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Marine Biological Laboratory Library
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Presented by
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January 9, 1964
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The
Alligator and Its Allies
By
Albert M. Reese, Ph.D.
Professor of Zoology 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
Tbe Knickerbocker Press
IQI5
COPYRIGHT, I915
BY
ALBERT M. REESE
The Knickerbocker Press, Hew Work
PREFACE
HE purpose of this volume is to bring
together, in convenient form for the use of
students of zodlogy, 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 trans-
lation 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.
baby
iv Preface
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.
MorGAntTown, W. VA.
May 1, 1915.
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Fic. 14. SKULL OF
GAVIAL.
(Lateral View.) «
Fic. 13. SKULL OF GAVIAL.
(Dorsal View.)
(Photograph from U. S. National
iVineoum) (Photograph from U. S.
National Museum.)
The Biology of the Crocodilia A3
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.
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 IgI10o.
In captivity it is savage and untamable. Dit-
mars, 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 ex-
tremely 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
44 The Alligator and Its Allies
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.» (Ele «says: Thesyoune ine wim
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. JI 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
The Biology of the Crocodilia 45
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 Monar-
des relateth.”
CHAPTER ai
THE SKELETON
A. THE EXOSKELETON
HE 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 arrange-
ment; 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 ele-
vated 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.
46
UNDER-SURFACE AND HORN-BACK.
ALLIGATOR SKINS;
nS)
(From Report of Commissioner of Fish and Fisheries, 1902.)
Fic.
The Skeleton 47
The dermal exoskeleton consists of bony scutes
that underlie the epidermal scales of the dorsal sur-
face 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 pit-
ted. 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 deriva-
tions 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
48 The Alligator and Its Allies
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
les 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 explana-
tion of their occurrence.
In the epidermis of young and half-grown ani-
mals 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 flat-
tened 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
The Skeleton AQ
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 differ-
entiated 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 cer-
tain 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 inter-
cellular 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
4
50 The Alligator and Its Allies
are near the hinder border of the scales. Only
occasionally are two pits
foundinonescale. These
pits are found in the gav-
ials but are absent in
some, probably all, alli-
cavors. “A small) knob
projects’ from the /cen-
ter of some of the pits.
These pits are not open-
ings of glands but have
about the same structure
as) the pits seen: mrthe
head.
ihe 1nvecumental
bones in the Crocodilia
originate in the connec-
tive 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.
Sc, scapula;
Sa, sacral region; Ri, ribs;
D
ey By
(From Claus & Sedgwick.)
L
' bige C)
iY
s
5 ye Y
Sa apn gi S e'
D, dorsal region; L, lumbar region;
H, humerus: R, radius; U, ulpa; Sta, sternum abdominale: Fe, femur; T, tibia; J, ischium;
Cc
\
C, caudal vertebre.
F
4
B. THE ENDOSKELETON
I. The Vertebral Column. SN
The vertebral column
A RICAT Pansists of about sixty-
~~ ve ¥
4 V4 + \
é geo "ONT
CSy2 \C
=~ :
Fic. 16.—Skeleton of Crocodile.
j weer
| LIBRARY
\ MiiGss
The Skeleton 51
five vertebrae, which may be separated into the
usual regions; there are nine cervical, ten dorsal, five
lumbar, twosacral, and about thirty-ninecaudal. It
is likely that the number of caudals may be sub-
ject to frequent variation; one complete skeleton
had sixty-five vertebre in all, another had sixty-
eight. A complete skeleton of the crocodile (species
not known) had sixty vertebrae. A thirteen-foot
skeleton at Western Reserve University had only
sixty-one vertebre, but some of the caudals were
evidently missing. Two skeletons of C. porosus in
the museum at Singapore had sixty and sixty-three
vertebre respectively. A skeleton of Tomzstoma
schlegali in the same museum had sixty vertebre.
The Cervical Vertebre. Since all of the cervical
vertebrz bear ribs, we shall assume the distinction
between them and the dorsal vertebre to be that
the ribs of the latter meet the sternum, while those
of the former do not reach to the sternum. As-
suming this distinction, there are, as was said
above, nine cervical vertebre.
With the exception of the first two, to be dis-
cussed 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,
52 The Alligator and Its Allies
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
Fic. 17. First FouR CERVICAL VERTEBR& OF A CROCODILE
(C. vulgaris). (From Reynolds, partly after Von Zittel.)
1. pro-atlas. 7. tubercular portion of fourth
2. lateral portion of atlas. cervical rib.
3. odontoid process. 8. first cervical rib.
4. ventral portion of atlas. 9. second cervical rib.
5. neural spine of axis. 10. convex posterior surface of cen-
6. postzygapophysis of fourth trum of fourth vertebra. ,
vertebra.
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. Pro-
jecting 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
The Skeleton 53
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. Pro-
jecting 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
54 The Alligator and Its Allies
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 fol-
lowing 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 dis-
tinctly visible in a lateral view of the neck (Fig. 17,
3). Like the rest of the cervical vertebre the poste-
rior 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-pos-
terior direction) neural spine. ‘The lateral pro-
cesses and those bearing the prezygapophyses are
also less strongly developed than on the following
vertebre.
The Thoracic Vertebre. The first thoracic verte-
bra 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 vertebre. The ribs
of the first two thoracic vertebrze articulate with
them by two processes, as in the typical cervical
vertebree; the other ribs articulate only with the
transverse process. The fourth thoracic may be
The Skeleton 55)
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 vertebre and has no
process for articulation with the head of the rib,
otherwise it is essentially the same. Like all of
Fic. 18. ANTERIOR VIEW oF A, A LATE THORACIC AND B, THE
First SACRAL VERTEBRA OF A YOUNG CROCODILE
(C. palustris). 3. (After Reynolds.)
neural spine. 4. sacral rib.
. process bearing prezygapophysis. 5. surface which is united with
. facet for articulation with the the ilium.
capitulum of the rib. 6. concave anterior face of centrum.
the vertebrae 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
vertebree are increasingly broad and truncated.
The transverse processes are very broad, long and
thin, and in the third to eighth vertebre they have
56 The Alligator and Its Allies
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. Uhese two surfaces ap-
proach each other as the vertebre are followed
caudad until, in the last two thoracic vertebre,
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 interverte-
bral foramina are smaller and more nearly circular
than in the cervical region, and are more closely
surrounded by bone.
The Lumbar Vertebre. The five lumbar vertebre
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 vertebrae has a small surface for articulation
with the antero-medial border of the transverse
process of the first sacral vertebra.
The Sacral Vertebre (Fig. 18, B). These are
two in number. The centrum of the first is con-
cave 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
The Skeleton 57
and neural arch, is a heavy bone shaped like a
truncated pyramid (4); the base of the pyra-
mid is ankylosed with the iliuw. These bones
seem to be much thickened transverse processes,
but since they are not completely fused with
their respective vertebre and are said to ossify
separately they should probably be called sacral
ribs. The two sacral vertebree do not seem to
be any more closely united than are any other two
vertebre.
The Caudal Vertebre (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 vertebre 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 vertebra. Towards the tip of the tail the
dorsal spines diminish in height and finally dis-
appear. The transverse processes of the first five
58 The Alligator and Its Allies
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 vertebre, but towardsthe 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 vertebre consisting
only of the centra.
tio” hesSeuly:
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 articula-
tion 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, Asa matter of convenience the
The Skeleton 59
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) ae AG 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 cra-
nial 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 fosse (sf).
On each side of the parietal and forming the
posterior corners 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 supratem-
poral 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 medi-
ally with the parietal and frontal, and ven-
trally with the alisphenoid and a small part of
the quadrate. It sends, in a ventro-lateral direc-
60 The Alligator and Its Allies
tion, a thick process that unites with a similar
process from the jugal to form the postorbital bar
(pb) which lies between the orbit (0) and the tem-
poral 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 em-
bryo. It is a heavy bone whose dorsal surface is
flattened posteriorly, deeply concave in the middle
region, and drawn out into a long projection anteri-
orly. 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.
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
Fic. 19, DorsAL VIEW OF THE SKULL OF THE
ALLIGATOR (A.
premaxilla.
maxilla.
lachrymal.
prefrontal.
jugal.
postfrontal.
squamosal.
quadrate.
Mississippiensis).
12. quadratojugal.
23. parictal.
24. frontal.
25. nasal. an,anterior nares;
0, orbit; pb, postorbital
bar; sf, supratemporal
fossa; if, lateral tem-
poral fossa.
The Skeleton 61
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 pre-
frontals 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 premaxillz
on the ventral side is the large anterior palatine
foramen.
62 The Alligator and Its Allies
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 guadratojugal (12) is a small bone, wedged in
between the jugal in front and the quadrate behind.
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
FIG. 20. VENTRAL VIEW OF THE SKULL OF THE
ALLIGATOR (A. Mississippiensis).
maxilla. 14. basioccipital. a, anterior
jugal. palatine vacuity; eu,
quadrate. opening of the median
palatine. Eustachian canal; pn,
pterygoid. posterior nares; pv,
transpalatine. posterior palatine
quadratojugal. vacuity.
The Skeleton 63
quadrate articulates with the quadratojugal; me-
dially with the basisphenoid and exoccipital; dor-
sally 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 premaxille, the maxilla, 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 fora-
mina, in a row parallel to the curved outer margin
64 The Alligator and Its Allies
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 inter-
locking 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 ante-
rior ends of the palatines. The postero-lateral
extremity of the maxilla articulates with the trans-
palatine. 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
The Skeleton 65
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 longitud-
inal 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
2
66 The Alligator and Its Allies
the pterygoids behind to the maxille 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, pre-
frontals, and vomers.
The plterygoids (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 immedi-
ately opens, which lies in the pterygoids, is divided
by a number of transverse, vertical septa. Poste-
rior 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 lat-
eral 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 articu-
lating ventrally with the pterygoid and dorsally
with the maxilla, the jugal, and the postfrontal.
Fic. 20A. LONGITUDINAL SECTION OF THE JAW AND TooTH OF A CROCODILE.
(After Bronn.)
c, cement; d, dentine; e, enamel; », pulp, of functional tooth; c’, cement; d’, dentine; e’,
enamel, of rudimentary tooth; ¢’’, epidermis; k, bone of jaw.
The Skeleton 67
The bastoccipital (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). Aswill 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 connec-
tion with the quadrate, with whose posterior mar-
gin 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
68 The Alligator and Its Allies
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 or 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 tri-
geminal 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 oculo-
motor and abducens nerves. Projecting caudad
is seen the rounded condylar part of the basz-
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 hypo-
glossal 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
69
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70 The Alligator and Its Allies
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.
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 quadrato-
jugal (12). The dorsal margin is formed by the
edges of the parietal (23) and the squamosals (7).
Immediately below the parietal is the supra-
occipital (26); it is a small, triangular bone, articu-
lating 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 exoccipi-
tal 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
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The Skeleton
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72 The Alligator and Its Allies
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 bound-
ary; through it, according to Reynolds, pass the
seventh nerve and certain blood-vessels.
The bastoccipital (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 ven-
trally curved condyle, and of a broader, irregular
ventral portion, between which and the basisphe-
noid is the single opening of the Eustachian canals
(eu). Dorsally and laterally the basioccipital artic-
ulates 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 maxille, 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:
The Skeleton 72
“Three bones, the epzotic, opisthotic, and pro-
otic, together form the auditory or periotic capsule
of each side. They are wedged in between the lat-
eral 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 epzotic
similarly becomes united with the supraoccipital,
the pro-otic (Fig. 23, 7)—seen in longitudinal
section to be pierced by the prominent trzgeminal
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
ovalts.
‘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 open-
74 The Alligator and Its Allies
ing 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 quad-
rate, 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 con-
cave 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
quadrateand 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 Jow (Wigs” 21, 23, and 2a). Mie
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
The Skeleton 75
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 splenzal
(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) isasmall bone, the coronoid (Figs. 23
anche24e 23 Shion 2h, 21) sit articulates; with ihe
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 splen-
ial, 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 quad-
rate, and sends back the large process so charac-
teristic of the crocodilian skull. On the dorsal side
76 The Alligator and Its Allies
of this process is a concavity that looks like another
articular surface. Laterally the articular articu-
lates with the supra-angular; ventrally and poste-
riorly 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, poste-
rior end forms a part of the prominent process men-
tioned 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 ante-
riorly 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 ex-
panded ends are formed of cartilage. They pass
at first backwards and then upwards and inwards.
They are homologous with part of the first bran-
chial arches of Selachians.”’
The Skeleton a4
ITI. The Ribs and Sternum.
The Cervical Ribs. As noted above, all of the
cervical vertebrz possess ribs. The first rib,
Fic. 25. Hyoms oF AN ALLIGATOR (Caiman latirostris)
(TO THE LEFT) AND OF A GREEN TURTLE (Chelone midas) (To
THE RIGHT). XX. (Brit. Mus.) (After Reynolds.)
The cartilaginous portions are dotted,
1. basilingual plate or body of 3. first branchial arch (anterior
the hyoid. cornu),
2. hyoid arch., 4, second branchial arch (pos-
terior cornu).
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,
78 The Alligator and Its Allies
with a well-marked vertebrarterial canal between
them. The ventral surface or capitulum articu-
lates 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 sur-
faces, as described in connection with the thoracic
vertebrae. 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 inter-
The Skeleton 79
2
Fic. 26. STERNUM AND ASSOCIATED MEMBRANE BONES OF A CrRO-
CODILE (C. palustris). 4. (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. 4. abdominal splint rib.,
2. sternum, 5. xiphisternal horn,
3. sternal rib.
80 The Alligator and Its Allies
mediate portion is present in all but the tenth rib,
and wherever present, except m the ninth rib,
it articulates distally with the sternal portion.
The sternal portions extend medio-cephalad in a
direction atright 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.
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 arrange-
ment 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 super-
ficial 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 Skeleton 81
the cartilaginous sternum proper (2), the xiphi-
sternal horns (5), and the bony episternum or inter-
clavicle (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 supra-
6
82 The Alligator and Its Allies
scapula. 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 ar-
ticulates with the
scapula and is
thickened to form
the anterior border
of the glenoid cav-
ity. ts? imedian
end is attached to
the sternum. Near
its scapular articu-
lation there is a
well-marked fora-
men that passes
entirely through
tae bone. The
episternum (e) or
Fic. 27. PECTORAL GIRDLE AND interclavicle was
ANTERIOR Lims. ; ;
c, coracoid; ce, centrale; cl, claw; e, episternum; described mt Ons
radius, 7. edule, a) seme a amg ection | with) the
ge eluate. sternum and ribs.
There is no clavicle nor other coracoid elements.
The anterior limb consists of the usual parts,—
the upper arm, forearm, and manus. The hu-
merus (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
The Skeleton 83
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 (uw) 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 asa 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 hour-
glass 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,
84 The Alligator and Its Allies
irregular bone, articulating with the ulna and the
ulnare; it is apparently connected by a long liga-
ment 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
metacar pals (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 forma-
tion 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
The Skeleton 85
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
Fic. 28. PELVIS AND SACRUM OF AN ALLIGATOR (Caiman
latirostris). X34. (Brit. Mus.) (After Reynolds.)
1. ilium. 6. neural spines of sacral verte-|
2. ischium, brae.
3. true pubis. 7. symphysis ischii.
4. epipubis (so-called pubis). 8. process bearing prezygapo-)
5. acetabular foramen. physis.
acetabulum. Its outer and lower border has two
surfaces, the larger and more posterior articulating
with the ischium, the other with the cartilaginous
pubis.
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.
86 The Alligator and Its Allies
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 ele-
ment 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 epipubts (4) is a slightly arched bone, some-
what 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 men-
tioned above, it is called by Wiedersheim and
others the pubis. Near the center of the acetabu-
lum 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
The Skeleton 87
of a division into two articular surfaces. The
ventral side of the femur near the proximal end
shows a fairly distinct trochantal eee:
The shin or crus is made
up of two well-developed
bones, the ézbza (t) and jib-
ula (fb), the former being
somewhat longer and con-
siderably thicker than the
latter.
The tibia consists of a
cylindrical shaft with en-
largements of about equal
size at the ends. The
proximal end forms most
Gr etae Hknce jomt, the
distal end articulates with
a tarsal element said by
Reynolds to represent the
fused astragalus and cen-
trale, by Wiedersheim
called the astragalus, and
said to represent the
united tibiale, interme-
dium, and centrale (tb,
Fic, 29. POSTERIOR LIMB.
ca, calcaneum or fibulare; cl, claw;
f, femur; fb, fibula; ¢, tibia;
13, 14-5, tarsalia; tb, 1b%, tibiale-
centrale; 1V, V, 4th and 5th
metatarsals.
tb’). The fibula articulates by a small enlargement
at its proximal end with the femur, and by an enlarge-
ment of about equal size, at its distal end, with the
fibulare or caleaneum (ca), and with a small facet on
the above-mentioned tibiale-centrale element.
88 The Alligator and Its Allies
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 men-
tioned 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 meta-
tarsal, and with the distal tarsal element said by
Reynolds to represent the fourth and fifth tarsa-
lia. 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
(t*), the other (t**) the fourth and fifth. Wieders-
heim 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
The Skeleton 89
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 ait
THE MUSCLES
HE description of the muscles here given is
taken from Bronn (11), who, in turn,
largely follows Gadow. The animal de-
scribed is the crocodile, but while Bronn does not
indicate the species, it is probable that the differ-
ences between the various members of the Croco-
dilia 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.
90
The Muscles QI
Pterygo-maxillaris (Pterygoideus) (Pterygoid-
ien, Aeusser Fluigelmuskel, 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 ptery-
goid 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 tempo-
ralis is weakly developed.
Occtpito-maxillaris (Digastricus maxilla) (Nie-
derzieher 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 (Intermax-
illaire, Mylo-hyoideus, Zwischenkiefermuskel, La-
tissimus colli). This muscle consists chiefly of
transversely running fibers, and has in its middle
third a small, median, longitudinal raphe or apo-
neurosis. In the posterior part of the neck it
92 The Alligator and Its Allies
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 vertebre, 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-hy-
oideus). 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 cesophagus and at-
taches itself to about the middle of the backwardly
turned border of the horn of the hyoid of that side.
Episterno-ceratoideus (Niederzieher des Zungen-
beins, or Brustbeinzungenbeinmuskel, Sterno-hy-
oideus). A flat and fairly broad muscle which
The Muscles 93.
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 re-
duced 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, Hyomax-
illaris, Hyoglossus, Hyomandibularis, Mylo-hyoid-
eus posterior). This muscle arises, very thin, from
the mandibular symphysis, goes thence immedi-
ately 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
94 The Alligator and Its Allies
inserted on the ventral surface of the coracoid at
the boundary of the scapula.
Costo-scapularis (Collo-scapularis superficialis,
Levator scapulz 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). Origi-
nates 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 vertebre.
Collo-capitis (Rectus capitis anterior). rises,
as a rule, from the cervical centra, at times from
the second thoracic vertebra (Gavialis). It ex-
tends 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
Occtpito-cervicalis Medialis (Complexus cervicis,
Biventer cervicis, Zweibauchiger Strecker or Zwei-
The Muscles 95
bauchiger Nackenmuskel, Splenius capitis). It
springs, by separate points, from the dorsal pro-
cesses of the four anterior body vertebre and the
six posterior neck vertebree; it is convex on its
dorsal, weakly concave on its ventral surface; it
leads cephalad asa short, strong tendon by which it
is attached to the angle between the upper hinder
border of the skull, z.e. to the superior and lateral
occipital region.
Squamoso-cervicalis Medialis (Kopfbauchmuskel
[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 pro-
cesses of the two anterior and six posterior cervical
vertebre; 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, lat-
erad to the occipito-cervicalis medialis muscle.
Eptstropheo-vertebralis (Splenius colli). This
muscle springs from the spinous processes of the
most anterior three body vertebrze and the last
cervical vertebra; it receives fibers from the articu-
lar processes and intermediate parts of the six
posterior cervical vertebree and is inserted on the
second cervical vertebra.
Collo-squamosus (Splenius capitis, Nackenwar-
zenmuskel, Trachelo-mastoideus). Springs from
96 The Alligator and Its Allies
the upper transverse processes of the last three
neck vertebre, and, becoming tendinous, is in-
serted on the hinder border of the squamosal.
Collo-occipitis. Arises from the transverse pro-
cesses of the posterior five cervical vertebre,
extends directly forwards on the ribs of the verte-
bree, 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-exoc-
cipital, 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 in-
serted on the upper side of the five posterior cervi-
cal 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
1., Figs. 1 and 2) 'cst))\(upper end! of ther head
nodder,’’ sterno-mastoideus, anterior part of
The Muscles 97
sterno-mastoideus, anterior part of atlanti-mas-
toideus); (b) a posterior part or sterno-atlanticus
(Plate I., Figs. 1 and 2, cst?) (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 ante-
rior 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 scap-
ularis inferior muscle.
Collo-scapularits Superficialis (Plate I., Fig. 1,
cssp) (Levator scapulz superficialis, Levator scap-
ule, Heber des Schulterblatts, Acromio-trachélien,
Teil des Serratus magnus, Levator anguli scap-
ula). A considerable muscle on the side of the
7
98 The Alligator and Its Allies
neck. It arises from the tips of the ribs of the
first and second cervical vertebrae (where it is
fused with the sterno-atlanticus muscle), and
also from the transverse process of the third and
fourth cervical vertebree; it goes with diverging
fibers to the entire anterior border of the scapula.
Thoraci-scapularis Superficialis (Serratus super-
ficialis, Pectoralis minor, Hinterer Theil des inneren
grosseren Rtickwartsziehers, 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, homogen-
eous 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 scapule et serratus pro-
fundus, Serrati anteriores, Serratus anticus major,
Vorderer Theil des inneren grésseren Ruckwarts-
The Muscles 99
ziehers 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 supra-
scapula, except on its forward part, and is made up
of two layers—a superficial and a deep one. The
former layer (Fig. 3, cthspr') is weakly developed
and is composed of two or three thin, distinct
bundles, that extend from the ribs of the eighth,
ninth, and eleventh vertebre (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 vertebre; 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 vertebre; after a short course it
inserts itself on the antero-dorsal angle of the
suprascapula.
Costo-coracoideus (Plate I., Fig. 3, cc) (Sub-
clavius et Triangularis sterni and Levator secunde
superioris costz, Petit dentelé, Pectoralis minor,
Pectoralis). This is a broad muscle of considerable
100 The Alligator and Its Allies
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.
Pecioralis (Plate \., Kies. 1 and 2, p) “(Pec-
toralis 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, Schltssel-
beinhalfte, Theil der Schulterblatthalfte des Hebers
ee
The Muscles 101
des Armes, Obergratenmuskel, 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 scapu-
laris 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, b*)
(Biceps, Coracoideus, Langer Kopf des langen
Beugers, Langer Kopf des Biceps, Biceps humeri,
Biceps brachii, Coraco-radialis). A slender and
102 The Alligator and Its Allies
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 ausge-
hender Beuger, Portion of Brachizus). 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) (Latissi-
mus dorsi, Breiter Ruickenmuskel, Humero-dor-
salis). It springs as an aponeurosis from the back
at the level of the first four or five dorsal vertebre,
and passes, with converging fibers, cephalo-ven-
trad to unite with the teres major muscle; in
common with the latter it extends along the exten-
sor surface of the humerus to be inserted between
the lateral and median processes.
The Muscles 103
Dorsalis Scapule (Plate I., Fig. 1, dss) (Deltoides
scapularis superior, Unterer Theil des dusseren
schulterblattmuskels, Untergratenmuskel, Supra-
scapularis, 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. anconei, 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 Infra-
spinatus, Theil der Schulterhalfte des Hebers des
Armes, Theil der oberen [Schulterblatt-] Abtheil-
ung des Deltoideus, Zweiter Hebemuskel des Ober-
armes, 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. a,
shpr) (Teres minor, Erster Teres major, Scapulo-
humeralis). A small muscle that springs from
the posterior border of the lower third of the scap-
ula, and passes, with converging fibers, to its
insertion on the humerus just distal to the medial
process.
Teres Major (Grosser runder Muskel oder kleiner
Riuckwartszieher des Oberarmbein, Zweiter teres
104 The Alligator and Its Allies
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.
Anconeus. This strong muscle lies on the ex-
tensor side of the upper arm. It is made up of two
layers: the superficial comes from the pectoral gir-
dle 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 medi-
ale. These five heads of the anconzus 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, Gewohnlicher [ausserer] langer Kopf
des dreikopfigen Streckers, Portion scapulaire
externe du triceps-brachial, Erster langer Kopf
des Triceps, [Zweiter] abducirender vom Schulter-
gerust entstehender Kopf des Streckmuskels des
Vorderarmes, Triceps Nr. 1, Triceps longus).
The Muscles 105
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 scapule
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 dreikopfigen Streckers, Portion
scapulaire interne du triceps-brachial, Zweiter
langer Kopf des Triceps, [Erster] abducirender vom
schultergertist entstehender Kopf des Streck-
muskels des Vorderarmes, Triceps Nr. 2, Triceps
longus secundus). Arises by two distinct tendi-
nous 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, [Acus-
serer| kurzer Kopf des dreiképfigen Streckers, Por-
tion huméral externe du triceps brachial, Aeusserer
vom Humerus ausgehender Kopf des Streck-
muskels 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. a,
ahp) (Longissimum caput m. brachiei internum,
Theil des inneren [kurzen] Kopfes des dreiképfigen
poreckers, Theil des Triceps Nr. 3, Theil des
106 The Alligator and Its Allies
Triceps internus, Theil der Portion humérale
interne du triceps brachial, [Mittler] vom Humerus
ausgehender Kopf des Streckmuskels des Vor-
derarmes). Springs from the middle of the ex-
tensor 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 dreikopfigen Streckers, Theil der Portion
humérale interne du triceps brachial, [Innerer] vom
Humerus ausgehender Kopf des Streckmuskels
des" Vorderarmes,__Vheil des) Uiriceps Nao en!
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 Brachizeus).
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
The Muscles 107
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 Vorwartswender, Pronateur, Pronator teres,
Pronator quadratus, Oberflachlich gelegener, lan-
ger runder Einwartsdreher). 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 Prona-
tor quadratus entsprect). 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
Rutckwartswender, Supinator radii longus). Among
the Crocodilia this and the following muscle are
well developed. This one springs from the con-
108 The Alligator and Its Allies
dylus 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 Rtickwartswender, Ex-
tensor 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 com-
munis 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). Origi-
nates 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, 6) (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 con-
dylus externus humeri and divides, on reaching
the carpus, into three thin, flat tendons, which in
part fuse with the carpo-phalangei muscle, and in
The Muscles 109
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, Gemein-
schaftlicher 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 Ab-
zieher 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-
110 The Alligator and Its Allies
radiale and with a thin tendon to the metacarpal
bone of the thumb. Rtidinger was not able to
find this muscle in Alligator cynocephalus.
Carpo-phalange: (Plate II., Fig. 1, 4) (Flexor
digitorum communis brevis, Oberflachlicher ge-
meinschaftlicher Fingerbeuger, Fléchisseur sub-
lime, 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 liga-
mentum 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-phalanget (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,
The Muscles iat
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) (Ab-
ductor pollicis). Springs from the os carpi-radiale;
is inserted on the first phalanx of the thumb.
Carpo-metacarpalis I, (Plate I1., 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.
Pistformi-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 digitti V. (Plate IL.,
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
112 The Alligator and Its Allies
inserted on the proximal end of the first phalanx
of the fifth finger.
Metacarpo-phalangeus I. cet 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 exter-
nus, Obliquus externus +internus + Serrati, Oblique
descendens). Springs, with a flat prong, from the
uncinate processes of the true tis, thence a
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
vertebrae, 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 consider-
ably 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.
The Muscles mes
Obliquus Abdominis Internus (Petit oblique, Ob-
liquus 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 por-
tion, 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 prox-
imal ends of the dorsal ribs but does not reach the
centra of the vertebrze 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 + pyra-
midenférmiger Muskel, Pyramidalis, Rectus ab-
dominis+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
8
114 The Alligator and Its Allies
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 inscrip-
tiones 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.
The Muscles 115
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 dia-
phragmatic 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 pro-
portion to the strength of the ribs, of slight struc-
ture; 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, 2.e.,
at right angles. The internal muscles are espe-
cially well developed in the breast region and pass
over into the internal oblique muscle.
QOuadratus Lumborum (Carré des lombes, Vier-
eckiger Lendenmuskel, Psoas major). (cilsioa9qs
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SHOULDER MuscLEs OF CrocopiLus AcuTus. (From Bronn after
Furbringer.)
Fic. 1. SHOULDER MUSCLES AFTER REMOVAL OF THE SPHINCTER
CoL_L1 MUSCLE (sphr).
Fic. 2. SHOULDER MUSCLES AFTER REMOVAL OF THE SPHINCTER
CoLL_1 MUSCLE (sphc).
Fic. 3. Drep LAYER OF THE INNER SHOULDER MUSCLES AFTER
REMOVAL OF THE HUMERUS AND ITS MUSCULATURE AS WELL
AS THE COLLO-SCAPULARIS SUPERFICIALIS MUSCLES
(¢ssp) AND THORACI-SCAPULARIS SUPERFICIALIS
Muscles (thcsp).
Fic. 4. SHOULDER MUSCLES AFTER REMOVAL OF THE PARS SCAP-
ULARIS OF THE SUPRA-CORACO-SCAPULARIS (sps) AND OF
THE Biceps MUSCLE (0).
Fic. 5. DIFFERENT VIEW OF FIG. 4.
Fic. 6. SHOULDER MUSCLES AFTER REMOVAL OF THE Pars Cora-
COIDEA OF THE SUPRA-CORACO-SCAPULARIS (spc) AND DEL-
TOIDES SCAPULARIS SUPERIOR (dss) MUSCLES.
LETTERING FOR ALL FIGURES OF THIS PLATE.
acs, ahl, ahp, asl, coraco-scapular, humerale laterale, humerale posticum, and scap-
ulare laterale externum heads of the anconzeus muscle; b, coraco-antebrachialis
(biceps); c, coracoid; cbb, coraco-brachialis; cc, costo-coracoideus; Cl, clavicle;
cssp, collo-scapularis superficialis (levator scapule superficialis): cst, capiti-
sternalis (sterno-mastoideus) ; cthspr, collo-thoraci-scapularis profundus (levator
scapule and serratus profundus); cu, dorso-scapularis (cucullaris); dh, dorso-
humeralis (latissimus dorsi); dsi, deltoides scapularis inferior; dss, dorsalis
scapule (deltoides scapularis supeiior); 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-
coiaco-scapularis; sphc, sphincter colli; SpS, spina scapule; SS, suprascap-
ulare; St, sternum; Sia, anterior part of sternum: Stp, posterior part of
sternum: thssp, thoraci-scapularis superficialis (serratus superficialis); U, ulna:
Vs, Vo, 5th and 6th vertebre.
NERVES SHOWN IN THIS PLATE.
3a. thoracicus VII. muscular branch of the
7. posterior branch of the tho- supra-coracoideus.
racicus superior VII for Ig. pectoralis.
the collo-thoraci-supra- 21. brachialis longus inferior.
scapularis profundus and 29b. teres major.
rhomboideus muscles. 31. dorsalis scapule (posterior).
7a. proximal. 32. cutaneus brachii and anti-
7b. distal thoracicus superior brachii superior lateralis.
VII. 32a. humero-radialis.
10a. thoracicus inferior. 33. deltoides inferior, (25 and
I2. supra-coracoideus. 42), cutaneus brachii and
I5. integumental, (13 and 14), antebrachii medialis.
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PLATE TT:
Fics. 1-4. MUSCLES OF THE FOREARM OF THE ALLIGATOR.
EG: I.
Fic. 2.
iG:
(From Bronn.)
Fics. 5-7. (From Bronn after Rathke.)
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, humeor-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.
I-5 as in Fic. 1; a, humero-carpi-radialis; b, humero-metacarpalis III,
IV, V (extensor digitorum longus); c, humero-carpi-ulnaris; d, carpo-
phalangei (extensor digitorum brevis).
a, b, humero-ulno-phalangei (flexor digitorum communis profundus).
a, ulno-carpi-radialis; 6, ulna; c, humero-ulnaris-lateralis (flexor carpi
ulnaris); d, humero-radialis brevis (supinator brevis).
HEAD, NECK, AND A PART OF THE Bopy OF A CROCODILUS
VULGARIS. (Ventral View).
a, lower jaw; 6, upper jaw; c, arch of palate; d, fold of palate; h,
pterygoideus internus (Rathke); i, pterygoideus externus (Rathke);
k, lomgus colli (Rathke); m, rectus capitis anticus major (Rathke);
n, sterno-mastoideus (Rathke); 0, levator scapule (Rathke); », scalenus
(Rathke).
6. PART OF A SIMILAR PREPARATION OF C. RHOMBIFER.
a, the hindermost of the superior maxillary teeth; b, lower jaw; ¢,
wings of palate; d, pterygoid; e, quadrate; g, intertransversalis (Rathke);
h, trachelomastoideus (Rathke); 7, levator scapule (Rathke); k, longus
colli (Rathke); m, pterygoideus externus (Rathke); m, rectus capitis
anticus major (Rathke).
Fic. 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
scapule (Rathke); g, cervicalis adscendens (Rathke); h, longus colli
(Rathke); i, intertransversalis (Rathke); k, trachelomastoideus (Rathke);
l, biventer cervicis (Rathke); m, splenius colli (Rathke); m, splenius
capitis (Rathke).
iv —
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PuaTE III.
Fic. 1. MuscLeES OF THE POSTERIOR EXTREMITY OF ALLIGATOR
MISSISSIPPIENSIS. LEFT SIDE, VENTRAL (PLANTAR) SURFACE.
Fic. 2. THE SAME, DORSAL AND LATERAL SURFACES.
Fic. 3. A. MISSISSIPPIENSIS; THE TENDONS OF THE FLEXOR TIBI-
ALIS MUSCLE IN THEIR RELATION TO THE GASTROCNEMIUS
MuscLe. RicGHT LEG, INNER SURFACE. (FIGS. I-3
from Bronn, after Gadow.)
amb, ambiens; cap, ext, gastr, external head of gastrocnemius; cap, int, gastr, in-
ternal head of gastrocnemius; cd, fm, caudali-femoralis; cd, il, fem, caudi-ilio-
femoralis; ex, il, 1b, extensor ilio-tibialis; ext, 1 (long), dig, extensor longus
digitorum; fi, tb, ext, flexor tibialis externus; ji, tb, int, flexor tibialis internus;
jm, tb, femoro-tibialis; il, cd, ilio-caudalis; il, cost, ilio-costalis; il, jib, 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 an-
terior; peron, post, peroneus posterior; p, is, f, ext. pubi-ischio-femoralis
externus; », is, f, int, pubi-ischio-femoralis internus; p, is, f, post, pubi-
ischio-femoralis posterior; gudr, 1b, quadratus lumborum; rect, rectus abdo-
minis; 1ib, ant, tibialis anticus; lib, 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, 7s, ischium;
0, pb, pubis; o, cl, cloacal bone; pr, 1, pb, lateral process of pubis; pr, tr, trans-
verse process; sp, ant, il, anterior spine of ilium; Sy, », symphysis pubis;
Sy, is, symphysis of ischium; /b, is, tubercle of ischium.
cap mt gastr_ -
Fl tb mt
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Nis fant, -
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PLatTe IV. (From Bronn, after Gadow.)
Fic. 1. ALLIGATOR MIUSSISSIPPIENSIS. INNER SURFACE OF THE
Petvic ReGIon, Lert Sipe. THE Pusis, ISCHIUM, AND
VERTEBR2& ARE CUT THROUGH THE MEDIAN PLANE,
XXVIII, 28th VERTEBRA.
Fic. 2. HATTERIA PUNCTATA.
Fic. 3. A. MIssISSIPPIENSIS. THE DEEPEST MUSCLES ON THE
PLANTAR SURFACE OF THE LEFT HIND Foot. ROMAN
NuMERALS IX-XII, SHORT ToE MUSCLEs.
Fic. 4. A. MIssIssIpPIENSIS. Lert LEG FROM THE POSTERO-MESIAL
Aspect. THE PLANTAR FLEXOR MUSCULATURE IN SITU,
AFTER REMOVAL OF THE GASTROCNEMIUS MUSCLE
AND THE ASSOCIATED MuscLes. ROMAN
NumeErRALs, VI-X, SHORT TOE
MUSCLES.
amb, ambiens; cap, ext, gastr, external head of gastrocnemius; cap, int, gasir, in-
ternal head of gastrocnemius; cd, fm, caudali-femoralis; cd, il, fem, caudi-ilio-
femoralis; ex, il, tb, extensor ilio-tibialis; ext, 1 (long), dig, extensor longus
digitorum; jl, tb, ext, flexor tibialis externus; Jl, 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 an-
terior; peron, post,|peroneus posterior; p, is, f, ext.. pubi-ischio-femoralis
externus; p, is, f, int, pubi-ischio-femoralis internus; p, 1s, f, post, pubi-
ischio-femoralis posterior; qudr, 1b, quadratus lumborum; rect, rectus abdo-
minis; /ib, ant, tibialis anticus; lib, post, tibialis posticus; trans, transversus
abdominis; itr, per, transversus perinei; m, post, il, posterior border of ilium;
ob, foramen in pubis for the obturator nerve; o, il, illum; 0, ts, ischium;
0, pb, pubis; o, cl, cloacal bone; pr, /, pb, lateral process of pubis; pr, iv, trans-
verse process; sp, ant, il, anterior spine of ilium; Sy, », symphysis pubis;
Sy, is, symphysis of ischium; /b, és, tubercle of ischium.
-transv
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PLATE V. (From Bronn, after Gadow.)
Fic. 1. HATTERIA PuncTATA. (Ventral View.)
Fic. 2. ALLIGATOR MissIssIPPIENsIs. Lert POSTERIOR EXTREMITY;
Foot IN PRONATION, HENCE SEEN FROM THE DorsAL SIDE.
lg, t, f, LIGAMENTUM TIBIO-FIBULARE.
Fic. 3. A. MissIssIpPIENSIS. MUSCLES OF THE DorsAL SURFACE
OF THE LOWER LEG AND Foor,
amb, ambiens; cap, ext, gastr, external head of gastrocnemius; cap, int, gastr, in-
ternal head of gastrocnemius; cd, fm, caudali-femoralis; cd, il, fem, caudi-ilio-
femoralis; ex, il, tb, extensor ilio-tibialis; ext, 1 (long), dig, extensor longus
digitorum; ji, 1b, ext, flexor tibialis externus; fl, tb, int, flexor tibialis internus;
jm, 1b, femoro-tibialis; il, cd, ilio-caudalis; il, cost, ilio-costalis; il, fib, ilio-
fibularis; il, fm (1, 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, 1b, pubi-tibialis; peron, ant, peroneus an-
terior; peron, post, peroneus posterior; p, is, f, ext. pubi-ischio-femoralis
externus; , is, f, int, pubi-ischio-femoralis internus; p, is, f, post, pubi-
ischio-femoralis posterior; gudv, 1b, quadratus lumborum; rect, rectus abdo-
minis; 11b, ant, tibialis anticus; tib, post, tibialis posticus; trans, transversus
abdominis; iv, per, transversus perinei; m, post, il, posterior border of ilium;
ob, foramen in pubis for the obturator nerve; 0, il, ilium; 0, is, ischium;
0, pb, pubis; o, cl, cloacal bone; pr, 1, pb, lateral process of pubis; pr, ir, trans-
verse process; sp, ant, il, anterior spine of ilium; Sy, », symphysis pubis;
Sy, is, symphysis of ischium; ib, is, tubercle of ischium.
t----Cap ext gastr
p— Peron post
peron.post— -
cap mt gastr - - - §
flew long. dig- ----
CHAPTER IV
THE NERVOUS SYSTEM
SPINAL CORD
HE spinal cord extends the whole length
AR of the vertebral canal and ends near the end
on tae: tail asa. thin, round! thread. “tt
varies in thickness and shape in cross section, being
nearly always elliptical, but at places approach-
ing 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,
131
132 The Alligator and Its Allies
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 longi-
tudinal. The tapering, cephalic end of each hemi-
sphere 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 cere-
bellum, HH, somewhat elliptical in outline as seen
. from above.
Extending caudad from beneath the cere: ellum
is the medulla, NH, with its triangular fourth
ventricle. The outlines of the medulla are some-
what obscured by the numerous roots of the
eighth to eleventh cranial nerves, VIII-XI, which
arise along its dorsal border. The medulla, as was
Fic. 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; J—XJI, cranial nerves; I, 2, first and second
spinal nerves; B.ol, olfactory bulb; Tyo, olfactory tract; G.p, para-
physis; Jnf, infundibulum; Hyp, hypophysis; Med, spinal cord.
The Nervous System 133
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.
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 plam. Beneath it
and extending forwards are the prominent optic
nerve, Dl and tract. Caudad) to the latter and
projecting ventrad and caudad are the infundibu-
lum, 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 dis-
tance 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
134 The Alligator and Its Allies
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. Ven-
tral 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, I 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 chi-
asma 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 ab-
ducens 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 ori-
The Nervous System 135
gins 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 cere-
bral 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 de-
scribed 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 discus-
sion 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
136 The Alligator and Its Allies
main divisions: (1) the ophthalmic branch, (2)
the superior maxillarv branch, and (3) the inferior
maxillary branch. (1) Phe: ophthatane> im fur
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 maxil-
lary 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 mnerves—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,
The Nervous System 7
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 trigemi-
nal nerve, (b) and (c) connecting with the trigem-
inal 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 cesophagus, (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, cesoph-
138 The Alligator and Its Allies
agus, and trachea; (3) goes to the cesophagus; (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 in-
to three twigs of which the first two lead to the
hyomaxillary and sterno-maxillary muscles respec-
tively, 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,
The Nervous System 139
and sends a small branch to the levator scapulz
superficialis muscle.
Vi The ventral branehwyor this, nerve sends
branches to the ventral muscles of the neck, to the
levator scapulz 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 scapulz superficialis muscle and to the
most anterior part of the collo-thoraci-supra-
scapularis 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 ven-
tral 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
140 The Alligator and Its Allies
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-coracoid-
eus 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.
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
Fic. 31.
TUS.
VII-XI. ventral
branches of
seventh to
eleventh
SD 7 sal ag ll
nerves.
3a. thoracicus
anterior
WAL
4. thoracicus
superior V.
Fo UDO RA CLOTS
superior
VI.
7a. proximally-
leading
thoracicus
superior.
7b. distally-lead-
ing thoraci-
cus superior
VIII.
g. thoracicus
superiorIX.
104, 101, 1042, 1003.
thoracicus
inferior.
18. cutaneuspec-
toralis.
19. pectoralis.
20 brachralis
longus in-
ferior.
BRACHIAL PLEXUS OF C. ACU-
(From Bronn, after Furbringer.)
22. Coraco-bra-
chialis.
22c. branchforthe
distal belly
of biceps
muscle.
2A US Catieleayets
branch for
thehumero-
antebrachi-
alisinferior.
(25+42). cutaneus
brachii and
antebrachii
medialis.
29. subscapu-
laris.
31. dorsalis scap-
ule (pos-
terior).
32. cutaneusbra-
chi1 superi-
or lateralis.
33. deltoides in-
ferior.
34. brachialis
longus su-
perior.
36. anconeus.
36a. scapulo-hu-
meralis pro-
fundus.
The Nervous System 141
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; (ec) coraco-
brachialis (22) to the like named muscle; (f) cutan-
eus 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 scapule (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
142
FIG.
SIDE OF A. MISSISSIPPIENSIS.
FAR AS THEIR ENTRANCE INTO THE MUSCLES.
IS MADE UP OF THE PRESACRAL STEMS 4d, J, c.
1s Bumi_t oF Two BRANCHES FROM STEMS A & B.
mab
CRURAL PLEXUS AND ISCHIADIC PLEXUS OF THE LEFT
THE NERVE BRANCHES ARE SHOWN AS
THE CRURAL PLEXUS
THE OBTURATOR NERVE
(FROM BRONN, AFTER
Bes
Gapow.)
SSVOOQMLE
ANE
BY
a, b,c. presacral nerves.
a& 6. postsacral nerves.
sacral nerve (26th
spinal nerve).
to extensor ileo-tibialis muscle.
to femoro-tibialis muscle.
to ileo-fibularis muscle.
to ileo-femoralis muscle.
to caudi-ileo-femoralis
cle.
caudi-femoralis muscle,
mus-
taud. Jat.
8.
The Alligator and Its Allies
s-
XXVI
to flexor tibialis externus
muscle.
to flexor tibialis internus
muscle.
to ischio-femoralis muscle.
pubo -ischio-femoralis in-
ternus. :
pubo - ischio- femoralis ex-
ternus muscle.
to pubo-ischio-femoralis pos-
terior muscle.
The Nervous System 143
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.
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 (¢);
the second postsacral shown in the figure appar-
ently 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
Bietiess2— — lhe: larce muscles, or the taily are
innervated by the regular, metameric nerves of
that region, and since there are usually thirty-nine
caudal vertebre, there are probably about that
many pairs of caudal nerves, although the last few
vertebrzee and the muscles of that region are so
small itvmmayewertaat some Ofetie imerves are
lacking. LORGIL
~~
144 The Alligator and Its Allies
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. ae
Lad |
‘br’ H vi. \
jpla \\ (
ised.
a Si ba co capr
} oe “ ,
vod a7 Heort 2 (29
Fic. 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.
tibial side of the shank. These two arteries give
off numerous branches to the muscles of the lower
216 The Alligator and Its Allies
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 tar-
sus it divides rather suddenly and, perhaps, vari-
ably, into four chief branches, leading to the toes.
A short distance caudad to the origin of the iliacs
the dorsal aorta gives off a pair of small pelvic arter-
ies, pat, going to the muscles of that region. Cau-
dal to these pelvic arteries is given off the unpaired
first hemorrhoidal artery, het, which divides into a
rectal, tt>, and a.cloocal,.cl, branch:
Caudal to the first hamorrhoidal arises the second
hemorrhoidal, he*?; also unpaired, leading to the
cloaca.
Posterior to the second hemorrhoidal, the aorta
continues into the tail as the large caudal artery, ca.
The Anterior Arteries. The origin of the great ar-
terial 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 deseribed, so that it now re-
mains to describe the distribution of the right sub-
clavian, 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 aris-
ing 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
The Vascular System 217,
wsophageal artery, oe, a small, caudally directed
vessel carrying blood to the posterior region of the
cesophagus. Close to the cesophageal arises an-
other 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 cesophagus, 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, im* (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, v*; it passes dorsad and caudad to the
region of the thoracic vertebre.
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, sct, going to the skin and muscles
218 The Alligator and Its Allies
of the shoulder; (b) the thoracic, t', carrying blood
to the posterior muscles of the shoulder and to the
posterior region of the upper arm; (c) the brachial,
br, which is really the continuation of the subclav-
ian 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, ra‘, and ulnar,
ult, 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 cesophagus.
Its distribution in the cervical and cephalic region
will be described later. The mate to the cesoph-
ageal branch, oe (near heart), of the right sub-
clavian 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 Vascular System 219
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, im’; the collateralis
colli, cc; the pleural, plu; and the vertebral, v‘)
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 sub-
clavian, it passes cephalad, at the side of the trachea
and cesophagus, 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
oesophagus, 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, x’,with a complicated group of branches
from the common carotid. Cephalad to the
connective x’, which extends dorsad and is hence
foreshortened in the figure, the collateralis colli
gives off a small vessel, y (too large in Fig. 62), tothe
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
220 The Alligator and Its Allies
(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, 1’, 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. Itis
seen, then, that the collateralis colli arteries supply
directly the lower side of the head—tongue, mandi-
ble, 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 cesophagus; in this
place it gives off a series of unpaired cervical
arteries, Fig. 62, ce, each of which almost imme-
diately divides into an anterior and a posterior
branch, that carry blood to the cervical vertebre.
At the base of the skull, in the region where it is
united by the first connective, x, with the collater-
alis colli, as described above, the primary carotid
divides into two similar branches, called by Bronn
The Vascular System 221
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 x* 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 com-
plete ellipse, as seen in Fig. 62; there is, however,
no-apparent connection between them at the an-
terior 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, x".
The short branch z quickly divides into three
parts: (1) a small anteriorly directed vessel which
may be called the zuternal carotid, ic, since it enters
the skull through the most ventral of the three
foramina in the exoccipital, and probably supplies
222 The Alligator and Its Allies
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, z*. The last-named branch, 2‘,
in turn, leads in three directions: (a) to the collater-
alis colli artery through the connective x’; (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, z?, to the anterior region of the skull,
giving off a fairly wide branch, jm’, to the large
jaw muscle, and then two branches, o* and 0’, to
the lateral surface of the eyeball and socket; it
then anastomoses, just cephalad and laterad to the
eye, with the forward continuation, cm’, of the
corresponding main stem, cm, of the common
carotid, already mentioned. The vessel cm’,
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, z?; at the
posterior-mesial border of the eye it gives off a
branch that divides into two twigs, one, o%, for
the posterior eye muscles, and one, e’, to the
region of the ear and the top of the skull.
The Vascular System 223
At the point of union of the branches cm‘ and
z’ 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, dm’,
extends cephalad along the mesial side of the
maxilla, dorsal to the palatine bone; it sends nu-
merous twigs into the maxillary bone among the
roots of the teeth. After passing nearly to the
end of the snout, the superior dental, dm, suddenly
forms a loop towards the median line and passes
as a straight branch, n’, directly caudad, near
and parallel to the median plane. The branch n’
extends along the floor of the nasal cavity and,
after giving off twigs to this chamber, ends in a
network of vessels, o*, on the anterior surface of
the eyeball and socket.
A pair of very small arteries, n?, may be seen in
the nasal chamber between and parallel to the
branches, n*; 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, dm’, though this could not be definitely
determined.
224
The Alligator and Its Allies
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.
br?, brachial artery.
c, cceliac artery.
ca, caudal artery.
capr, primary carotid.
cc, collateralis colli artery.
ce, cervical artery.
cl, cloacal artery.
cm, cm!, common carotid artery.
cr, crural artery.
cv, caudal vein.
da, dorsal aorta.
dm, inferior dental artery of max-
illa.
dm', superior
maxilla.
dental artery of
e, et, artery into skull, perhaps
to ear.
ej, external jugular vein.
ep, internal epigastric vein.
f, femoral vein.
f', fibular artery.
fb, fibular vein.
g, gastric vein of portal.
g', gastric vein of epigastric.
ga, gastric artery.
ghi, gastro-hepatico-intestinal
artery.
h, hepatic portal vein.
he?, he2, hemorrhoidal arteries.
i, intestinal vein.
ic, internal carotid artery.
id, inferior dental vein.
il, iliac vein.
il', iliac artery.
im, internal mammary vein.
im?, internal mammary artery.
is, ischiadic vein.
ist, ischiadic artery.
j, internal jugular vein.
jm, jm’, artery to jaw muscle.
1, lingual vein.
1', lingual artery.
lg, laryngeal artery.
lu, 1-7, lumber arteries (numbers
on left side of figure).
m, mesenteric vein.
m?, mesenteric artery.
md, mandibular artery.
mg, artery to musk gland.
ms, muscular vein.
n, artery to posterior nasal region.
nt, artery to anterior and mid-
nasal region.
n?, artery to anterior nasal region.
o'-o4, arteries to eye.
oc, artery to muscles at base of
skull.
oe, cesophageal arteries.
pa, pelvic artery.
pat, second pelvic artery.
pb, post brachial vein.
pc, post cava.
pd, right pulmonary artery.
pi, pancreo-intestinal artery.
pl, plt, pelvic vein.
plu, pleural artery.
ps, left pulmonary artery.
q, artery to muscle at angle of jaw.
ra, radial vein.
rat, radial artery.
re, reproductive vein or artery.
rp, renal portal vein.
rt, rectal vein.
rt*, rectal artery.
rv, renal vein.
s, subclavian vein.
sc, sciatic artery (Fig. 61).
——. ee
The Vascular System 225
sc, subscapular vein (Fig. 60). v, vertebral vein.
set, subscapular artery. vi, vertebral artery.
se.d., se.s., right and left subclavian | vca, anterior vena cava.
arteries. vh, hepatic vein.
si, spleno-intestinal artery. vpd, vps, right and left pulmonary
sp, Splenic vein. veins.
S.v., sinus venosus.
x, x?, connectives between collater-
t, thoracic vein. lcrenitianc crocs
t', thoracic artery.
tb, tibial artery. ‘
th, thryoid artery. y, artery to shoulder and skin.
tr, tracheal vein.
Zz, z* z?, branches of common
u, I-4, urogenital arteries (num-| carotid.
bers on right side of figure). I?, 2', 3', branches of ischiadic
ult, ulnar artery. artery.
TS
CHAPTER IX
THE DEVELOPMENT OF THE ALLIGATOR
(A. mississippiensis)
INTRODUCTION
ITH the exception of S. F. Clarke’s well-
known paper, to which frequent refer-
ence 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 nec-
essary embryological material. Clarke, some
twenty years ago, made three trips to the
swamps of Florida in quest of the desired ma-
terial. 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 Oke-
finokee 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
226
The Development of the Alligator 227
which a liberal grant of money to defray the ex-
penses 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 fix-
ation 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 7m 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.
dhsia kee
_ FIGURES I, I@ (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
228 The Alligator and Its Allies
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 5.14mm. Norelation
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
eachother. Noair-chamber, such as is found in the
hen’s egg, is found in any stage in the development.
The Development of the Alligator 229
In most—probably all normal—eggs a white
band appears around the lesser circumference a
shert time aiter being laid.) his chalky band,
which is shown at about its maximum development
in Fig. 1a, 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. 1a); 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 some-
times 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
230 The Alligator and Its Allies
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 chalaze are present.
The yolk is a spherical mass, of a pale yellow
color, lying in’ the “center-ot the whites ies di—
ameter 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 pro-
tection 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 vascu-
The Development of the Alligator 231
lar area and the allantois begin to function, secures
a better aération 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 accom-
plished 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
232 The Alligator and Its Allies
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 trans-
portation 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
The Development of the Alligator 233
quote directly Clarke’s paper in the Journal of
Morphology (17) in description of these surface
views.
STAGE ot
FicureES 2-2f (PLates VI., VII.)
The youngest embryo that we have for descrip-
tion is shown in Figures 2 and 2a. 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 nar-
rower 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. 2a), 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 2b—2f.
Figure 2b, through the anterior region of the
blastoderm, shows a sharply defined ectoderm (ec)
which is composed of three or four layers of cells
234 The Alligator and Its Allies
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 2c is about one fifth of the length of the
blastoderm posterior to the preceding and rep-
resents approximately the same conditions, except
that there is an irregular thickening of the entoderm
in the median region (e7). This thickening appar-
ently marks the anterior limit of the mesoderm, to
be discussed shortly.
Figure 2d represents the condition of the blasto-
derm 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 blasto-
pore becomes separated somewhat from the ento-
derm proper as a middle layer or mesoderm (Fig.
2e, 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. 2e).
Figure 2e is a section through the region just
The Development of the Alligator 235
mentioned, where, medially, the ectoderm, meso-
derm, 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 2f is one of the four sections that were cut
through the blastopore (b/p), 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 3m
and 67 and will be described in their proper order.
STAGE II
FicurEs 3-30 (PLATE VII., VIII., IX.)
The next stage to be described is shown in sur-
face views in Figures 3 and 3a. Of this stage Clarke
says:
236 The Alligator and Its Alles
“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. 3a) shows that the medul-
lary 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 3b-m are drawn from transsections of an
embryo of about this stage of development. Fora
short distance in front of the beginning of the head-
fold, there is a mass of cells of considerable thick-
ness between the ectoderm and entoderm. In
Figure 30 these cells appear as an irregular thicken-
ing of the entoderm, while in Figure 3c 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 3d passes through the head-fold, which
The Development of the Alligator 237
in this embryo was probably not so far developed
as it was in the embryo shown in Figures 3 and 3a.
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 3e is a short distance posterior to the
preceding. It shows a marked thickening of the
ectoderm in the medial region (ec), which is con-
tinuous posteriorly with the anterior ends of the
medullary folds that are just beginning to differ-
entiate (Figs. 3f-/).
Figure 3g 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 amedullary groove. The entoderm (ev)
is considerably thickened in the medial region, this
thickening being continuous posteriorly, as in the
preceding stage, with the mesoderm.
In Figure 3h, cut in a plane at some distance
posterior to the preceding, the medullary groove
(mg) is well marked; its bordering folds grad-
ually 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
238 The Alligator and Its Allies
dense mass of cells (/), apparently the anterior
end of the notochord in process of formation.
Figure 37, 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 (mt). It is evidently difficult to
decide whether this group of cells (zt), 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 ento-
derm. There is here absolutely no line of demar-
cation between the cells of the notochord and those
of the mesoderm and entoderm.
In Figure 37 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 (nf) and
entoderm (e7). The notochord is a clearly defined
mass of cells, distinct, as has been said, from the
mesoderm, but still closely united with the under-
lying 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.
The Development of the Alligator 239
In Figure 3k there is no sign of the medullary
groove, though the ectoderm (ec) 1s still much thick-
ened in the middle line. |The section passes, pos-
terior to the notochord, through the anterior edge
of the ventral opening of the blastopore (dlp).
The mesoderm (mes) is here again continuous with
the entoderm, around the edge of the blastopore,
but is distinct from the ectoderm.
Figure 3/ represents the third section posterior
to the preceding. The blastopore, which passes
upward and backward through the blastoderm, is
seen as an enclosed slit (b/p). Itissurrounded bya
distinct layer of compactly arranged cells contin-
uous with the thickened ectoderm (ec) above, with
the thin entoderm (en) below, and laterally with
the gradually thinning and scattering mesoderm
(mes).
Figure 377 is the next section posterior to the one
just described. It passes through the dorsal
opening of the blastopore (b/p), 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 poste-
rior to this one will be described in the next stage,
where they have essentially the same structure
and are better preserved.
Figures 372 and 30 are sagittal sections of an
embryo of about the stage under discussion. In
both figures the head-fold is seen as a deep loop
240 The Alligator and Its Allies
of ectoderm and entoderm, while the head-fold of
the amnion is seen at a.
The beginning of the foregut is seen in Figure 37
(fg), which is the more nearly median of the two
sections, Figure 30 being a short distance to the
side of the middle line.
In Figure 30 the thin entoderm (ev) is separated
from the much thicker ectoderm (ec) by a consider-
able layer of rather loose mesoderm (mes). In
Figure 37, which is almost exactly median in posi-
tion, 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 blasto-
pore (d/p) 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 blasto-
pore the lower side of the ectoderm is continuous
with a considerable mass of cells, the primitive
streak (ps).
STAGE III
FIGURES 4, 44, 5, 5@, AND 6-67 (PLATES X., XI.)
“‘Figures 4 and 4a 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 second-
The Development of the Alligator 241
ary fold taking place in the head-fold. This grows
posteriorly along the median dorsal line, forming
a V-shaped process with the apex pointing back-
ward 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 4a, 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, end-
ing 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. 5a) 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 5a.
“This is so unexpected a method of formation
16
242 The Alligator and Its Allies
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 pos-
terior to the V, in the embryos shown in Figures 4.
and 5, show the medullary groove and the medul-
lary 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 sec-
tions of an embryo of about this age is represented
in figures 6a-1.
Figure 6a 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 Development of the Alligator 243
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 (em) is here
flat and takes no part in the secondary folds.
In Figure 6), 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 6c represents a section which passes
back of the head-fold on both sides. The second-
ary 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 thicken-
ing (en) that was shown on the left side of the
244 The Alligator and Its Allies
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 6d is a short distance posterior to tne
preceding. Ihe secondary folds are Were imiueh
less pronouncedly arched and the deep groove
between them is reduced to a line (/). The ento-
derm (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 6e. The
line (1) 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
The Development of the Alligator 245
secondary folds of the anterior end of the embryo
and the just forming medullary folds of the pos-
terior end, so that it is impossible to say whether
the thickenin~ of ectoblast in this figure should be
called secondary folds or medullary folds. Asa
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 6f is about one sixth of the length of the
embryo posterior to the preceding. The medul-
lary thickening of the ectoderm (ec)is still marked
and the shallow medullary groove (mg) is fairly dis-
tinct. The entoderm (ez) is medially continuous
with both mesoderm (mes) and notochord (nt),
though these two tissues are otherwise distinct
from each other.
Figure 6g 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.
246 The Alligator and Its Allies
Figure 6/ passes through the blastopore (dlp).
The appearance of the section is almost identical
with that of Figure 2f, already described.
Figure 67 is five sections posteric~ to the preced-
ing and has about the same structure as the corre-
sponding sections in the preceding two stages, where
this region of the embryo was injured, and hence
not drawn. Continuous with the posterior bor-
der of the blastopore (seen in the preceding fig-
ure) 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 (ez)
is here entirely, distinc irom (he mesoderm. menL
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 ento-
blast, as is the case anterior to the blastopore.
The primitive groove gradually becomes more and
more shallow, as it is followed toward the poste-
rior, until it is no longer discernible; back of this
point the primitive streak may be traced for a con-
siderable 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.
The Development of the Alligator 247
STAGE IV
FiGuREsS 7a-7h (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 8a.
Figure 7a 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 7b 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 (e7) caused by the head-fold is
plainly seen. In this depression le 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 nerv-
ous layer (vl), 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 7c is some distance posterior to the pre-
ceding, though the exact distance could not be
248 The Alligator and Its Allies
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 7d 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 ecto-
derm (ec) curves over to form the medullary folds
it becomes much more compact and somewhat
thicker. The notochord (zf) is large and distinct,
but is still fused with the entoderm (en). The
mesoderm (mes) forms a well-defined layer, en-
tirely 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 7e 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 (nfé)
The Development of the Alligator 249
is of about the same diameter as before, but is here
quite distinct from the entoderm (em) as well as
from the mesoderm (mes).
Figure 7f is seven sections posterior to Figure
ze. 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 7g passes through the blastopore and
shows essentially the same structure as was de-
scribed in connection with the corresponding
section of Stage I (Fig. 2f).
Figure 7/ represents the region of the primitive
groove (pg) and primitive streak (ps). The sec-
tion 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
Ficures 8-87 (PLATEs XII., XIII., XIV.)
On opening the egg this embryo (Figs. 8 and 8a)
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
250 The Alligator and Its Allies
theegg. 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 trans-
mitted 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 sepa-
rated; 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
The Development of the Alligator 251
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. 8a) 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 (zt) 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 8b and 8c. 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 8d
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
252 The Alligator and Its Allies
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 noto-
chord (mt) 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 (b/p) 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 8c 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 80; anteriorly it is
pushed down and back as the head-fold, and pos-
teriorly 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 (8b). The most
striking feature of the section is the presence of
five mesoblastic somites (s). Each somite, espe-
The Development of the Alligator 253
cially 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 myoccel may be seen.
A series of transverse sections of the embryo
shown in Figures 8 and 8a is represented in Figures
8d-7. ,
Figure 8d 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, sepa-
rated 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 8e 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 (/).
Figure 8f 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 (7/) layers of the ectoderm, but no
mesoblast is yet to be seen.
254 The Alligator and Its Allies
Figure 8g passes through the middle part of the
head-fold, and shows that the medullary folds in
this region are fused below, but are widely sepa-
rated 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 condi-
tions 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 ante-
rior 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
The Development of the Alligator 255
will show the medullary canal open both above
and below, as in Figure 8d, while sections farther
caudad pass through the short region where the
folds are fused, and we have the appearance repre-
sented in Figure 8e. In Figure 8f 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 8g represents a
section through the tip of the bent-under region of
the medullary folds, which are here fused below
and open above.
Figure 81 passes through the posterior part of
the head-fold, between the limits of the fold of the
ectoderm and that of the entoderm. The medul-
lary groove (mg) is here very wide and compar-
atively shallow; its walls are continued laterally as
the gradually thinning ectoderm (ec). The enteron
(ent) iscompletely enclosed, and forms a large, some-
what 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. Inthe 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 ecto-
derm is a considerable mass of mesoderm (mes),
which here consists of more scattered and angular
cells than in the preceding section.
256 The Alligator and Its Allies
Figure 82 shows the appearance of a section
through the mesoblastic somites, in one of which
a small myoccel (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 sec-
tions, 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 myoccel may
be seen and cleavage is well marked in sections
between this one and the preceding.
The notochord (nt) has about the same appear-
ance as in the preceding section, but is more
distinctly separated from the surrounding cells.
Figure 87 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 Fig-
ure 8, the medullary groove is continued without
any line of demarcation into the primitive groove,
and the medullary folds into the edges of the primi-
tive 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
The Development of the Alligator 257
it be here so called, 1s 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 indis-
tinguishable in the middle line, and in the center
of this mass of cells the blastopore (d/p) or neu-
renteric 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 medul-
lary 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 9a-gm (PLATES XIV., XV.)
The embryo represented by this series of trans-
verse sections is intermediate in development
between those represented in surface views by
17
258 The Alligator and Its Allies
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 dis-
tinguisnable somites.
Figure 9a 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 push-
ing of the head under the yolk is shown at its com-
mencement in Figure 11. The process continues
until nearly the entire anterior half of the embryo
is covered; but when the embryo attains a consider-
able 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 an-
terior 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
g and 9b. 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 sur-
The Development of the Alligator 259
rounds 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 com-
pactly arranged cells of the nervous layer (/), from
which it is separated by only a line.
In Figure 9), which shows a section a short dis-
tance posterior to the preceding, the medullary
canal (mc) is somewhat deeper and is still open
ventrally. There is a distinct space between
the nervous (z/) and epidermal (ep) layers of the
ectoderm, in which space a few mesoblast cells
(mes) may be seen. The section is cut just poste-
rior to the edge of the amnion, so that there is now
neither amnion nor yolk above the embryo.
Figure 9c is about ten sections posterior to Figure
9b. 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 embry-
onic spinal cord. The space between the super-
ficial (ep) and nervous (zl) layers of the ectoderm is
quite extensive and is largely filled by a fairly
compact mass of mesoderm (mes).
Figure 9d, although only five sections posterior
to the preceding, shows a marked change in struc-
ture. The medullary canal (mc) is here of the
typical outline for embryos of this age. A large,
260 The Alligator and Its Allies
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 noto-
chord (nt). The space surrounding the notochord
and enteron is filled with a fairly compact mass
of typical, stellate mesoblast cells. The depres-
sion 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 9e represents a section passing through
the posterior edge of the head-fold. The epider-
mal ectoderm is here continuous with the thin layer
of superficial ectoderm (ec) of the blastoderm, while
the entoderm (ez) of the blastoderm is still contin-
uous 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) isa 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 9f is about twenty sections posterior to
The Development of the Alligator 261
the preceding section, and passes through the point
of separation of the folds of the entoderm (ez).
From this point the entoderm gradually flattens
out, leaving the enteron unenclosed. The medul-
lary canal (mc) and notochord (ni) 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 9g, the twenty-fifth section posterior
to that represented in Figure 9f, 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 hori-
zontal 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 medul-
lary 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 dor-
sally. The enteron is here entirely open ventrally,
262 The Alligator and Its Allies
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 sepa-
rated 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
Figuresggandgz. This is the region of the meso-
blastic somites, and in this region, as has been
above stated, the medullary canal is completely
enclosed. It is evident then that the entire ante-
rior two thirds of the medullary canal is enclosed
except for the short region represented in Figure 8g.
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 9h 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 97 is through a region nearly one hundred
sections posterior to the preceding, and cuts the
The Development of the Alligator 263
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 consider-
ably 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 9j is about twenty sections posterior
to Figure 97. The medullary groove (mg) is con-
siderably 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 9k shows the medullary groove (mg)
in about the same position as in the preceding
section. The blastopore (b/p) 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/ is four sections back of the preceding;
264 The Alligator and Its Allies
the wide, dorsal opening (b/p) 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 9k, b/p), inthe 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 essen-
tially likethat 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 poster-
iorly 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 9m, 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
The Development of the Alligator 265
groove, mg in Figure 9/, into the shallow slit, pg,
Figure 9m.
There is no line of demarcation between the
typical medullary groove region of Figure 9/ and
the equally typical primitive groove region repre-
sented in Figure 9m. As was noted in the pre-
ceding 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 ento-
derm (en) and the lateral regions of the ecto-
derm (ec) and mesoderm (mes) in Figure 9m are
about as they were in Figure 9/, but in the mid-
dle line is seen a compact mass of cells form-
ing 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 an-
gular 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 9m the primitive streak may
be seen, then it disappears, and only the ecto-
derm and entoderm remain as thin sheets of
tissue above the yolk.
266 The Alligator and Its Allies
STAGE VII
FIGURES IO AND I0a (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 extremeend. Transverse sec-
tions, however, of Figure 12, in which the medul-
lary 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 aslight enlargement (v”) posteriorto the
first probably represents the second cerebral vesicle.
There are now eight pairs of somites (s).
The head-fold (2) 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 preced-
ing 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 Development of the Alligator 267
The head at this stage begins to push down into
the yolk in a strange way that will be described
later.
STAGE VIII
FIGURES II-IIk (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 (qa)
now covers nearly two thirds of the embryo. The
heart (/it) is seen as a dark, rounded object pro-
jecting 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
268 The Alligator and Its Allies
noted in the preceding stage has advanced so far
that a considerable part of the embryo now pro-
jects forward under the blastoderm. In some
cases it is almost concealed in a dorsal view; in other
cases 1t 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 11a 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 (zl) 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 110 is represented a section, behind the
preceding, which passes through the posterior tip
of the turned-under anterior end (mc’). Here the
The Development of the Alligator 269
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 meso-
blast (mes), between the nervous (/) and epidermal
(ep) layers of ectoderm.
Figure 11c 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 (qa),
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 (mt) 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
270 The Alligator and Its Allies
is found to open ventrally and medially into the
anterior end of the heart.
Figure 11d is about a dozen sections posterior
to the preceding. The appearance of the over-
hanging 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 dis-
tinct endothelium (e7’) whose exact origin has not
yet been worked out.
Figure Ile is just back of the heart, and shows
in its place the two vitelline veins (vv). The depres-
sion 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 poste-
rior to this shows where this depression opens ven-
trally to the yolk sac. The other structures
shown in the figure are not markedly different
from what was seen in Figure 11d.
Figure 11f is about one tenth the length of the
embryo posterior to Figure 11e. The chief differ-
ences here noticed are in the enteric and ccelomic
The Development of the Alligator 271
cavities. The former is no longer enclosed, a dor-
sal 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 meso-
blast. A thickening of the mesoblast on either
side of the notochord, especially on the left, repre-
sents a mesoblastic somite. The medullary canal
(mc) is more open than in the more anterior sec-
tions.
For about one third of the length of the embryo
posterior to Figure 11f there is a gradual flattening,
in a dorso-ventral direction, with loss of the am-
nion, until the condition represented in Figure 11g
isreached. 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 fol-
lowed 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
272 The Alligator and Its Allies
in thickness between the ectoderm and entoderm.
It shows laterally a slight separation to form the
body cavity.
Figure 11/ is about ten sections posterior to
Figure 11g, and differs from it chiefly in that the
notochord (ut) is continuous with the lower side
of the medullary canal (mc), though still distinct
from the underlying entoderm (en).
Figure 112, 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 pesterior to the notochord and passes
through the anterior edge of the blastopore or, as
it may now perhaps better be called, the neuren-
teric 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 117 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
The Development of the Alligator 273
similar to that shown in Figure 11k, which is about
the twentieth section posterior to Figure 117. The
break (ec’) in the ectoderm is here seen as a com-
pact group of cells which at first glance seem to be
continuous with a rounded mass of cells below (ps).
Examination under greater magnification, how-
ever, 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 dis-
tinct 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 12-12g (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 (v?), 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 thick-
ness, especially in the anterior region, where the
enlargement of the cerebral cavity is considerable.
18
274 The Alligator and Its Allies
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 thicken-
ing is the optic vesicle (e). The auditory vesicle,
though of considerable size, does not show in this
surface view. The head-fold (2) 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 re-
gions there is not very much difference in size.
Figure 12a passes through the region of the fore-
brain. 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
The Development of the Alligator 275
yolk (y). The great size of this and the following
figure is due partly to the increase in size men-
tioned above and partly to the fact that the sections
pass through the region of cranial flexure. The
present figure (12a) 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. Sur-
rounding the brain is a considerable mass of meso-
blast (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 thick-
ness in different regions, the thicker parts being at
the sides. The amnion (a) has the usual appear-
ance, and in this region of course completely sur-
rounds the embryo.
Figure 12) 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
276 The Alligator and Its Allies
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 out-
line. 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 con-
tact 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 (nb) of this
section is probably the hinder part of the midbrain,
though it may be the anterior part of the hind-
brain; there is no sharp line of demarcation be-
tween 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 (nf), surrounded by the mesoblast.
At various places throughout the mesoblast irregu-
lar open spaces may be seen; these are blood-
vessels. The ectoderm (ec) and amnion (a) have
The Development of the Alligator 277
about the same appearance as in the preceding
figure, though the former seems somewhat thinner.
Figure 12c is just back of the bent-under fore-
brain 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 (0), 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 (p/); 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 root of the pharynx is flat and.
278 The Alligator and Its Allies
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 men-
tioned 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 11c, 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 (qa) is still
a continuous envelope entirely surrounding the
embryo.
Figure 12d, about twenty sections posterior to
Figure 12c, 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 (vt) 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 aorte (ao),
now smaller and more ventral to the notochord
The Development of the Alligator 279
than in the preceding figure. Ventral to the
enteron is the large heart (it), projecting below
the body cavity, which is no longer enclosed. The
mesodermic wall (mes’) of the heart is still com-
paratively 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 dis-
cussion 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 inter-
rupted 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) extend-
ing around the enteron (et) and heart (ht), and
the latter (so) being carried dorsalward as the
280 The Alligator and Its Allies
mesoblastic part of the amnion (a). The amnion
may be traced through about 130 of the 200 sec-
tions into which this embryo was cut.
Figure I2e is nearly one fourth the length of the
embryo posterior to Figure 12d; it 1s 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 (em) 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 itssextemn,
the ectoderm is very thick. Under the notochord
the dorsal aortz (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 myoccel (myc). The meso-
blastic layer of the amnion (so) is distinct through-
out 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
The Development of the Alligator 281
about forty sections, or one fifth of the length of the
embryo. Their exact length is difficult to deter-
mine 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 12f is just posterior to the head-fold of
the amnion, passing, im fact, on) the leit 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 (zt), aorta (ao), and entoderm (e7)
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 aorte,
where there seems scarcely enough tissue to hold
these vessels in place. The body cavity (bc) is
large, and a small myoccel (myc) is seen on the left.
282 The Alligator and Its Allies
Figure 12¢ 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 myoccel (myc),
though in surface view the mesoblastic somites do
not extend this far toward the tail.
STAGE X
FIGURES 13-13g (PLATES XIX., XX., X XI.)
This embryo (Fig. 13) is about 5mm. in length,
and hence is slightly smaller than the preceding
stage, though somewhat more advanced in develop-
ment. ‘The medullary canal is still apparently
unclosed for a short distance at the extreme poste-
rior end; this appearance is probably due to the
neurenteric canal (zc) 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 (0)
marks the position of the ear. There are now
about twenty pairs of somites, though it is difficult
The Development of the Alligator 283
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 (wv) 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 13a 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
284 The Alligator and Its Allies
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 benweem
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 pos-
terior 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 might -sidevalso vis
noticed a marked thickening of the ectoderm,
which is invaginated to form a small pit, the lens
vesicle (Jv); 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
The Development of the Alligator 285
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 hind-
brain 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 com-
paratively 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 de-
scribed 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 130, about forty sections posterior to
Figure 13a, passes through the hindbrain in the re-
gion 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
286 The Alligator and Its Allies
entoderm around the mandibular folds (md);
there it is somewhat thickened. The most strik-
ing feature of the section is the presence of two
large auditory vesicles (0). 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 (nut), 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 aorte (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 hyomandibu-
lar cleft was not cut through its external opening.
In each mandibular fold a large aortic arch (ar) is
The Development of the Alligator 287
seen, and also a slight condensation of meso-
blast, the latter probably being the forerunner of
cartilage.
Figure 13c passes through the anterior part of
the heart about seventy-five sections posterior to
Figure 13b. 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 deter-
mined. 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 aorte 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, 7. 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
288 The Alligator and Its Allies
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 espe-
cially 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 (e2’), 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 13d is about one hundred and twenty-
five sections posterior to Figure 13c. 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 repre-
sented in the last three figures, is very evident
here. The spinal cord (sc) is considerably smaller
here than in the preceding figure, while the noto-
chord (zt) is not only relatively but actually larger
than in the more anterior regions. Beneath the
notochord is the aorta (ao), now a single large
The Development of the Alligator 289
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 (7. e., at the end of the reference line
ec) each muscle plate is separated from the over-
lying 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 con-
tain 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
19
290 The Alligator and Its Allies
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 13e is about one hundred and forty sec-
tions posterior to the section just described. The
embryo is here very slender, so that the contrast
between this and the first figure (13a) 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 (wi),
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 13f, 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
The Development of the Alligator 291
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 (2),
somewhat flattened and relatively and actually
smaller than in the preceding figure. A few scat-
tered blood-vessels may be seenin 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 13g. 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 magni-
fication 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, espe-
cially at the extreme anterior end; the wall of the
midbrain (mb), where the marked cranial flexure
takes place, is somewhat thinner, and it gradually
292 The Alligator and Its Allies
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 struc-
ture 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 (hf), 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.
The Development of the Alligator 293
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 les
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 out-
line 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 (g™°) 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 (g*) 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
294 The Alligator and Its Allies
through the pharyngeal wall. The outlines of the
visceral folds, especially of the mandibular, begin
to be apparent. The nasal pit (7) now shows as a
round depression in front of the more definitely
outlined eye (e). The auditory vesicle (0) is so
deep beneath the surface that it may be seen only
by transmitted light.
Figures 15a—e represent transverse sections of an
embryo of about this general state of develop-
ment, except that the gill clefts are not so definitely
open as in the surface view.
Figure 15a, 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 invagin-
ation 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 ec-
toderm (ec), which is comparatively thin. The
hindbrain (/b) has the usual shape for that struc-
ture. Its ventral wall is dense and thick, while its
The Development of the Alligator 295
roof is reduced to the usual thin, wrinkled mem-
brane. 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 (cz) is one of the
cranial nerves, which is connected with the hind-
brain a few sections anterior to the one under
consideration. The pharynx (pf), which is cut
near its extreme anterior end, is represented by
three irregular cavities near the base of the fore-
brain. Scattered throughout the mesoblast, which
makes up the greater part of the section, are nu-
merous blood-vessels (bv).
Figure 150 is twenty sections posterior to Figure
15a 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 (7); 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 (2b) is somewhat
narrower than in the preceding figure, but is other-
wise about the same; the origin of a cranial nerve
is seen on its left side (cz). The notochord (7)
has the same appearance as in the preceding section.
296 The Alligator and Its Allies
A number of blood-vessels may be seen, the pair
lying nearest the notochord being the aorte (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 struc-
ture, made of a dense mass of mesoderm surrounded
by arather 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 15c is posterior to the region affected by
The Development of the Alligator 297
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 (0); 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, some-
what wrinkled wall. The notochord (md?) is slightly
larger than in the more anterior sections. Numer-
ous 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 15d, as is evident, is a section through
the region of the heart, which appears as three ir-
regular 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
298 The Alligator and Its Allies
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 15e 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 consider-
ably smaller than in the preceding, while the
notochord is very much larger; in fact the noto-
chord here seems abnormally large when compared
to correspondine ‘sections*jof other “Seniess iG
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
The Development of the Alligator 299
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 de-
veloped 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 (w#) 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 irregu-
lar, thick-walled outgrowth from the hindgut.
A horizontal section through the anterior end
of an embryo of this age is shown in Figure 15f.
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 (zt), muscle plates (mp), aortz
(ao), and anterior cardinal veins (ac) need no spe-
cial description. The appearance of the pharynx
300 The Alligator and Its Allies
(ph), with its gill clefts and folds, is quite similar
to that of the corresponding structures in the chick.
None of the four clefts (g™*) show, in the plane at
which the section was cut, any connection with the
exterior; in fact the fourth cleft (g*) 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 16-16g (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 (0) have
about the same superficial appearance as in the
preceding stage. The nose is not shown in this
hgure. About) thishy ysomutes vare =present- uae
exact number cannot be determined in surface view.
The amnion is complete, though not shown in the
figure, and the tail (¢) is well formed. The umbili-
cal stalk was torn in the removal of the embryo, so
that it is not shown in the figure. The dim out-
line 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
The Development of the Alligator 301
anterior to the tail. Four gill clefts (g*‘*) 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 16a 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 (7) 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 160 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. deseribed 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 (kb) at the
302 The Alligator and Its Allies
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 (cv); and the other
is one of the auditory vesicles (0), which is still
more irregular in outline than it was in the pre-
ceding stage. The only blood-vessels to be seen
are a few very small ones that lie close to the wall
of the brain. Vhevectodermuis) quite? thaneab act
points.
Figure 16c, the largest section of this series,
passes through the forebrain in the region of the
eyes and through the gill clefts. The forebrain (fd)
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 (0c), in whose opening lies the lens vesicle (/v),
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 \cletts) »Only fa
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
The Development of the Alligator 303
figure: one pair opens widely on either side, so
that there 1s a large areayot. the section that. is
distinct from the two still larger portions and con-
tains 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 aorte
(ao), which are of very unequal size. The spinal
cord (sc) and muscle plates need no special de-
scription.
Figure 16d is in the region of the heart (At) and
lungs (uz). The former is an irregular cavity whose
walls, especially on the ventral side (mes’), are be-
coming very thick and much folded. Although
thin, the body wall completely surrounds the heart,
as would be expected, since this was true of the pre-
ceding stage. The lung rudiments (J) and the fore-
gut from which they have arisen have the same ap-
pearance 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
304 The Alligator and Its Allies
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 kid-
neys 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 nar-
row channels with the dorsal aorta; no such con-
nection 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 16e is in the region of the liver and the
Wolffian bodies; it also shows the tip of the ven-
tricular end of the heart. The liver (/z) is a large
irregular mass, of a blotchy appearance under this
magnification, lying between the heart (vm) and
The Development of the Alligator 305
the intestine (7). 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 com-
paratively 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 (wé) in their usual location. The
heart is cut through the ventricle (v7), 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 appareutly due
to an artificial break and not to a lack of fusion.
The only point that need be mentioned in connec-
tion with the structures of the dorsal part of the
section is that the distinctness of the myoccel
(myc) on the right side is somewhat exaggerated.
Figure 16f is in the middle region of the embryo
where both splanchnopleure and somatopleure are
unfused. Owing chiefly to the unclosed condition
of the midgut (7) and to the increase in length of
20
306 The Alligator and Its Allies
the mesentery (7s), 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 16g 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 indi-
cated, 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 (#) ; it is much
vacuolated and gradually increases in thickness
toward the posterior, though its outline is quite
The Development of the Alligator 307
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 (0),
and back of that is the edge of the ventricle (v7).
Posterior and dorsal to the ventricle the liver (/z)
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 17-17g (PLATES XXIII., XXIV.)
Body flexure has increased until now the fore-
brain 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 crescenticin shape. The hyomandib-
308 The Alligator and Its Allies
ular 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, 1s in the appearance of the
appendages (aa and pa); they have the char-
acteristic 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 (ft) is, perhaps,
somewhat abnormal. The umbilical stalk (wz) 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 17a—-g, drawn under a
lower magnification than were any of the preceding
figures.
Figure 17a 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 distin-
guished. Under greater magnification the pres-
The Development of the Alligator 309
ence 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 (In) 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
(0) 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 an-
other slit which is simply the anterior margin of the
stomodeum. On the right neither of these open-
ings is in the plane of the figure, though the gill
cleft (hyomandibular), which les 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 noto-
chord (né) and the floor of the brain, is noticeable
310 The Alligator and Its Allies
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) is also through the pharyngeal re-
gion, 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 (7) 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 (mt) 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 neighbor-
hood of the notochord, and a more marked conden-
sation (mp) farther toward each side is the curiously
shaped muscle plate.
Figure 17c¢ 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 differ-
ence in the thickness of the ventricular and auric-
ular walls. As in the preceding stage, the body
The Development of the Alligator 311
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 oesophagus 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 1s cut at the
place where it turns downward to empty into the
heart. Dorsal to the cesophagus are the aortz
(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 (mt) 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 de-
fined nerve roots. The condensation of mesoblast
around the notochord is quite evident, and 1n 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 myoccel at the dorsal end.
are The Alligator and Its Allies
Figure 17d 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
(7), 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 sepa-
rated 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
Cuviert, 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 glcmeru-
lus, 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
(Ju), and the cesophagus (oe) are seen as three
small, circular openings; that of the oesophagus is
somewhat smaller than the other two. The noto-
chord (7#), spinal cord (sc), and muscle plates (n7p)
have almost the same appearance as in the preced-
ing 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 (s7), which may
be traced ventrally as far as the end of the muscle
The Development of the Alligator 313
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 17e 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 (7) may be called the intestine, while
the larger is evidently the stomach (2’). 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 con-
nection 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 enlarge-
ments 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
314 The Alligator and Its Allies
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 17f 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 (z), 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 Wolf-
fian 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 poste-
rior 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 (sz), 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
The Development of the Alligator 315
knob contains a distinct cavity (not shown in
the figure), the myoccel.
In Figure 17g, 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. 17d), 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 (¢), being cut
through one of its curves, is seen as an elon-
gated body with a section of the spinal cord, noto-
chord, 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 enlarge-
ments or projections of the mesoblast (covered,
of course, with ectoblast) of the Wolffian ridge.
They are, as shown in this section and in the sur-
face view of this stage (Fig. 17), bluntly pointed pro-
jections from the sides of the body. The anterior
appendage seems to be slightly more developed
than the posterior, as was noted in describing the
316 The Alligator and Its Allies
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 contin-
uous with the ectoderm that covers the ventral pro-
jection 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 (g—°). 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 rudi-
The Development of the Alligator 317
mentary, gill cleft had not Clarke (17) found a
fen pair of clefts" bis vmaternal. Whe nasal
pit has advanced in development somewhat and
shows the beginning of the groove that connects
iGewith the mouthe ps loethisemeurethe erescentic
hyomandibular cleft shows its connection with
the groove between the mandibular and the hyoid
folds.
STAGE XVI
FIGURE I9 (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 man-
dibular fold has not changed appreciably. The
nasal pit (7) is now connected by a distinct groove
with the stomodeum. 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. Thestalk of the umbilicus (#), 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.
318 The Alligator and Its Allies
STAGE XVII
FIGURES 20-207 (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 (7) 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 20a-.
Figure 20a 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 differen-
The Development of the Alligator 319
tiation than was seen in the preceding sections,
though this cannot be shown under the low magni-
fication used. The hindbrain (hb), which is cut
near its anterior border, exhibits the usual mem-
branous 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 (ep), 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, densewalls. Themesoblast,in which
several blood-vessels (bv) are seen, exhibits three dis-
tinctareas—a median, lighterzone, witha moredense
area on either side. The significance of this varia-
tion in the density of the mesoblast is not apparent.
Figure 200 is only a few sections posterior to the
section just described. It is drawn chiefly to show
the appearance of the forebrain, the other struc-
tures being about as in the preceding figure, except
that both eyes (e) are here represented. The sec-
tion passes through the wide opening between the
third (tv) and the lateral ventricles (ch) and cuts the ~
anterior edge of the pineal body: (efz). The
t 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.
320 The Alligator and Its Allies
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). Itis shown in Figure 17a 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 preced-
ing figure are seen here.
Figure 20c, 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 (0), 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 (cz), apparently the eighth,
since in another section it comes in close contact
with the wall of the larger part of the auditory vesi-
cle 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 sec-
tions 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),
The Development of the Alligator 321
except that the tip of the wall of one of the cerebral
hemispheres (ch) is cut. The left nasal chamber
(7) is shown: it will be noted again in the following
section. The eye on the right side shows no
remarkable features; its lens (/m) is large and lies
well back of the lips of the optic cup, which may
now be called the iris (iv). A thin layer of meso-
blast 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 pig-
mented.’ Ihe inner wall of the opticycupmis be-
ginning 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 appear-
ance issdue 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 pre-
ceding the present, andina stage slightly older it has
disappeared; so that it would seem to beavery tran-
sient structure. It apparently is formed at about
the time that the optic stalk, as such, disappears.
It isintheregionof the choroid fissure, if not through
it, that the opticnerve (o7) enterstheeye. 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.
2I
322 The Alligator and Its Allies
Figure 20d represents a section through the
hindbrain (hb), pharynx (pf), and tip of the snout.
On either side of the hindbrain are a convoluted au-
ditory vesicle (0), and several blood-vessels and
nerves, while ventral to it is seen the anterior end
of the notochord (zt), 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 (pm) 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 otherend. The un-
usual 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 20e is only a short distance posterior to
the preceding. On the left side the pharynx (ph)
is connected with the exterior through the stomo-
dzeum, and on the right the hyomandibular cleft
(g’) iscut almost through its opening to the exterior.
The Development of the Alligator 323
The auditory vesicle (0) on the right is cut near its
middle region, while that on the left is barely
touched by the plane of the section. The noto-
chord (zt), with its condensed area of mesoblast,
is somewhat larger than in the preceding section.
The nasal canal on the right (7) is cut through
neither anterior nor posterior opening, while on the
left side the canal shows the anterior opening(a7).
Figure 20f, 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 (p/h) is here reduced to an irregular, trans-
versely 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 deter-
mined. 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 (Ja), 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 (vz), above which is the
bulbus (0) and the tip of the auricle (aw). The
324 The Alligator and Its Allies
bulbus is nearly circular in outline, though its cav-
ity is very irregular. A fewsectionsanterior to this,
the opening of the bulbusinto the ventricleis seen.
In Figure 20g the section represented is only a
short distance posterior to the one represented by
Figure 20f. 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
cesophagus (0e)—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 (Ja) that was mentioned in connection
with the preceding figure, but the condensation is
here more marked. From the bulbus (6) anaortic
arch (ar) extends upward for a short distance on the
right side, while to the left of the cesophagus an aor-
ticarch(ar)is cut through the upper part of its course.
Ventral to the bulbus the ventricle (v7) and two auri-
cles (au) are seensurrounded by the pericardial wall.
Figure 20h is in the region of the liver (Ji), which
has about the same position in relation to the auri-
cles (au) that was occupied by the ventricle in the
last figure. The auricles are connected with
each other by a wide passage. The trachea (ia)
and the cesophagus (oe) are entirely distinct from
each other; the former is a small, nearly circular
hole, while the lumen of the latter is obliterated and
The Development of the Alligator 325
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 cesophagus had been
closed by the simple shrinkage of its walls; higher
magnification, however, fails to show any sign of
a collapsedlumen. It is doubtless the problematic
and temporary closure of the cesophagus that is
noticed in other forms. On. each side of the
cesophagus, in close relation with the anterior
cardinal vein (ac), is noticed a nerve (cm) cut
through a ganglionic enlargement. When traced
forward these nerves are seen to arise from the re-
gion 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 aortz (ao) the sympathetic nerves (sy)
will be noticed. The mesoblast surrounding the
spinal cord (sc) and notochord (v#) is distinctly
condensed (more so than the figure shows) to
form what may be called the centrum (c) and neu-
ral arch (ua) of the vertebre. 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 vertebre just mentioned. The spinal cord
326 The Alligator and Its Allies
(sc) is here differentiated into three areas—a
dense, deeply stained area immediately around
the neuroccel; 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 207 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 (Ju) and liver (Jz) 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), Wolf-
fian 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 tubuies.
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 rudi-
ments (Ju). Filling most of the body cavity (dc)
and making up the greater part of the middle of the
figure are the liver (Jz), now a very large organ; the
stomach (7’), also quite large; the pancreas (pan), a
small body lying near the stomach; and the lungs
The Development of the Alligator 327
(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 207 is through the base of the posterior
appendages (pa), in which the cartilages are already
being outlined by condensations of mesoblast. The
intestine (z) is cut in two regions—at a more an-
terior 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.
328 The Alligator and Its Allies
STAGE XVIII
FIGURE 21 (PLATE XXVII.)
This embryo, as may be seen, for example, by the
form of the appendages, is slightly further devel-
oped 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 posi-
tion of both elbow and knee joints, and in the
The Development of the Alligator 329
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 com-
pletely formed, the upper projecting far beyond
the lower. The elliptical outline of the eyes is
noticeable, but the lids are still too little devel-
oped to be seen in this figure. The surface of the
embryo is still smooth and white.
STAGE XX
FIGURES 23-230 (PLATE XXVII.)
In this surface view (Fig. 23) several changes are
seen, though no very great advance in develop-
ment has taken place. The outlines of the digits
(five in the manus and four in the pes) are now
well defined; 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 themiddle region of the tail,
just before it crosses the nose.
A sagittal section of the entire embryo (except
330 The Alligator and Its Allies
the tail) of this age is shown in Figure23a. Inthe
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 com-
plicated nasal chamber (7) is not here seen, nor is
the connection of the nasal chamber with the
posterior nares (pz). The pharynx (ph), is ante-
riorly connected with the exterior through the
mouth (m) and the nares, while posteriorly it
opens into the oesophagus (oe); the trachea (ta),
though distinct from the cesophagus, does not yet
open into the pharynx. In the lower jaw two
masses of cartilage are seen, one near the sym-
physis (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 (hd) are seen as large
cavities, while the cerebral hemispheres (ch) appear
nearly solid, only a small portion of a lateral
ventricle showing. The paraphysis (epz) 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 (27) is seen as an elon-
The Development of the Alligator 331
gated 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. Inits 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 vertebre are cut to
one side of the middle line, and hence show the
neural arches (za) 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 (v7) seems to be closely sur-
rounded, both in front and behind, by the auricles
(au). The liver (Jz) is the large, reticular mass
back of the heart. Dorsal and anterior to the
liver is the lung (Jw), now of considerable size and
development. The enteron is cut in several places
(oe, 2) 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) extend-
ing from the pelvic region, where the mass is
greatest, to the region of the lungs. The Wolf-
flian tubules stain darkly and the whole structure
forms a very striking feature of the section. Dor-
sal to the posterior end of the Wolffian body is a
332. The Alligator and Its Allies
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). On the right side the
plane of the section cut through the lens of the eye
(In); on the left side the section was anterior to
the lens. The upper (/) and lower (Il) 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 (wr) and inferior (/7) 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 spheneth-
moidal cartilage and a dorsally evaginated part of
the pharynx are two small openings (pu); when
traced forward these tubes are found to open into
the convoluted nasal chamber, while a short dis-
tance posterior to the plane of this figure they unite
The Development of the Alligator 333
with each other and open almost immediately into
the pharynx. The rather complicated structures of
the nasal passages of the alligator have been de-
scribed by the writer in another paper (57). Inthe
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 (tz).
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 project-
ing with a loop of the intestine from the abdominal
wall; this is shown more clearly in the next stage.
334 The Alligator and Its Allies
The embryo now begins to exhibit some of the
external characteristics of the adult alligator.
STAGE XXII
FIGURE 25 (PLATE XXVIu.)
This embryo needs no particular description. It
has reached in its external appearance practically
the adult condition, although there is still con-
siderable 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
The Development of the Alligator 335
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 ori-
gin, though in the posterior regions, where the
germ layers are continuous with each other, it is
difficult to decide with certainty.
336 The Alligator and Its Allies
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 exte-
rior and probably a fourthalso. 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 Development of the Alligator 337
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 cen-
tral nervous system.
The lumen of the cesophagus is for a time obliter-
ated 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.
22
en’, endocardium.
ent, enteron.
ep, epidermal layer of ectoderm.
epi, paraphysis.
es, embryonic shield.
f, fronto-nasal process.
fb, forebrain.
fg, foregut.
gi-s, gill clefts.
gft—°, gill folds.
gl, glomerulus.
h, head-fold.
hb, hindbrain.
ht, heart.
z, intestine.
2’, stomach.
in, infundibulum.
tr, iris.
at, iter.
k, kidney (metanephros).
1, remains of groove between
secondary folds.
la, larynx (cartilages of).
li, liver.
Il, lower lid of eye.
In, lens.
lr, inferior rectus muscle of eye.
338
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, myoccel.
n, nasal invagination or cavity.
na, neural arch of vertebra.
nc, neurenteric canal.
nl, nervous layer of ectoderm.
nt, notochord.
0, ear vesicle.
oc, optic cup.
oe, cesophagus.
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.
The Alligator and Its Allies
pr, pericardial cavity.
ps, primitive streak.
pi, pecten.
rt, retina.
Ss, somites.
sc, spinal cord.
se, sphenethmoid cartilage.
sf, secondary fold.
Sg, Spinal ganglion.
sm, splanchnic mesoblast.
sn, spinal nerve.
so, somatic mesoblast.
st, stomodeum.
sy, sympathetic nervous system.
t, tail.
ta, trachea.
ig, thyroid gland.
th, thickening and posterior limit
Ole Ge
in, tongue.
to, tooth anlage.
tr, truncus arteriosus.
tv, third ventricle of brain.
tv’, third ventricle of brain.
uw, 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 acameralucida,
The Development of the Alligator 339
The magnification of each figure, except those from Clarke, is indi-
cated 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 IIL., 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 I. Surface view of egg. X 2.
Ia. Egg with part of the shell removed to show the chalky
band in the shell membrane. X 2.
FIGURES 2 and 2a. Dorsal and ventral views respectively of the
blastoderm before the formation of the notochord,
medullary folds, etc. After Clarke.
2b-2f. Transverse sections of an embryo of the age represented
in Figures 2 and 2a. X 43.
3 and 3a. Ventral and dorsal views respectively of an embryo
a few days older than that represented in Figures 2 and
2a. After Clarke.
3b-3m. Transverse sections of an embryo of the age shown
in Figures 3 and 3a. X 43.
FIGURES 3 and 30. Two sagittal sections of an embryo of the same
stage as Figures 3 and 3a. X 43.
4and 4a. Dorsal and ventral views respectively of a slightly
older embryo than the one shown in Figures 3 and 3a.
Figure 4a shows only the head region. After Clarke.
5 and 5a. Dorsal and ventral views respectively of an embryo
of almost the same age as the preceding, to show the fur-
ther 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 6a-67. A series of transverse sections of this stage. X 43.
Ficures 7a-7h. A series of transverse sections of an embryo slightly
older than the one shown in Figures 4-6. X 43. (No
surface view of this stage is figured.)
8 and 8a. Dorsal and ventral views respectively of an embryo
with five pairs of mesoblastic somites. > 20. (Drawn
by transmitted light.)
340 The Alligator and Its Allies
8b and 8c. Two sagittal sections of an embryo of this stage.
X 43.
Figures 8d-8j. A series of transverse sections of the embryo repre-
sented in Figures 8 and 8a. X 43.
ga-gm. Aseries of transverse sections of an embryo somewhat
more advanced in development than the one represented
in the last series. XX 43.
FIGURES 10 and 10a. Dorsal and ventral views respectively of an
embryo with eight pairs of mesoblastic somites. X 20.
(Drawn chiefly by transmitted light.)
FIGURE 11. Dorsal view of an embryo with fourteen pairs of meso-
blastic somites. The area pellucida and the developing
vascular area are shown, the latter having a mottled
appearance. The pushing of the head under the blasto-
derm is also shown. X 20. (Drawn chiefly by trans-
mitted light.)
FicurEs tta-11k. A series of transverse sections of an embryo of this
stage. X 43.
FiGurRE 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.) X 13.
FIGURES I2a-12g. A series of transverse sections of an embryo of this
Stageumerdee
FiGuRE 13. Surface view of an embryo with about twenty pairs of
mesoblastic somites. >< (about)15. (Drawn with both
reflected and transmitted light.)
FicureEs 13a-13f. A series of transverse sections of an embryo slightly
more developed than the one shown in Figure 13.
S207
FicuRE 13g. A sagittal section of an embryo of about the age of the
one represented in Figure 13. X 20.
14. Head of an embryo with one pair of gill clefts; ventro-
lateral view. XX 13.
15. Profile view of the head of an embryo with three pairs of
gill clefts. XX 13.
FicurEs 15a-15e. A series of transverse sections of an embryo of
about the age of the one represented in Figure 15. X 20.
Ficure 15f. A horizontal section through the anterior region of an
embryo of the age of that shown in Figure 15. X 20,
16. Surface view in profile of an embryo with four pairs of gill
clefts. (about) 12.
The Development of the Alligator 341
Ficures 16a-16f. A series of transverse sections of an embryo of the
approximate age of the one represented in Figure 16.
<\20;
FiGurE 16g. A sagittal section of an embryo of the age (possibly
slightly younger) of the one represented in Figure 16.
2a:
17. Surface view in profile of an embryo at the time of origin of
the limbs. XX (about) 5.
FiGcureEs 17a-17g. A series of transverse sections of an embryo of the
age of the one represented in Figure 17. X 7.
FiGurRE 18. Surface view in profile of the head of an embryo slightly
larger than, though of about the same state of de-
velopment as, the one represented in Figure 17. Re-
produced here chiefly to show the gill clefts. xX
(about) 3.
19. Surface view of an embryo somewhat more developed
than the one just described. X (about) 3.
FIGURE 20. Surface view of an embryo older than the one represented
in Figure 19; with well-developed manus and pes.
X (about) 5.
FIGURES 20a-20j. A series of transverse sections of an embryo of the
age of the one represented in Figure 20. X 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.
x F.
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. X (about) 1.
23a. 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. XX (about) 3.
23b. A vertical section through the head of an embryo of
about the size (perhaps slightly smaller) of the one
shown in Figure 23. XX (about) 3.
24. A photograph of an older embryo in which the pigmen-
tation of the scales is evident, though not shown in the
figure. XX (about) 1.
25. A photograph of an embryo in which the pigmentation and
the development of the body form are practically
342 The Alligator and Its Allies
complete. The allantois, unabsorbed yolk, etc., have
been removed. XX (about) 34.
26. A photograph of a just-hatched alligator, of an alligator
egg, and of a young alligator in the egg just before hatch-
ing. Xx (about) #.
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PLATE X.
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PLATE XII. 7b-7h, Stace IV; 8, Sa, STAGE V.
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PLATE XX. STAGE X.
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PLATE XXII. 75d-15f, StaGE XII; 16-r6d, STAGE XIII.
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PLATE XXIII. 1r6c-r6g, StaGE XIII; 17, Stace XIV.
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PLATE XXIV. r7a—1r7g, STAGE XIV; 728, STAGE XV; 79, STAGE XVI.
20c
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iS OP ie
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26 ee
PLATE XXVIII. 25, StaGE XXII, ALLIGATOR
EmsBryo; 26, STAGE XXIII, ALLIGATOR
Just HatcHeD AND RELATIVE SIzE
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15.
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BIBLIOGRAPHY
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1867.
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17:
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33-
34-
35-
The Alligator and Its Allies
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—— “Untersuchungen tiber die Bauchmuskeln der Crocodile,
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—— “Das Gehororgan der Crocodile,” in dessen Anat. Studien,
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”
26;
36a.
37-
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41.
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43
43a.
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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
Cyanocephalus, 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
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 focd, 35
mississippiensis, 3, 7
taw 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 tens
unknown to ancients, 40
use of tail, 14
value of live, 34
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
Alligatoride, 1
Amnion, 236, 247, 251, 259, 266,
267, 268, 269, 270, 274, 275,
278, 290, 334
Ampulle, 149
Ancestry, 4
Annulus tympanicus, 149
349
350
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
coeliac, 212
collateralis colli, 217, 219
common carotid, 219, 221
crural, 216
dorsal aorta, 212
fibular, 214
first heemorrhoidal, 216
gastric, 212
gastro-hepatico-intestinal, 212
iliac, 214
inferior dental, 223
injection of, 201
internal carotid, 70, 221
internal mammary, 217
ischiadice, 214
lingual, 220
lumbar, 213
mandibular, 220
mesenteric, 213
oesophageal, 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 hemorrhoidal, 216
spleno-intestinal, 212
subclavian, left, 219
subscapular, 217
superior dental, 223
thoracic, 218
thyroid, 217
Index
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
coranoid, 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
Bones— Continued
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
Index 351
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. EF :,°226, 227, 228. 230,
231, 232, 233, 236, 243, 247,
250, 274, 293, 317, 318
Classification, I
Claws, 46, 84
development of, 333
Cleavage of mesoblast, 263
Clitoris, 196
Cloaca, 155
embryonic, 316, 327
Cloacal glands, 156
Cocoa, Pla:, 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
352
Crocodile— Continued
African, 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
thombiferus, 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, I
Crocodilide, 1, 2, 3
Deaths by crocodiles in Africa, 40
in India, 43
Deltoid ridge, 83
Dentine, 65
Dermal skeleton, 47
Diaphragm, 115
Index
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
Fund, 226
El lagarto, 40
Embryo, development of, 231
earliest stages of, 232, 233
position of, 230
removal of, from egg, 231
stages! I.) 233-5 Uli 2s5 lies
240; IVs, 2473. Ves 24950 Vl
257° VII, 2663) VILI., 3267;
EX, 273 Nicy, 282s OL eOaT
XI, 293) XAll. 300; 51V5
2073 Via IOs OV Sigs
ANAOL BR DAVIWU Ia, Seth
XEN, 3293 DX" 33207) Deke
2337 SOXII., 133402 oi
Science
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
Index
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,
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
291,
Gastroliths, 44, 45, 153
Gavial,
food of, 44
Indian, distribution of, 43
eggs and nest of, 44
meaning of, 44
Gavialide, I, 3
Gavialis gangeticus, 2
character of, 43, 44
size of, 43
Genital ducts, 156
Geographical distribution of
Crocodilia, 6
Gescmackwarzchen, 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
23
3953
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,
309, 322
Hypophysis, 53, 133
development of, 306, 320, 331,
337
286, 307,
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
develcpment of, 323
Lateral disks of stomach, 153
Lateral ventricle, developing, 319
Laying season of alligator, 18
Lens, 147
Lens ean ae 284,
309, 321,
“Povathan® xt Book of Job, 40
294, 302,
354 Index
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,
27 OV 27272 2 Oe er
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
anconeus, 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, IIT
carpo-metacarpalis V., I11
carpo-phalangei, 109, 110
carpo-phalangeus, III
carpo-phalangeus primus digiti
Woo dit
caudali-ilio-femoralis, 120
caudi-femoralis, I21
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, 10I
coraco-brachialis, 1or
coraco-ceratoideus, 92
costo-coracoideus, 93, 99
costo-scapularis, 94
costo-vertebralis lateralis, 94
costo-vertebralis medialis, 94
deltoideus scapularis infericr,
103
diaphragmatic, 115
dorsalis scapule, 103
of dorsal neck region, 94
dorso-humeralis, 102
dorso-seapularis, 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
Index 355
Muscles— Continued
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, III
metacarpo-phalangeus I., digiti
Ve ,ar2
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., III
of posterior appendages, 118
pterygo-maxillaris, 91
pubi-ischio-femoralis externus,
12
pubi-ischio-femoralis internus,
124
pubi-ischio-femoralis posterior,
124
quadratus lumborum, I15
rectus abdominis, 113
rectus internus, I15
rectus lateralis, 114
rectus ventralis, 113
retractor oculi, 137
thomboideus, 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
Myoccel, 256, 280, 281, 282, 311
Nasal passages, I51
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 (radi-
alis), 141
brachial plexus, distribution of,
140
coraco-brachialis, 141
crural and ischiadic plexuses,
142, 143
cutaneus brachii et antebrachi-
alis medialis, 141
cutaneus pectoralis, 141
dorsalis scapula (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
pneumcgastric, 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
356
Nerves—Continued
trigeminal, 68, 133, 135, 136,
oe een
trigeminal, inferior
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, 2c
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 Zodlogical Park, croco-
dilians in, 39
Nictitating membrane, 144
Notochord, 236, 238, 245, 248,
249, 251, 255, 256, 260, 263,
266, 269, 270, 285, 295, 306,
335 j
Nuchal shield, 47
maxillary
Obex, 133
Oblique muscles, 146
Odontoid process, 52, 54
(Esophagus, 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
Index
Olfactory tract, 132, 133, 134
Olivary enlargement of cesopha-
gus, I51
Optic chiasma, 134
CUP, 302, 309, 310, 321
lobes\132) 123) 135
nerve, development of, 321
stalk, 302
tracts, 134
vesicle, 274, 276, 282, 294
Oral cavity, 150
Ora serrata, 147
Osteolemus 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
Papille 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, develop-
ment 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
Proccelia, I
Pulp cavity of tooth, 65
Pupil, 146
Index
Recessus cavi tympani, 149
Recessus scale tympani, 149
Rectum, 155
transsection of (fig.), 186
Rectus muscles of eye, 146
development of, 332
Respiratory organs, 197, 200; (fig.)
19
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
papille 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
357
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, I
Sternum, 77, 80
Stomach, 152, 153
development of, 305, 326
glands of, 177, 179
histology of, 174
transsection of (fig.), 176
Stomodzeum, 317, 322
Stratum corneum, 48
Supratemporal fossa, 57
Sympathetic nerves, 314, 325, 337
Systemic arch, 202
Tail, 317, 318, 329
Tailfold, 274
Tapetum lucidum, 146
Tarsus, 88
Taste papilla, 165
Tear dots, 145
Teeth, 47, 64
Teleosauria, I
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
papille 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
358
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
Index
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
Vertebrze, 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
fA Selection from the
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2 h thie
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An Introduction to
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