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  [Transcriber's Note:

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  Smithsonian Miscellaneous Collections
  Volume 56 Number 11


  DEVELOPMENT OF THE DIGESTIVE CANAL
  OF THE AMERICAN ALLIGATOR

  With Fifteen Plates

  By
  ALBERT M. REESE
  Professor of Zoology, West Virginia University


  [Illustration: Smithsonian symbol]

  (Publication 1946)


  City of Washington
  Published by the Smithsonian Institution
  1910




  The Lord Baltimore Press
  BALTIMORE, MD., U. S. A.




DEVELOPMENT OF THE DIGESTIVE CANAL OF THE AMERICAN ALLIGATOR

By ALBERT M. REESE

Professor of Zoology, West Virginia University


In a previous paper (6) the writer described the general features in the
development of the American Alligator; and in other papers special
features were taken up in more detail.

In the present paper the development of the enteron is described in
detail, but the derivatives of the digestive tract (liver, pancreas,
lungs, etc.) are mentioned only incidentally; the development of these
latter structures may be described in a later paper.

No detailed description of the histological changes taking place during
development has been attempted, though a brief description of the
histology is given for each stage discussed.

The material upon which this work was done is the same as that used for
the preceding researches. It was collected by the author in central
Florida and southern Georgia by means of a grant from the Smithsonian
Institution, for which assistance acknowledgment is herewith gratefully
made.

Various methods of fixation were employed in preserving the material. In
practically all cases the embryos were stained in toto with Borax
Carmine and on the slide with Lyon's Blue. Transverse, sagittal, and
horizontal sections were cut, their thickness varying from five to
thirty microns, depending upon the size of the embryos.

The first indication of the formation of the enteron is seen in the very
early embryo shown, from the dorsal aspect, in figure 1. The medullary
folds and notochord are evident at this stage, but no mesoblastic
somites are to be seen.

A sagittal section of approximately this stage, shown in figure 1A,
represents the foregut, _fg_, as a shallow enclosure of the anterior
region of the entoderm, while the wide blastopore, _blp_, connects the
region of the hindgut with the exterior. No sign of a tail fold being
present, there is, of course, no real hindgut. The entoderm, which has
the appearance of being thickened because of the fact that the notochord
has not yet completely separated from it, is continuous, through the
blastopore, with the ectoderm. Posterior to the blastopore the primitive
streak, _ps_, is seen as a collection of scattered cells between the
ectoderm and the entoderm, apparently formed by proliferation from the
ventral side of the ectoderm.

A slightly later stage is shown in figure 2, a dorsal view of an embryo
with five pairs of mesoblastic somites. A sagittal section of this stage
is shown in figure 2A. The foregut is here more inclosed, and the
notochord, _nt_, having separated from the entoderm, _en_, is seen as a
distinct layer of cells extending from the foregut to the blastopore.

A transverse section through the headfold of this stage is shown in
figure 2B. The foregut is seen as a wide cavity, _ent_, depressed
dorsally, apparently, by the formation of the medullary groove and the
notochord; it is wider laterally than in a dorso-ventral direction, and
its walls are made up of about three layers of closely arranged,
irregular cells; the wall is somewhat thinner on the dorsal side, just
below the notochord.

Figure 3 is a dorsal view of the next stage to be described; about
fifteen pairs of somites are present.

Figure 3A is a transverse section through this embryo near the anterior
end of the enteron, _ent_, which cavity, cephalad to this region, is
bluntly pointed. As seen in the figure the enteron is here wide from
side to side, and is depressed dorso-ventrally except for a wide groove
in the ventral wall. This groove is lined with rather more closely
arranged cells, and marks the region where the mouth will break through
at a somewhat later stage. A short distance caudad to this region the
groove disappears and the pharynx is reduced to a shallow slit extending
almost to the superficial ectoderm on either side; then the slit-like
pharynx becomes suddenly reduced in a lateral and increased in a
dorso-ventral direction, to assume the outline shown in figures 3B and
3C. At a point about one-third of the length of the embryo from the tip
of the head, the enteron opens to the yolk-sac, so that what now may be
called the foregut has this considerable extent. There is, however, not
the slightest indication of a tail-fold, so that there is no inclosed
hindgut at all. As is shown in figure 3D, the neurenteric canal, _nc_,
still opens ventrally, though the medullary canal, _mc_, has now no
dorsal opening to the exterior. The medullary canal continues for a
short distance (about fifteen sections of five microns thickness)
posterior to the opening of the neurenteric canal.

Figure 4 is a surface view of the next stage to be described. There are
here about twenty pairs of somites, though the exact number cannot be
determined. Although not visible externally in the surface view shown,
the gill clefts are beginning to form, and the first one opens to the
exterior as will be seen in sections of another embryo of this stage.
The mouth has now broken through, putting the wide pharynx into
communication with the exterior; probably the mouth opening is formed at
about the time of the opening of the first gill cleft.

Figure 4A represents a transverse section through the head of an embryo
of the approximate age of the one just described; it passes through both
forebrain, _fb_, and hindbrain, _hb_; through the extreme edge of the
optic vesicles, _ov_, and through the anterior end of the notochord,
_nt_. It is just cephalad to the anterior end of the pharynx and to the
hypophysis. The chief purpose in showing this section is to represent
the two large head-cavities, _hc_. The origin of these cavities may be
discussed at a later time. They are irregularly oval in cross section,
and extend in an antero-posterior direction for a distance about equal
to their long axis as seen in cross section. The two cavities project
towards each other in the middle line, and are almost in contact with
the notochord, in the region figured, but they do not fuse at any point.
These two head-cavities are the only ones to be seen, in this animal,
unless the small evaginations from their walls represent other cavities
fused with these. Their walls are thin but distinct, and consist of a
single layer of cells. These cells are completely filled with their
large, round nuclei, so that the wall has the appearance, under higher
magnification than is used in this figure, of a band of closely strung,
round beads.

Figure 4B represents the eighteenth section caudad to the one just
described. It passes through that region of the enteron, _ph_, which may
be called the pre-oral gut, since it lies cephalad to the now open
mouth. Owing to the plane of the section the upper angle of the first
gill cleft, _g^1_, is seen on the left, although this would not
naturally have been expected in a section through the pre-oral gut. The
evagination to form the hypophysis, _p_, is seen against the floor of
the forebrain, _fb_. The wall of this region of the enteron is
comparatively thin, and consists of not more than two layers of
compactly arranged cells with round nuclei.

Figure 4C is about forty sections caudad to the one just described. It
passes through the mouth, seen as a vertical opening between the two
mandibular arches, _md_. The hyomandibular cleft, _g^1_, the only one
which opens to the exterior in this embryo, is very wide, and may be
traced through a number of sections; in this section the opening is seen
only on the left. The pharynx, _ph_, is very wide; as it is followed
caudad its ventral opening is gradually closed by the approach of the
two mandibular folds. The dorsal wall of this region of the pharynx is
very thin, consisting of a single layer of flat cells with round nuclei;
while the ventral wall, leading through the mouth and lining the
mandibular folds, is composed of two or three layers of compactly
arranged cells.

Figure 4D is through a plane sixteen sections caudad to the last. In
this region, which is just caudad to the otic vesicles, the pharynx has
still its rectangular outline, and its walls are of the same character
as in the preceding figure. The posterior edges of the hyomandibular
clefts are seen projecting in a ventro-lateral direction, _g^1_; while
dorsal to these are the wider, second pair of clefts, _g^2_. Where the
mandibular folds come together posterior to the mouth, they fuse first
at their outer or ventral border, which leaves a deep, narrow groove in
the anterior floor of the mouth. As this groove is followed caudad its
ventral wall is seen to become much thickened, _tg_, to form the
_anlage_ of the thyroid gland. In the present section the walls of the
groove are just fusing, to cut off the cavity of the gland from the
dorsal part of the groove. The next section caudad to this shows the
thyroid as a round, compact mass of cells, with a very small lumen,
still closely fused with the bottom of the oral groove. The lumen may,
in this embryo, be traced for only a few sections, caudad to which the
thyroid is seen as a small, solid mass of cells unattached to the oral
groove. Close to the sides of the thyroid are seen two large blood
vessels, _ar_, the mandibular arches, which unite into the single
ventral aorta just caudad to the posterior end of the thyroid. High
power drawings of the thyroid just described are shown in figures 4E
and 4F.

Figure 4G is about fifty-five sections caudad to the preceding figure,
and passes through the middle region of the heart, _ht_. The enteron,
_ent_, is cut caudad to the last gill cleft, but it is nearly as large
as in the pharyngeal region described above; its walls are of a more
even thickness than in the more anterior sections, though there is an
area, just below the aorta, where the wall is still but one cell thick.
In the ventral wall of this part of the enteron, and, to some extent, in
the lateral walls, there seems to be a tendency for the nuclei to become
collected toward the side of the wall away from the digestive cavity;
this condition cannot be well seen in the figure owing to the amount of
reduction in reproduction.

Figure 4H is seventy-nine sections posterior to the last, and passes
through the foregut, _ent_, just cephalad to the anterior intestinal
portal and caudad to the heart. The outline of the enteron is here
almost a vertical slit, and the lining entoderm consists, in its dorsal
and lateral regions, of a single layer of columnar epithelium, while in
its ventral region, where it adjoins the liver trabeculae, it is made up
of several layers of cuboidal or irregular cells. The nuclei in the
dorsal and lateral regions of the entoderm are arranged in a very
definite layer at the basal ends of the cells, though an occasional
nucleus may be seen near the center of the layer. The mesoderm that
extends ventrad from the mesentery, on each side of the entoderm just
described, consists of a thick layer of compactly arranged cells. The
ventral end of the entodermal wall is fused with the wall of a small
cavity, _li_, which may be traced several sections cephalad to this
plane. This cavity is a part of the system of hollow liver trabeculae
seen as a group of irregular masses of cells ventrad to the enteron at
the opening of the anterior intestinal portal. The large blood vessel,
_bv_, is the meatus venosus.

Figure 4I is just four sections caudad to the preceding. It passes
through the anterior intestinal portal, _aip_. The medial liver
trabecula into which the enteron was seen to open, in the preceding
figure, now opens ventrally to the yolk-sac as the anterior intestinal
portal. A few liver trabeculae are to be seen on either side of the
portal, but they show no lumena, and may be traced through only a few
sections. The extent of this uninclosed region, the midgut, is very
difficult to determine with accuracy, but, at this stage, it comprises
about one-half of all the sections of the series. The difficulty is due
partly to the unavoidable tearing of the tissues in removing the embryo
from the yolk-sac, and partly to the indefiniteness of the posterior
intestinal portal, where the walls of the enteron are very thin. As seen
in figure 4I the location of the anterior intestinal portal is very
distinct.

A short distance caudad to the anterior intestinal portal there is
constricted off from the roof of the midgut a narrow diverticulum,
figure 4J, _i_, the meaning of which is not apparent; it extends through
only ten to fifteen sections, tapering caudad till it disappears. The
region of the hindgut, at this stage, is about one-fifth of the entire
length of the embryo. Its anterior portion is wide and, as has been
said, rather indefinite in outline.

Figure 4K represents a typical section through the midgut region of an
embryo of about the age of the one from which the preceding figures were
drawn. This and the following figures of this stage were drawn from an
embryo in which the posterior region was in better condition than in the
embryo from which the other figures of the stage were taken. The
mesentery, _ms_, is here of considerable length and continues around the
yolk in a layer of diminishing thickness. The epithelium of this region
of the enteron consists of a single layer of fairly regular cells, which
are columnar in the dorsal region, just beneath the mesentery, and
cuboidal or even flattened in regions more distant from the median
plane.

Figure 4L, through the region of the hindgut, shows at _i_ the
completely inclosed intestine; it is a comparatively narrow tube, lined
with columnar epithelium outside of which is a dense layer of mesoblast
continuous with the mesentery. In the center of the figure the
allantois, _al_, is seen as an irregular cavity, lined with a single
layer of columnar or cuboidal cells, and surrounded by a thick mass of
loosely arranged, stellate mesoblast cells. The allantois is probably
somewhat larger here than in the other embryos used for this stage, in
which it was torn away. The tail, _t_, of the embryo is shown at the
lower side of the figure, surrounded by the amnion; it is cut in the
region of a curve so that the caudal intestine, _i_, is cut
longitudinally and has the outline of an elongated ellipse. In this
embryo the caudal intestine could be followed to the end of the tail,
through several dozen sections; for some distance posterior to the
allantois it is extremely narrow, so that its lumen is almost
obliterated, and its walls are made up, in any one place, of not more
than a dozen cuboidal cells. Towards the posterior end of this region
the intestine is considerably enlarged as seen in figure 4L.

Figure 4M passes through the region where both the allantois and the
Wolffian ducts open into the hindgut. The union of the allantois and the
gut accounts for the elongated outline of the enteron in this section.
The openings of the Wolffian ducts, _wdo_, are seen at the lower end of
the section of the enteron. The cells lining the Wolffian ducts are
smaller than those lining the enteron. In the lower side of the figure
are seen the structures of the tail, including the outline of the tiny
caudal intestine, _i_, mentioned above. No sign of a cloacal
invagination could be made out with certainty.

The next stage to be studied is shown in surface view in figure 5.

Figure 5A represents a section through the head region of this embryo.
Owing to the obliquity of the plane of the section the figure is quite
asymmetrical. The pharynx, _ph_, is lined with a comparatively thin
epithelium and opens, on the left, at two places, one the mouth and the
other the second gill cleft, _g^2_. In the dorsal wall of this cleft, as
well as in the corresponding wall of the opposite cleft, is seen a
thickening of the epithelium; these thickenings, _ty_, are the rudiments
of the thymus gland, whose development may be described in detail in
another paper. Compared to the size of the gill clefts the cavity of the
pharynx is, at this stage, comparatively small.

Followed caudad the pharynx becomes depressed until, in the region shown
in figure 5B, it is a mere narrow slit, _g_, extending transversely
across the embryo and opening through the gill clefts to the exterior on
each side.

Figure 5C passes through the posterior region of the pharynx, _ph_, the
tip of the forebrain, _fb_, the anterior edge of the heart, _ht_, and
the curve of the tail, _t_. The chief point of interest in this section
is the thyroid gland, _tg_. It now lies deep in the tissue of the floor
of the pharynx, entirely separated from the pharyngeal epithelium. It
consists of a compact mass of cells, now showing a bilobed structure in
its anterior end, and extending through about twenty-five ten-micron
sections. It is solid throughout most of its extent, but, in the section
figured, which is near the anterior end, the lobe on the right side
shows a small but distinct cavity scarcely visible in the figure.

Caudad to the region just described the pharynx contracts suddenly to
form the oesophagus, a narrow, V-shaped slit, which soon divides into an
upper and a lower cylindrical tube, figure 5D, _ent_.

Followed caudad the lower of these tubes divides into the two bronchial
rudiments, figure 5E, _lu_, which, in the embryo here figured, extend
through nearly one hundred sections. In the region shown in figure 5E
the three tubes, _oe_ and _lu_, lie at the angles of an imaginary
equilateral triangle, while in the region of the liver, where the
bronchial rudiments end, the tubes lie in the same horizontal plane.

A short distance caudad to the ends of the bronchial rudiments the
oesophagus turns suddenly ventrad and becomes much enlarged to form the
stomach, figure 5F, _i´_, which may be traced through twenty-five or
thirty sections in this series. The epithelium of the stomach is fairly
thick, and consists of five or six layers of compact, indistinctly
outlined cells with spherical nuclei. Ventrad to the stomach is seen, in
figure 5F, a section of the duodenum, _i_, which extends, with gradually
diminishing caliber, for twenty-five or thirty sections caudad to the
posterior limit of the stomach, where it opens to the yolk-sac and is
lost.

The section that cut this embryo in the posterior region of the stomach
also passed through the hindgut in the region of the posterior
appendages, figure 5G. There the intestine, _i_, is a distinct,
cylindrical tube which extends, with not much variation in caliber, and
with little variation in position, from this point to the cloaca.
Followed cephalad, towards the posterior intestinal portal, it gradually
diminishes in caliber, as did the foregut on approaching the anterior
intestinal portal. The epithelium consists here of three or four layers
of compactly arranged cells, and has about the same appearance as in the
oesophagus and duodenum.

Figure 5H represents a section through the cloacal region, _cl_, showing
the openings into the cloaca of the Wolffian ducts, _wdo_. Just anterior
to these openings the cloaca opens ventrally into a small,
anteriorly-projecting pouch, the rudiment of the allantois.

Caudad to the openings of the Wolffian ducts the cloaca extends ventrad
as a narrow, solid tongue of epithelium towards the exterior, figure 5I,
and fuses with the superficial ectoderm at the caudal end of a prominent
ridge that lies in the mid-ventral line between the posterior
appendages. In this embryo the cloaca has no actual opening to the
exterior; the walls of the part that projects towards the exterior are
in close contact, except in the region of the openings of the Wolffian
ducts, as is shown in figure 5H.

Owing to the coiling of the end of the long tail the plane of the
section, as is seen in figure 5I, passes through the posterior end of
the embryo no less than four times. In the most posterior of these four
sections of the tail, beginning slightly caudad to the section here
shown, is seen a small cavity which may be called the post-anal gut,
_pag_. It has thick walls, and extends for about thirty-five sections in
the series under discussion. Its lumen is very large in its caudal
region, figure 5I, _pag_, and tapers gradually cephalad until it
disappears. Posteriorly the post-anal gut ends quite abruptly not very
far from the extreme tip of the tail.

Figure 5J is a composite drawing from reconstructions of the enterons of
two embryos of approximately this stage. One of these reconstructions
was plotted on paper from a series of transverse sections; the other was
made in wax from a series of sagittal sections. For the sake of
simplicity the gill clefts are not represented, and the pharynx, mouth,
and liver are represented in outline only. For the same reason the lung
rudiment of one side only is shown.

The relative size of the pharynx, _ph_, as seen in the figure, is
smaller than it is in reality because of the small dorso-ventral
diameter (the only one here shown) compared to the lateral diameter. The
end of the lung rudiment, _lu_, is slightly enlarged and lies in a plane
nearer to the observer than that of the oesophagus, _oe_, though this is
not well shown in the figure.

The oesophagus, _oe_, diminishes slightly in caliber for a short
distance caudad to the origin of the lungs, then gradually increases in
caliber until it suddenly bends to the side (towards the observer) and
merges into the wide stomach, _i´_. The stomach, which is irregularly
conical in shape, lies in a place slightly nearer the observer than the
end of the lung rudiment mentioned above.

Lying to one side of the stomach and duodenum, and extending cephalad
beyond the end of the lung rudiment is the liver, _li_, whose outline is
only roughly shown here by the broken line. The stomach opens rather
abruptly into the duodenum, _d_, which slopes back towards the plane of
the oesophagus (away from the observer).

The projection from the side of the duodenum, _pan_, not well figured
here, indicates the position of the pancreas, better shown in the next
reconstruction. The duodenum extends only a short distance caudad to
this point and then opens, _aip_, to the yolk-sac.

The yolk-stalk, or unclosed region of the enteron, is still of
considerable extent, though its exact boundaries are not easy to
determine. The distance between the anterior and posterior intestinal
portals is approximately shown in the figure under discussion.

The hindgut is cylindrical in cross section and of about the same
diameter throughout, except for a slight enlargement in the cloacal
region.

The post-anal gut is not shown here; it will be described in connection
with the next reconstruction where it is figured.

Figure 6 is a surface view in profile of an embryo of the next stage to
be studied. The manus and pes are here well developed, and the general
development of the embryo is in considerable advancement over the last
stage studied.

Figure 6A represents a reconstruction, from a series of transverse
sections, of the enteron of an embryo of about the age of the one shown
in figure 6. The outlines of the entire embryo, of the eye, _e_, and of
the anterior, _aa_, and posterior, _pa_, appendages are shown by broken
lines. Its position being coincident with that of the stomach, liver,
and pancreas, the anterior appendage can scarcely be seen. The enteron,
including one lung only, for the sake of simplicity, is shaded solid
black, while the liver and pancreas, with their ducts, are outlines with
unbroken lines. As in the preceding reconstruction no attempt is made to
show the gill clefts, and only the dorso-ventral profile of the enteron
is shown. Caudad to the pharynx, the enteron being more or less
cylindrical in section, this profile gives a good idea of its shape, but
in the pharyngeal region, where the lateral diameter is so much greater
than the dorso-ventral, the reconstruction gives but a poor idea of the
size of that part of the enteron.

The widely-open mouth, _m_, leads, with no line of demarkation, into the
pharynx, _ph_, which is of irregular outline and, as has been said, of
much greater lateral than dorso-ventral diameter.

The pharynx becomes gradually constricted to form the oesophagus, _oe_,
a very long and slender structure, which, as will be seen in cross
section, is, at this stage, solid for the greater part of its length. As
in the case of the pharynx, the lateral diameter of the oesophagus is
generally greater than the dorso-ventral diameter.

From the floor of the caudal part of the pharynx is pushed out the
trachea, _ta_. In the reconstruction, especially in the anterior end,
the trachea appears several times the diameter of the oesophagus; this
is due to the great thickness and indefiniteness of its walls rather
than to a greater diameter of its lumen.

At about the position of the line _ta_ the trachea divides into the two
bronchi (only one shown in the figure), which are somewhat enlarged at
the ends to form the lung rudiments, _lu_. While the trachea and bronchi
lie ventrad to the oesophagus, the lungs lie laterad and even dorsad to
the oesophagus and cardiac end of the stomach. Caudad to the heart and
in the region of the anterior appendages, _aa_, the oesophagus suddenly
enlarges to form the stomach, _i´_, which has now quite the outline of
the typical human stomach.

From the stomach the duodenum, _d_, extends, following a sort of
V-shaped course, towards the yolk-stalk, _ys_. In the region of the
yolk-stalk it is somewhat enlarged and ends in a blind sac like a
caecum. At the side of this sac is seen the opening of the enteron to
the yolk-stalk; the anterior and posterior intestinal portals are not
distinguishable from each other. From this point the hindgut, _hg_,
extends cephalad until it lies laterad to the middle region of the
duodenum, then bends through 180° and extends, in an almost straight
line, to the cloaca, _cl_, lying in the region of the posterior
appendage, _pa_.

The allantois, _al_, extends cephalad for some distance from the floor
of the cloaca. Some distance caudad to the cloaca, near the end of the
much coiled tail, is seen the post-anal gut, _pag_. This structure as
has been noted above, is quite distinct from the other parts of the
enteron. It is of elongated, pyriform outline, with the pointed end
extending cephalad.

In the narrow space between the stomach and the duodenum is the
elongated pancreas, _pan_, opening by two or more short ducts into the
duodenum.

The liver, _li_, in the figure under discussion, has about twice the
area of the stomach. It extends caudad and dorsal about the same
distance as the latter organ, but it extends ventrad and cephalad far
beyond the boundaries of the stomach.

Extending along the ventral border of the liver is a long narrow duct,
apparently the bile duct, _bd_. It connects, caudally, with the anterior
end of the pancreas, while at its other extremity, near the
antero-ventral corner of the liver; it ends blindly.

The transverse sections now to be described have been selected from the
series from which the reconstruction, just described, was made.

Figure 6B represents a typical section through the pharynx. Its plane is
approximately shown by the line 400 of figure 6A though the plane
apparently does not cut the eye, _e_. The pharynx, _ph_, has here the
outline of an irregular V. Its walls, except at the outer angles of the
clefts, _g^1_, are composed of but a single layer of cells. In the
dorsal wall these cells are flattened, while in the ventral wall they
are more rounded. This difference in the shape of the cells accounts for
the slightly greater thickness of the floor over that of the roof of the
pharynx. The gill clefts no longer communicate with the exterior.

Figure 6C represents the caudal half of the embryo in the plane 475 of
figure 6A. The section of the pharynx, _ph_, is here crescentic in
outline, and the pharyngeal walls, especially the floor, are somewhat
thicker than in the more anterior section just described. Lying a short
distance dorsad to the pharynx are seen two small, thick-walled
openings, _ty_; these are the rudiments of the thymus glands. They are
here quite distinct from the enteron, and may be traced through a large
number of sections, being in some regions solid and of a smaller
diameter than in the present section.

Figure 6D is in the region of the line 500 in figure 6A. The thymus
rudiments, _ty_, have about the same appearance as in the preceding
figure, except that they are somewhat larger. The pharynx, _ph_, is much
smaller than in the last section, and though somewhat crescentic in
outline, its convex side is dorsal instead of ventral in position. The
pharyngeal walls are here thicker, and consist of two or three layers of
cells, instead of the single layer of more anterior sections.

In the median plane the floor of the pharynx is pushed down, as a solid
tongue of cells, _gs_, the anterior edge of the glottis. Ventrad and
laterad to the glottis a crescentic condensation of mesoblast represents
the beginning of the laryngeal cartilages, _la_.

Two or three sections caudad to the one just described, the two layers
of which the tongue of cells from the floor of the pharynx is composed
separate slightly at the bottom to form a small cavity, the trachea,
_ta_; this condition is shown in figure 6E, which represents part of a
section through the plane 532 of figure 6A.

The oesophagus, _oe_, is here a solid, crescentic mass of cells, the
lumen being completely obliterated. The dorsal part of the tongue of
cells, mentioned above, connects the ventral side of the oesophagus with
the trachea, like a sort of mesentery. Above the oesophagus, on either
side, is the thymus rudiment, _ty_, in this section practically a solid
mass of cells instead of a tube. The epithelium of the trachea here
consists of three or four layers of compactly arranged cells; this
epithelium is surrounded by a dense mass of mesoblast which is
responsible for the greater thickness of the trachea as seen in figure
6A. As has been said, the oesophagus here has no lumen, and when
examined under high magnification its walls are found to be completely
fused, not merely in close contact. The same is true of the tongue of
cells between the oesophagus and trachea. Two or three sections caudad
to the one under discussion this tongue of cells loses its connection
with the trachea, and the latter structure is entirely independent of
the oesophagus.

The solid condition of the oesophagus continues through about fifty
sections of this series, the horns of the crescent gradually shortening
until only the central part remains as the hollow cylinder seen in
figure 6F, _oe_, which is a section through plane 650 of figure 6A. From
about this point to its opening into the stomach the oesophagus has
essentially the same structure. Its epithelium is of the simple columnar
type, the cells being long, with generally basally located nuclei.

In the section under discussion the trachea, _ta_, is of about the same
size as the oesophagus, but its epithelium is thicker and consists of
two or three layers of cells. The trachea extends, as a separate and
distinct structure, through about one hundred and fifteen sections, and
then, at a point four or five sections caudad to the present section, it
divides suddenly into the two bronchial tubes. Each bronchus, like the
trachea, is lined with an epithelium of three or four layers of cells;
but the epithelium is surrounded by a thin layer of much condensed
mesoblast. The bronchi continue caudad, with slightly increasing
caliber, through about fifty sections, when they suddenly enlarge to
form the lungs. As seen in figure 6A the lungs are irregularly conical
in outline and lie on either side of the posterior end of the
oesophagus.

Figure 6G is a section through the plane 750 of figure 6A. The
oesophagus, _oe_, is seen as a small, circular opening between two much
larger openings, the lungs, _lu_. The epithelium of the oesophagus is
the same here as in the more anterior regions described above; that of
the lung rudiments is very variable in thickness, even in different
parts of the same section, being in some places composed of a single
layer of cuboidal or even flattened cells, in other places consisting of
four or five layers of cells (not well shown in the figure). Surrounding
the epithelium of the lung rudiments is a thin layer of quite dense
mesoblastic tissue. A fairly well defined mesentery, _ms_, is now
present in this region.

Filling the greater part of the body cavity, below the oesophagus and
lung rudiments, is the liver, _li_; and ventrad to the liver the section
passes through a loop of the duodenum, _d_.

The epithelium of the duodenum consists of four or five layers of
compactly arranged cells, near the center of an oval mass of fairly
dense mesoblast. In a lateral projection of this mass of mesoblast lies
a small, circular opening, the bile duct, _bd_. Its epithelium consists
of a single layer of columnar cells. In more anterior sections the bile
duct is larger in cross section, being about one-half the diameter of
the oesophagus. As has been said it ends blindly at a point a short
distance anterior to the antero-ventral edge of the liver. A few
sections caudad to the one under discussion the bile duct connects with
the liver, figure 6A, _bd´_; and some distance caudad to this the duct
opens, _bd´´_, into the duodenum so close to the opening, _pan´_, of the
pancreas that it is difficult to determine whether the latter organ has
a separate opening into the duodenum or opens into the bile duct.

At some distance ventrad to the structures just described the intestine
is cut, by the plane of the section, in two places, _i_. The more dorsal
of these is inclosed and has, under this magnification, the same
appearance as the duodenum, _d_; a higher magnification, however, shows
that its epithelium consists of a single layer of tall, rather clear,
columnar cells. The more ventral of the two sections, above mentioned,
which is continuous with the dorsal section a very short distance caudad
to this point, is in the region that opens to the yolk--in fact a number
of yolk-granules, _y_, may be seen in the opening. The epithelium of
this part of the intestine consists of a single layer of clear, columnar
cells, which, around the borders of the opening, are thrown into
numerous folds and are almost of goblet form.

Figure 6H represents a section through the plane 820 of figure 6A. The
section is caudad to one lung and cuts the extreme tip of the other,
_lu_. The liver, _li_, and pancreas, _pan_, are seen at the side of the
stomach, _i´_, here cut through its greatest transverse diameter. The
epithelium of the stomach varies somewhat in thickness and consists of
two or three layers of cells, the variation in thickness being due to a
variation in the length of the cells rather than to a variation in the
number of layers.

Ventrad to the stomach the intestine, _i_, is cut in three places, of
which the most dorsal section is the largest. The epithelium of these
intestinal sections, especially the lower two, consists of usually a
single layer of columnar cells which are clearer than those of the
stomach. A fairly thin mesentery, _ms_, supports this region of the
intestine.

In the region of the posterior appendages, _pa_, the section passes
through the hindgut, _hg_, and allantois, _al_. The former is of about
the same size as the more anterior sections of the intestine, but its
epithelium is less clear and is composed of two or more layers of cells.
The allantois is cut near its opening into the hindgut; its walls are
thin, the epithelium consisting of but a single layer of more or less
flattened cells.

Figure 7 represents a reconstruction of the enteron of an embryo of 42
mm. crown-rump length. Because of the body flexure and large size of the
embryo the head was amputated, in the plane _a-b_, and cut sagitally,
while the body was cut transversely in the direction shown by the
section planes. In the present figure the outline of the embryo,
including the eye, appendages, and umbilical stalk, is shown by fine
dotted lines; the outlines of the lungs and liver are shown by heavier,
broken lines; while the outlines of the enteron proper and the trachea
are shown in solid lines, filled in which fine stippling. For the sake
of simplicity only one lung and one bronchus are shown.

Since the head has now quite a reptilian form, the oral cavity, _m_, has
more or less of the adult outline. A transverse groove near the anterior
end of the lower jaw marks off the tongue, _tn_; and the rudiments of
teeth are seen but not shown in the figure because of the low
magnification used.

The pharynx, _ph_, is a very extensive cavity that is sharply separated
from the mouth by a prominent transverse fold of skin, the velum
palitum, _vp_, just in front of the posterior nares, _pn_, and by a less
marked fold from the base of the tongue; it is these two valves that
enable the adult alligator to open its mouth under the surface without
getting water into the lungs. The mouth and pharynx are lined at this
stage with a thin, stratified epithelium, which consists of a basal
layer of rather tall columnar cells and one or two superficial layers of
flattened cells. The pharyngeal epithelium is rather thicker than that
of the oral cavity.

In the embryo from which this reconstruction was made the pharynx was in
direct communication with neither the oesophagus nor the trachea, though
the separation in each case was by a mere membrane. The trachea, _ta_,
opens, except for this membrane, into the pharynx a short distance back
of the transverse, dorsal and ventral folds mentioned above, and almost
directly ventrad to the posterior nares. The anterior end of the
oesophagus, _oe_, is in contact with the extreme postero-ventral wall of
the pharynx.

The trachea, which is already surrounded by distinct cartilaginous
rings, is long, and of about the same diameter throughout. In the region
of the anterior appendage, at the point marked _X_, it divides into the
two very short bronchi, which almost immediately open into the lungs,
_lu_. The lungs, whose structure will be shown in the sections of this
stage, are large, irregular bodies, extending about equal distances
cephalad and caudad to their openings into the bronchi. The caudal ends
of the lungs overlap the cephalic end of the liver, _li_.

The oesophagus, _oe_, is large, and is laterally compressed so that its
dorso-ventral diameter, the one shown in the present figure, is two or
three times as great as its lateral diameter. This gives the impression,
in the reconstruction, that the oesophagus is nearly as large as the
stomach.

As has been said, the oesophagus does not open directly into the
pharynx, but is separated from it by a membrane which consists of the
flattened epithelial layers of both cavities separated by a thin layer
of mesoblast. This partition between the pharynx and the oesophagus is
not a mere fold of mucous membrane, but is a complete, though thin,
wall, easily seen in the series of sagittal sections from which this
region of the embryo was drawn. The anterior end of the oesophagus is
suddenly constricted so that the actual opening closed by this partition
is not large.

Followed caudad the dorso-ventral diameter of the oesophagus varies
somewhat, as does the lateral diameter, but it remains large throughout
and opens into the stomach with no sharp line of demarkation. The
character of the epithelium of the enteron caudad to the pharynx will be
discussed in connection with the sections to be described below.

The stomach, _i´_, is very different in outline from what was seen in
the last stage described, figure 6A. Instead of having approximately the
form of the typical mammalian stomach it is now so elongated that the
opening into the duodenum, the pylorus, _py_, seems to be nearer the
anterior than the posterior end. While the position of the pylorus is
very distinct it is difficult to distinguish the line of demarkation
between the stomach and the oesophagus.

The extreme caudal region of the stomach is enlarged to form a blind
sac, representing the gizzard, _gz_. A slight enlargement in the region
of the pylorus may represent the glandular region of the adult stomach.
The stomach opens, in a rather curious way, into the side of the
duodenum, _d_, the anterior end of the latter structure having the
appearance of a sort of caecum, to be seen in the next stage of
development.

The duodenum, _d_, makes a U-shaped bend at the side of the stomach, and
then, in the region of the caudal edge of the gizzard, _gz_, dips
suddenly ventrad and caudad towards the umbilical cord, _u_, where it
apparently ends blindly, though this appearance is probably due to an
artifact in the embryo from which the reconstruction was made. It is
likely that, in removing the embryo from the yolk, the connection
between the two loops, _i_, of the intestine was broken.

The ascending intestinal loop is of slightly less caliber than the
descending loop above mentioned; it passes dorsal and cephalad to the
posterior border of the gizzard where its lumen is continuous, for a
short distance, with that of the descending loop above described. This
unusual condition is probably abnormal, but owing to lack of material
only one series of this stage was studied.

At the dorso-caudal angle of the gizzard the small intestine, _i_, opens
into the ventral side of a larger tube which may be called the large
intestine, _il_. The blind end of the large intestine, cephalad to the
opening of the small intestine, projects forward, dorsal to the gizzard,
as a sort of caecum, _ce_, though this structure is generally stated to
be wanting in the crocodilia, and is not seen in the next stage.

From the caecum the large intestine passes in a ventro-caudal direction,
with gradually decreasing caliber, to the cloaca, from whose anterior
wall the intromittent organ, _io_, projects.

From the ventral wall of the large intestine, at a point about one-third
the distance from the cloaca to the caecum, projects ventrad and
cephalad the stalk of the allantois, _al_. Owing to its thin walls and
small lumen the allantois was traced only a short distance into the
umbilical stalk.

The profile of the liver, _li_, has, at this stage, about the same area
and even outline as that of the lung. It lies, of course, on both sides
of the enteron proper, and overlaps, anteriorly, as has been said, the
posterior end of the lung.

Figure 7A represents a section through the plane 305 of figure 7.
A considerable advance in the general development of the organs is seen
over the last stage studied. The spinal column is well outlined in
cartilage, and the ribs are cut at various places, _r_. In the body wall
a considerable differentiation of muscular tissue has taken place, but
it is only faintly shown in this series of figures. The scales,
especially along the mid-dorsal line, are shown as an area of less
closely dotted tissue.

The lungs, _lu_, cut here through their anterior ends, are large, but do
not nearly fill the cavities, _bc_, in which they lie; they have the
sacculated appearance characteristic of embryonic lung tissue.

The oesophagus, _oe_, is cut through about its middle region, where its
caliber is greatest. As was said above, its dorso-ventral diameter is
more than twice its lateral diameter, caused partly by the oblique angle
at which it was cut. Its wall, figure 7H, is very thin and exhibits a
dense layer of mesoblastic tissue, in which circular and longitudinal
muscle layers are beginning to differentiate. It is lined by an
epithelium which here consists of a single layer of columnar or cuboidal
cells with large nuclei. On the ventral side, where the oesophageal wall
is in contact with that of the trachea the epithelium is somewhat
thickened by an increase in the number of cell layers. With the low
magnification used these details could not, of course, be shown.

The trachea, _ta_, is of much smaller caliber than the oesophagus,
especially in its dorso-ventral diameter. While its epithelial lining is
not yet appreciably different from that of the oesophagus, its
connective tissue wall is much thicker and shows numerous condensations,
the rudiments of the cartilaginous rings. In the region represented by
this figure the connective tissue layers of the trachea and oesophagus
are continuous with each other, but cephalad and caudad to this point
they are distinct, though sometimes in contact. Several large blood
vessels, _bv_, on each side of the oesophagus probably represent the
carotids and jugulars, but they were not worked out to determine with
certainty which they were.

Eighty-five sections (figure 7, _X_) caudad to the one under discussion
the trachea divides into the two bronchi. These bronchi gradually
separate from each other until, at the point at which they open into the
lungs, about eighty sections caudad to their point of separation, they
lie on either side of the ventral third of the oesophagus.

Figure 7B represents a section through the plane 480 of figure 7. The
section is just cephalad to the heart, and passes through the caudal
third of the lungs, _lu_, which have the same appearance as in the
preceding figure; also through the extreme cephalic end of the liver,
_li_. The lungs here much more nearly fill the body cavity than in the
preceding figure. The section being caudad to their openings into the
lungs the bronchi do not, of course, show.

The oesophagus, _oe_, is here of much less diameter than in the
preceding figure, but is still laterally compressed. Its wall is
somewhat thicker than in the more cephalic region, the increase being
mainly due to the greater thickness of the connective tissue layer,
though the epithelium is also slightly thicker because of an increase in
the length of the lining cells. Instead of lying almost entirely ventrad
to the lungs, as in the preceding figure, the oesophagus here lies
directly between them.

Figure 7C represents a section through the plane 627 of figure 7. The
plane of the section passes through the opening of the stomach, _i´_,
into the duodenum, _d_. The cross section of the stomach is somewhat
larger than that of the oesophagus, but it differs from the more
anterior region mainly in the character of its walls. These are much
thicker than in the oesophagus; in the mesoblast which forms the greater
part of their thickness, muscle fibers are beginning to differentiate.
The epithelial layer also is thicker than in the oesophagus; it consists
of tall columnar cells that, at places, are thrown into small folds,
figure 7I. These folds, even under the low magnification used, are more
evident than is shown in the present figure. The pylorus, _py_, is wide
and, as has been noted in connection with figure 7, is situated far
cephalad to the caudal end of the stomach. It opens into the side rather
than into the end of the duodenum, which projects cephalad as a short
blind pouch, _d_. The stomach and duodenum, in this section, are almost
completely surrounded by the liver, _li_.

Figure 7D represents a section through the plane 680 of figure 7.

The stomach, _i´_, which is cut through its middle region, is somewhat
larger than in the preceding figures, though its walls have about the
same character. Its outer walls are continuous, to a considerable
extent, with the tissue of the surrounding body wall, especially in the
region just caudad to the plane of the present section.

The duodenum, being cut through a double loop (see figure 7), is seen in
two places, dorsally where it is cut through the edge of one loop, and
ventrally where it is cut square across. In both sections the structure
is the same, as might be expected, figure 7J. The surrounding mesoblast
is differentiated into muscle fibers, figure 7J, _ml_, which form a
fairly distinct layer; inside of this layer is a tall columnar
epithelium, _ep´_, which is thrown into prominent folds. A thin layer of
mesoblast, probably the submucosa, _sl_, lies beneath the epithelium and
projects up into the folds. About ten or twelve folds are seen in any
one section; only the larger ones are well seen in figure 7D.

Figure 7E shows a section through the plane 770 of figure 7. It is in
the region of the umbilicus, _u_, and the extreme caudal end of the
stomach which has been called the gizzard, _gz_. The small size of the
gizzard is due to its being cut near its caudal margin. The enteron is
here cut in no less than seven places: the reason for this will be
evident on examination of the plane of the section as shown in figure 7.
Dorsal to the gizzard the section cuts the so-called caecum, _ce_,
a little nearer its anterior end than is shown in figure 7. The
duodenum, _d_, is cut at five points, and has about the same structure
as in the preceding figure. The character of the duodenal loops that
causes the rather curious appearance of the present figure will be
readily understood by reference to figure 7, though the reconstruction
is not mathematically accurate. The ventral projection of the lower
loops of the duodenum into the umbilicus is seen both in the present
figure and in the reconstruction. The loop of the duodenum that, in the
sections, is seen to lie directly ventrad to the gizzard, in the
reconstruction is shown too much to the side of the latter organ. The
descending loops of the duodenum are cut in such a way that the
surrounding mesoblast forms a continuous mass of tissue.

Figure 7F represents a section through the plane 901 of figure 7. The
section passes through the kidneys, _k_, the edge of one posterior
appendage, _pa_, the large intestine, _il_, and two regions of the small
intestine, _i_.

The large intestine is here a thick walled, cylindrical structure, _il_,
hanging from a thin mesentery, _ms_, in the much reduced body cavity.
The layers of its wall are much more fully differentiated than in the
more anterior regions of the enteron. The epithelium is here stratified
instead of simple columnar, and the folds into which it is thrown are
broader and less numerous than in the duodenum above described.

Ventrad to the large intestine, and almost in contact with it, is seen
the allantois, _al_, whose general outline was noted in connection with
figure 7. It is an irregular structure, consisting of a very thin outer
layer of mesoderm, lined with a single layer of flattened epithelial
cells.

Lying at a considerable distance ventrad to the main body of the
section, are seen the two sections of the small intestine, _i_,
surrounded by irregular strands of tissue from the umbilicus. The
structure of these two intestinal loops is about the same as in the more
anterior region described above.

Figure 7G, the last of this series, represents a section through the
cloaca, caudad to the urinary openings, in the plane 1060 of figure 7.
The epithelium of the cloaca is, of course, simply a continuation of
that of the surface of the body, somewhat thickened, perhaps, in the
deeper regions.

The intromittent organ, _io_, which projects cephalad from the wall of
the cloaca, is here seen as a three-pointed body of considerable size,
projecting ventrally from the body.

Figure 8 shows in outline the enteron, from the ventral aspect, of an
embryo of 20 cm. total length, or at about the time of hatching. The
drawing was made from a dissection and, for the sake of simplicity, only
the enteron, respiratory organs, heart, and thymus are shown. The jaw is
cut through on the left side and is turned over to the right, thus
bringing into view the roof of the mouth, _m_, and the dorsal side of
the tongue, _tn_. At the same time the pharynx, _ph_, and the wide
anterior end of the oesophagus, _oe_, are cut open, exposing the
glottis, _gs_, and vocal cords, _vc_.

The lungs, _lu_, and trachea, _ta_, which are now fully formed, are
dissected loose and drawn over to the right side of the animal, together
with the heart, _ht_, and the thymus, _ty_; only one side of the thymus
is shown, the other half being hidden by the trachea.

The mouth has reached nearly the outline of the adult. The lips are
formed and, in the anterior part of the lower jaw, four tooth rudiments,
_to_, are externally visible. The mucous membrane of the roof of the
mouth, _m_, is covered with rounded papillae, easily seen with a lens
but not shown in the figure. The tongue, _tn_, is fully formed, and is
free anteriorly and laterally to about the extent that is seen in the
adult; the papillae with which it is covered are not so prominent as
those seen on the roof of the mouth. At the base of the tongue is the
prominent transverse fold, noted in connection with figure 7, that meets
above the velum palitinum, not shown here but shown in figure 7. Caudad
to these folds is seen the glottis, _gs_, a triangular opening with the
vocal cords, _vc_, at its base.

The mucosa of the inside of the pharynx and the anterior end of the
oesophagus, exposed by the dissection, is thrown into numerous
longitudinal folds, not shown in the figure; these well-marked folds
extend throughout the length of the oesophagus.

The oesophagus, _oe_, tapers gradually from the wide pharynx, _ph_, and
then continues as a cylindrical tube of uniform diameter to the right
side of the anterior end of the stomach, where it opens into the latter
organ. Its walls are thick, and its lumen is almost obliterated by the
longitudinal folds of the mucosa, mentioned above.

The stomach, _i´_, is oval in outline, though somewhat flattened
laterally; it is depressed, dorso-ventrally, to a little more than half
the lateral diameter. As has been said, the oesophagus enters its right
anterior border; the pylorus is on the right side, 3 or 4 mm. caudad to
the oesophageal opening. The wall of the stomach is comparatively thin
except in the region of the oesophageal and pyloric apertures, and at a
point, opposite these apertures, on the left side. At the latter point
is an oval or disc-shaped area that is several times as thick as the
surrounding wall; it probably represents the gizzard structure of the
adult. The thickening mentioned in the region of the two apertures seems
to be mainly due to a wrinkling of the mucosa which, in other parts of
the stomach, is nearly smooth, so far as can be seen with the naked eye.
A sphincter thickening around the oesophageal and, to some extent,
around the pyloric aperture, causes each of these structures to project
into the stomach like an ileo-caecal valve.

The pylorus, _py_, opens into a small, pointed, thin-walled
diverticulum, _di_, and, at the same time, into the duodenum, _d_. The
diverticulum noted, also, in connection with figure 7, has relatively
thick, wrinkled walls; its significance is not known to the writer. From
this diverticulum the duodenum, _d_, leads caudad and laterad for a
short distance as a narrow tube, then suddenly expands into the widest
part of the entire intestine. Into this wide part of the duodenum, 3 or
4 mm. from the pylorus, opens the bile duct, _bd_. The bile sac, _bs_,
is an elongated oval body with thin walls, lying to the right of the
pylorus, its connection with the liver was not seen.

Lying between the anterior end of the duodenum and the posterior end of
the stomach, and extending caudad for 10 to 15 mm., in the median plane
of the animal is the pancreas, _pan_. It is a long narrow body of a
whitish color; its duct or ducts could not be determined by dissection.
The duodenum extends caudad, with gradually diminishing caliber, from
the enlarged region mentioned above. About 10 to 15 mm. caudad to the
stomach it makes a sort of double loop to the right, a wide loop, _lp_,
and a close one, _lp´_, nearer the median plane. From the latter loop
the intestine extends straight to the left, for a distance of about 10
mm., where it makes a small loop cephalad, _lp^2_, and then opens to the
yolk-sac, _y_. The yolk-sac is shown here simply as an irregular piece
of tissue, the yolk having been removed.

The anterior intestinal portal, _aip_, and posterior intestinal portal,
_pip_, are in close proximity with each other.

From the posterior intestinal portal the intestine extends straight
cephalad to the posterior end of the stomach, dorsal to which it forms a
double loop, a wider one, _lp^3_, and a narrow one, _lp^4_. From the
latter loop, _lp^4_, the intestine extends straight caudad, parallel and
near to the straight region leading from the posterior intestinal
portal, until it reaches the region of the loop _lp^2_, dorsal to which
it forms a small loop, _lp^5_. From loop _lp^5_ the intestine, which is
here of very small caliber, extends caudad for about 10 mm., where it
forms another indistinctly double loop, _lp^6_.

From loop _lp^6_ the large intestine, _il_, extends, with gradually
increasing caliber, to the cloaca, _cl_, a distance of 10 to 15 mm.

Except in the enlarged region near the pylorus the lumen of the
intestine is almost obliterated by the folding of its thick walls, so
that little or nothing can be told of its lining with the naked eye.

A distinct mesentery holds the loops of the intestine in position and
binds the entire enteron close to the dorsal body wall. Because of the
lack of properly fixed tissue no sections of the enteron of this stage
were made.




REFERENCES


1. BRONN, H. G.: Klassen des Thier-Reichs. (Vols. on reptiles.) 1890.

2. CHAFFANJON, V.: Observations sur Alligator mississippiensis (Tractus
intestinalis und Mesenterium). Ann. Soc. Linn. Lyon, vol. 28, p. 83 ff.,
1881.

3. EISLER, P.: Zur Kentniss der Histologie des Alligatormagens.
Archiv f. Mikr. Anat., vol. 34, pp. 1-10, 1889.

4. HERTWIG, O.: Comparative Embryology of Vertebrates. Especially
vol. 2, pp. 1-241, 1906.

5. REESE, A. M.: The Nasal Passages of the Florida Alligator. Proc.
Acad. Nat. Sc. Phila., 1901.

6. REESE, A. M.: The Development of the American Alligator. Smith. Misc.
Coll., vol. 51, No. 1791, pp. 1-66, 1908.




DESCRIPTION OF FIGURES 1-8, PLATES 1-15

  [Transcriber's Note:
  The word "Plate" refers to the physical pages on which the Figures
  were printed. The word is not used as an illustration identifier.
  Based on the author's age at time of publication, "Miss C. M. Reese"
  is more likely to have been his sister than his daughter.]

  The surface views were drawn, under the author's direction, by
  Miss C. M. Reese. The first two of these views were copied, by
  permission, from S. F. Clarke; the others were drawn from the
  specimens themselves.

  All of the figures of any one stage are given the same number,
  followed by distinguishing letters, so that it is possible to tell
  at a glance what figures belong together.

  All of the figures except those from Clarke were drawn under a
  camera lucida.

Figure 1. A surface view of an embryo, from the dorsal aspect, at the
beginning of the formation of the enteron.

Figure 1A. A sagittal section of an embryo of approximately the age of
the one shown in figure 1. × 43.

Figure 2. A dorsal view of an embryo with five pairs of mesoblastic
somites.

Figure 2A. A sagittal section of an embryo of the stage shown in
figure 2. × 43.

Figure 2B. A transverse section through the headfold of an embryo of the
stage shown in figure 2. × 43.

Figure 3. A dorsal view of an embryo with about fifteen pairs of
somites. × 20.

Figures 3A-3D. A series of transverse sections through an embryo of the
stage of the one shown in figure 3. × 43.

Figure 4. A surface view of an embryo with about twenty pairs of
somites. × (about) 15.

Figures 4A-4D. A series of transverse sections through the anterior end
of an embryo of the approximate age of the one shown in figure 4. × 20.

Figures 4E and 4F. Two transverse sections through the thyroid gland of
this stage; more highly magnified. × 102.

Figures 4G-4M. A series of transverse sections caudad to the preceding.
Figure 4H, × 43; other figures, × 20.

Figure 5. A surface view, in profile, of an embryo at the time of the
origin of the limbs. × (about) 5.

Figures 5A-5I. A series of transverse sections through an embryo of the
age shown in figure 5. × 7.

Figure 5J. A composite drawing of reconstructions of the enterons of two
embryos of the age of the one shown in figure 5. One reconstruction was
in wax, from sagittal sections, the other was a plotted reconstruction
from transverse sections. × 14.

Figure 6. A surface view, in profile, of an embryo with well developed
manus and pes. × (about) 5.

Figure 6A. A reconstruction, plotted from transverse sections, of the
enteron of an embryo of about the age of the one shown in figure 6.
× 14.

Figures 6B-6H. Part of a series of transverse sections from which the
preceding reconstruction was made. × 7.

Figure 7. A reconstruction of the enteron of an embryo of 42 mm.
crown-rump length.

Figures 7A-7G. A part of the series of transverse sections from which
the preceding reconstruction was made. × 7.

Figure 7H. A high power drawing of a portion of the wall of the
oesophagus in the region of figure 7A. × 190.

Figure 7I. A high power drawing of a portion of the wall of the stomach
in the region of figure 7C.

Figure 7J. A high power drawing of a portion of the wall of the duodenum
in the region of figure 7D.

Figure 8. An outline drawing, from the ventral aspect, of the enteron of
an embryo of 20 cm. length, at about the time of hatching; made from a
dissection. × 1.


LETTERING FOR ALL FIGURES

  _a_, head-fold of amnion.
  _aa_, anterior appendage.
  _ac_, anterior cardinal vein.
  _aip_, anterior intestinal portal.
  _al_, allantois.
  _an_, anterior nares.
  _ao_, aorta.
  _ar_, aortic arch.
  _au_, auricle.
  _b_, bulbus arteriosus.
  _bc_, body cavity.
  _bd_, bile duct.
  _bd´_, opening of bile duct to liver.
  _bd´´_, opening of bile duct to duodenum.
  _blp_, blastopore.
  _bp_, basilar plate.
  _bs_, bile-sac.
  _bv_, blood vessel.
  _c_, centrum of vertebra.
  _ca_, caudal artery.
  _ce_, caecum.
  _ch_, cerebral hemisphere.
  _cl_, cloaca.
  _cm_, circular muscle layer.
  _cn_, cranial nerve.
  _cp_, posterior choroid plexus.
  _cv_, cardinal vein.
  _d_, duodenum.
  _dc_, ductus Cuvieri.
  _di_, diverticulum of stomach.
  _e_, eye.
  _ec_, ectoderm.
  _ec´_, thickening of ectoderm.
  _en_, entoderm.
  _en´_, endocardium.
  _ent_, enteron.
  _ep_, epidermal layer of ectoderm.
  _ep´_, epithelium.
  _epi_, pineal body.
  _es_, embryonic shield.
  _f_, fronto-nasal process.
  _fb_, forebrain.
  _fg_, foregut.
  _g^1-5_, gill clefts.
  _gf^1-6_, gill folds.
  _gl_, glomerulus.
  _h_, head-fold.
  _gs_, glottis.
  _gz_, gizzard.
  _hb_, hindbrain.
  _hc_, head cavity.
  _hg_, hindgut.
  _ht_, heart.
  _i_, intestine.
  _i´_, stomach.
  _il_, large intestine.
  _in_, infundibulum.
  _io_, intromittent organ.
  _ir_, iris.
  _it_, iter.
  _k_, kidney.
  _la_, larynx.
  _li_, liver.
  _lm_, longitudinal muscle layer.
  _ln_, lens.
  _lp_, _lp´_, etc., loops of intestine.
  _lu_, lungs.
  _lv_, lens vesicle.
  _m_, mouth.
  _ma_, manus.
  _mb_, midbrain.
  _me_, medullary canal.
  _me´_, tip end of medullary canal.
  _md._, mandibular folds.
  _mes_, mesoderm.
  _mes´_, myocardium.
  _mf_, medullary fold.
  _mg_, medullary groove.
  _mk_, Meckel's cartilage.
  _ml_, muscle layer.
  _mp_, muscle plate.
  _ms_, mesentery.
  _mv_, meatus venosus.
  _mx_, maxillary fold.
  _myc_, myocoel.
  _n_, nasal cavity.
  _na_, neural arch of vertebra.
  _nc_, neurenteric canal.
  _nl_, nervous layer of ectoderm.
  _nt_, notochord.
  _o_, ear vesicle.
  _oc_, optic cup.
  _oe_, oesophagus.
  _on_, optic nerve.
  _os_, optic stalk.
  _ov_, optic vesicle.
  _p_, pituitary body.
  _pa_, posterior appendage.
  _pag_, post-anal gut.
  _pan_, pancreas.
  _pan´_, opening of pancreas.
  _pc_, posterior cardinal vein.
  _pe_, pes.
  _pg_, primitive groove.
  _ph_, pharynx.
  _pip_, posterior intestinal portal.
  _pl_, pelvis.
  _pn_, posterior nares.
  _pr_, pericardial cavity.
  _ps_, primitive streak.
  _pt_, pecten.
  _py_, pylorus.
  _r_, rib.
  _rt_, retina.
  _s_, somites.
  _sc_, spinal cord.
  _se_, spenethmoid cartilage.
  _sg_, spinal ganglion.
  _sl_, submucosa.
  _sm_, splanchnic mesoblast.
  _sn_, spinal nerve.
  _so_, somatic mesoblast.
  _st_, stomodaeum.
  _sy_, sympathetic nervous system.
  _t_, tail.
  _ta_, trachea.
  _tg_, thyroid gland.
  _tn_, tongue.
  _to_, tooth anlage.
  _tr_, trunchus arteriosus.
  _tv_, _tv´_, third ventricle of brain.
  _ty_, thymus gland.
  _u_, umbilical stalk.
  _v´_, _v´´_, _v´´´_, first, second, and third cerebral vesicles.
  _va_, vascular area.
  _vc_, vocal cords.
  _vm_, vitelline membrane.
  _vn_, ventricle of heart.
  _vp_, velum palitum.
  _vv_, vitelline blood vessels.
  _wd_, Wolffian duct.
  _wdo_, opening of Wolffian duct.
  _wr_, Wolffian ridge.
  _wt_, Wolffian tubule.
  _x_, point of origin of bronchi.
  _y_, yolk.
  _ys_, yolk-stalk.


  [Transcriber's Note:

  The Figures were printed on a series of 15 pages ("Plates") at the
  end of the book. Since the caption text has already been given in
  full, the individual Figure numbers will not be repeated here.]


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Errata

  As seen in figure 6A the lungs are irregularly conical
    _printed as 6a_
  at the point marked _X_
    _text [two occurrences] unchanged: capital letter not explained_
  _ep_, epidermal layer of ectoderm. [. for ,]
  _epi_, pineal body. [_epi_ not italicized]