odontoid process, 52, 54

Œsophagus, 151, 152, 324, 330
cilia of, 174
epithelium of (feeding), 173
epithelium of (hibernating), 172
histology of, 168
transsections of (figures), 169, 170

Okefinokee, 10, 226

Olfactory bulb, 132, 133

Olfactory lobes, development of, 332

Olfactory tract, 132, 133, 134

Olivary enlargement of œsophagus, 151

Optic chiasma, 134
cup, 302, 309, 310, 321
lobes, 132, 133, 135
nerve, development of, 321
stalk, 302
tracts, 134
vesicle, 274, 276, 282, 294

Oral cavity, 150

Ora serrata, 147

Osteolæmus tetrapis, 3

Otic vesicle, _see_ Auditory vesicle

Outer ear, 148

Ova, 193

Ovary, 193

Oviducts, 156, 193, 194

Palm Beach, Fla., 9

Pancreas, 154
development of, 326

Papillæ of tongue, 150

Paraphysis, 132, 133
development of, 319, 320, 330

Pecten, 147, 321

Pectoral girdle, 81

Penis, shaft of, 194

Pericardium, 323

Periotic capsule, 73

Pes, 317, 318, 329

Petromyzon marinus, 157

Pharynx, development of, 277, 285, 292, 295, 299

Pigment, 333, 334

Pineal body, 132, 319

Pits in scales, 49

Pituitary body, _see_ Hypophysis

Plover and crocodile, 40

Posterior cardinal vein, development of, 298

Postorbital bar, 60

Prickle cells, 49

Primitive groove, 246, 249, 250, 256, 265, 267

Primitive spinal column, 326

Primitive streak, 233, 240, 246, 249, 250, 256, 265, 267, 275

Procœlia, 1

Pulp cavity of tooth, 65

Pupil, 146

Recessus cavi tympani, 149

Recessus scalæ tympani, 149

Rectum, 155
transsection of (fig.), 186

Rectus muscles of eye, 146
development of, 332

Respiratory organs, 197, 200;
(fig.), 198

Rete Malpighii, 48

Retina, 147
development of, 321

Retractor oculi muscle, 146

Ribs, 77

Rima auditoria, 148

Ring muscle, 157

Roof of mouth, 165
covering of (fig.), 166
glands of, 167
papillæ of, 167

Saccus naso-lachrymalis, 146

Scales, development of, 333

Sclera, 146

Scutes, 47

Semicircular canals, 149

Semilunar valves of stomach, 154

Sexual characteristics, 19

Sexual maturity, 17

Shell membrane, 228

Shell-tooth, 334

Sinus venosus, 202

Size of alligator, 16
at hatching, 16

Skeleton, 46, 50

Skin, 87

Skull, 58
dorsal aspect of, 59
lateral aspect of, 67
posterior aspect of, 70
sagittal section of, 72
ventral aspect of, 63

Smaller part of stomach, 153

Small intestine, 154
histology of, 179, 186
mucosa of (fig.), 184
transsection of (figs.), 181, 182, 183, 185

Smithsonian Institution, 226, 227

Somatapleure, 279

Somites, 251, 252, 256, 266, 267, 274, 282, 292, 300

Special sense organs, 144

Spinal cord, 131, 290, 291, 298, 311, 325
development of, 298

Spinal ganglion, development of, 299, 311, 312, 314, 331

Spinal nerves, 138

Splanchnopleure, 279

Stenosauria, 1

Sternum, 77, 80

Stomach, 152, 153
development of, 305, 326
glands of, 177, 179
histology of, 174
transsection of (fig.), 176

Stomodæum, 317, 322

Stratum corneum, 48

Supratemporal fossa, 57

Sympathetic nerves, 314, 325, 337

Systemic arch, 203

Tail, 317, 318, 329

Tailfold, 274

Tapetum lucidum, 146

Tarsus, 88

Taste papillæ, 165

Tear dots, 145

Teeth, 47, 64

Teleosauria, 1

Temperature range in swamps, 23

Tendon Achilles, 126, 127

Testes, 194

Thoracic ribs, 78

Thyroid gland, development of, 78, 269

Tomistoma schlegeli, 2
skeleton of, 51

Tongue, 150
covering of (fig.), 160, 161
development of, 333
epithelium of, 161
glands of, 160, 162, 163, 164
histology of, 157
papillæ of, 160, 165

Tooth, development of, 333
socket of, 64
structure of, 65

Torsion of body, 274, 336

Trachea, 197
development of, 324, 330
rings of, 198, 199

Trigeminal foramen, 78

Tuberculum of rib, 78

Tympanic cavity, 70, 73, 148, 149

Umbilical stalk, 300, 308, 317, 318, 333, 334

Ureter, 156, 193
development of, 290

Urogenital organs, 192, 196

Valves of outer ear, 148

Vasa deferentia, 156

Vascular system, 201
lettering for, 224

Veins, of anterior region (fig.), 209
anterior vena cava, 208
axillary, 210
brachial, 210
caudal, 207
coronary, 202
external jugular, 211
femoral, 206
hepatic, 202, 205
hepatic portal, 205
iliac, 206
inferior dental, 211
internal epigastric, 206
internal jugular, 208
internal mammary, 208
ischiadic, 207
lingual, 211
mesenteric, 205
muscular, 211
pancreatic, 205
of pes, 207
postbrachial, 210
postcaval, 202, 204
of posterior region, 204
precaval, 202, 208
pulmonary, 203
radial, 211
rectal, 207
renal portal, 207
subclavian, 210
subscapular, 210
superior dental, 211
thoracic, 210
vertebral, 208

Velum palatinum, 151

Venous system, 204

Ventricle, 203

Vertebræ, cervical, 51, 52
caudal, 57
lumbar, 56
sacral, 56

Vertebral column, 50

Vertebrarterial canal, 78

Vitelline blood-vessels, 267, 283

Vitelline veins, 270

Vitreous humor, 332

Vocal cords, 197

Voeltzkow, A., 228, 230, 231, 232, 334

Voice of alligator, 18
before hatching, 25

Warts, 49

Wolffian body, 289, 299, 304, 305, 307, 314, 316, 326, 331, 336
ducts, 280, 281, 289, 299, 314, 316
ridge, 299, 306, 314
tubules, 306, 313, 331

Xiphisternal horns, 81

Yolk of egg, 230

Transcriber’s Notes

Language inconsistencies and errors in spelling, capitalisation and
hyphenation and in the translations of terms have been retained,
except as mentioned under Changes below. The textual differences
between the list of illustrations and the illustration captions have
not been standardised.

Page 123, paragraph Pubi-ischio-femoralis Externus: (Plate III., Fig.
1, p. is. f. int., Plate IV., Fig. 1, p. is. f. int.) should possibly
read (Plate III., Fig. 1, p. is. f. ext., Plate IV., Fig. 1, p. is.
f. ext.).

Page 273-274: ... to the region of the midbrain (v²) ...: v² does not
occur in Plates XVIII. or XIX.; ... at the side of the forebrain (v′)
...: v′ occurs in Plate XVII.

Some of the numbers in round brackets refer to the bibliography at
the end of the text. In some bibliographical references the source
document shows blank spaces rather than first names.

Changes made

Footnotes and illustrations have been moved out of text paragraphs;
some ditto marks have been replaced with the dittoed text.

The references to plates (for example those after page 130) have been
standardised regarding the use of commas and italicised abbreviations.

Some minor typographical and punctuation errors have been
corrected silently, as have some minor lay-out and typographical
inconsistencies.

Page vii, items 10 and 11: captions of individual photographs have
been added.

Page 3: repeated footnote marker [2] removed from last column of
Caiman niger.

Page 51: ... skeleton of Tomistoma schlegali ... changed to ...
skeleton of Tomistoma schlegeli ....

Page 72, caption with Fig. 24: supraangular changed to supra-angular.

Page 87, caption with Fig. 29: tb¹ changed to tb′ cf. illustration.

Page 107: entsprect changed to entsprecht.

Page 165: Gescmackwärzchen changed to Geschmackwärzchen.

Page 258: ... as shown in Figures 9 and 9b. changed to ... as shown
in Figures 9a and 9b.

Page 343, item 12: Das Skelet der Krokodiliner changed to Das Skelet
der Krokodilinen.

Page 345, item 36a: experimentallen changed to experimentellen; item
47: struttura del cruore changed to struttura del cuore.

Page 347, item 67: des Menchen changed to des Menschen; item 70:
Auricular höcker changed to Auricularhöcker; item 80: Het. Institute
changed to Het Instituut.

Index: some spellings corrected to reflect the spelling as used
in the text (coranoid to coronoid, Cyanocephalus to Cynocephalus,
Gescmackwärzchen to Geschmackwärzchen).

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