stage under discussion. In both figures the head-fold is seen as a deep
loop of ectoderm and entoderm, while the head-fold of the amnion is
seen at _a_.
The beginning of the foregut is seen in Figure 3_n_ (_fg_), which is
the more nearly median of the two sections, Figure 3_o_ being a short
distance to the side of the middle line.
In Figure 3_o_ the thin entoderm (_en_) is separated from the much
thicker ectoderm (_ec_) by a considerable layer of rather loose
mesoderm (_mes_). In Figure 3_n_, which is almost exactly median in
position, there is, of course, no mesoderm to be seen in front of
the blastopore, and the entoderm shows a considerable increase in
thickness, due to the formation of the notochord (_nt_). The blastopore
(_blp_) is the most striking feature of the figure, and is remarkable
for its great width in an antero-posterior direction. Its anterior and
posterior borders are outlined by sharply defined layers of ectoderm
and entoderm. Posterior to the blastopore the lower side of the
ectoderm is continuous with a considerable mass of cells, the primitive
streak (_ps_).
STAGE III
FIGURES 4, 4_a_, 5, 5_a_ AND 6-6_i_ (PLATES X., XI.)
“Figures 4 and 4_a_ are of an embryo removed, on June 18th, from an egg
which had been taken out of an alligator two days before. Figure 4, a
dorsal view, is of special interest in that it shows a secondary fold
taking place in the head-fold. This grows posteriorly along the median
dorsal line, forming a V-shaped process with the apex pointing backward
toward the blastopore. There is quite a deep groove between the arms of
the V. The head-fold on the ventral side, as seen in Figure 4_a_, made
from the same embryo as Figure 4, grows most rapidly on the mid-line,
and also becomes thicker at that place. The medullary folds now begin
to form on either side of the medullary groove, ending posteriorly on
either side of the blastopore and anteriorly on either side of the
point of the V-shaped process in the middle of the head-fold. This is
seen in Figure 5, which is a dorsal view of an embryo from an egg three
days after it was taken out of an alligator. A ventral view of the same
embryo (Fig. 5_a_) represents the thickened process on the mid-line at
its greatest development. For some reason the notochord did not show in
this embryo, possibly owing to particles of the yolk material adhering
about the mid-line.
“In an embryo a day or two older, the V-shaped fold of the head-fold
is seen to have broken through at the apex, and each of the arms thus
separated from one another unites with the medullary fold of its
respective side. This can be seen in Figure 6, which is a dorsal view
of part of an embryo a day or two older than the one represented by
Figures 5 and 5_a_.
“This is so unexpected a method of formation for the anterior part
of the medullary folds that I have made use of both Figures 4 and 5.
They were made from very perfect specimens, and the sections of both of
them, and of the specimen from which Figure 6 was drawn, proves that
the structure is what it is indicated to be in surface appearance. That
is, the transverse sections posterior to the V, in the embryos shown in
Figures 4 and 5, show the medullary groove and the medullary folds; the
several sections passing through the apex of the V show neither groove
nor folds, but only a median thickening; and in front of the point or
apex of the V the successive sections discover a gradually widening
groove between the arms, which is also much deeper than the shallow
groove found posterior to the V. While I have not seen, and from the
nature of the conditions one cannot see, the change actually proceeding
from the form of Figure 5 to that of Figure 6, still the explanation
given appears to be the only one possible” (17).
A somewhat extended series of transverse sections of an embryo of about
this age is represented in figures 6_a_-_i_.
Figure 6_a_ is a section through the head-fold; it passes through the
extreme anterior end of the secondary folds (_sf_) that were described,
in surface view, above (Figs. 5 and 6). The section was not quite at
right angles to the long axis of the embryo, so that the fold on the
right was cut farther toward its anterior end than was the fold on
the left. The pushing under of the head causes a forward projection
of the secondary folds, so that the fold to the right appears as a
rounded mass of cells with a small cavity near its center. On the
left the plane of the section passes through the posterior limit of
the head-fold, and shows the cells of the secondary fold continuous
with the dorsal side of the ectoderm (_ec_). As pointed out above by
Clarke, the secondary folds are here some distance apart, and gradually
approach each other as we proceed toward the posterior. The entoderm
(_en_) is here flat and takes no part in the secondary folds.
In Figure 6_b_, a short distance back of the one just described, the
secondary folds (_sf_) are much larger and are closer together. On the
right the section passes through the extreme limit of the head-fold, so
that the secondary fold of that side is still a closed circle, with a
few scattered cells enclosed. On the left the section is posterior to
the head-fold; on this side the secondary fold is seen as a high arch
of ectoderm, with a thick mass of entoderm beneath it.
Figure 6_c_ represents a section which passes back of the head-fold on
both sides. The secondary folds (_sf_) are seen as a pair of ectodermal
arches continuous with each other in the middle line of the embryo.
The ectoderm of the folds is much thickened and gradually becomes
thinner distally. On the right the entoderm shows the same thickening
(_en_) that was shown on the left side of the preceding figure. This
thickened appearance of the entoderm is due to the fact that the
section passes through the anterior limit of a tall fold of that layer,
which underlies the similar fold of the ectoderm that has already been
described. This secondary fold of the entoderm is seen on the left side
of the section. It may be traced through several sections, but soon
flattens out posteriorly.
Figure 6_d_ is a short distance posterior to the preceding. The
secondary folds are here much less pronouncedly arched and the deep
groove between them is reduced to a line (_l_). The entoderm (_en_) is
no longer markedly arched and is closely adherent, along the median
plane, to the ectoderm, where there is seen the thickening (_th_) that
has been mentioned by Clarke (see above). Springing from the entoderm
on each side of this thickening is a small mass of mesoderm (_mes_).
The section immediately posterior to the one just described is
represented in Figure 6_e_. The line (_l_) which separated the two
secondary folds in the preceding section is no longer present, so
that the ectoderm (_ec_) is continuous from side to side, with only
a shallow depression (_mg_), which may be considered as the extreme
anterior end of the medullary groove. The median thickening (_th_) is
cut near its posterior limit and still shows a close fusion of the
germ layers. There is no line of demarcation between the gradually
flattening secondary folds of the anterior end of the embryo and
the just forming medullary folds of the posterior end, so that it is
impossible to say whether the thickening of ectoblast in this figure
should be called secondary folds or medullary folds. As a matter of
fact, the secondary folds become, of course, the anterior ends of the
medullary folds. The mesoblast (_mes_) is here of considerable extent,
and its entodermal origin is beyond doubt, though not well shown in the
figure.
Figure 6_f_ is about one sixth of the length of the embryo posterior to
the preceding. The medullary thickening of the ectoderm (_ec_) is still
marked and the shallow medullary groove (_mg_) is fairly distinct. The
entoderm (_en_) is medially continuous with both mesoderm (_mes_) and
notochord (_nt_), though these two tissues are otherwise distinct from
each other.
Figure 6_g_ is nearly one third the length of the embryo posterior to
the preceding and passes through the posterior third of the embryo.
The medullary thickening of the ectoderm (_ec_) is marked, but shows
no sign of a medullary groove; in fact, the median line is the most
elevated region of the ectoderm. The notochord (_nt_) is larger in
cross-section than in the more anterior regions. It is still continuous
with the entoderm (_en_) and is fairly closely attached to, though
apparently not continuous with, the mesoderm (_mes_) on each side.
Figure 6_h_ passes through the blastopore (_blp_). The appearance of
the section is almost identical with that of Figure 2_f_, already
described.
Figure 6_i_ is five sections posterior to the preceding and has about
the same structure as the corresponding sections in the preceding two
stages, where this region of the embryo was injured, and hence not
drawn. Continuous with the posterior border of the blastopore (seen in
the preceding figure) is the deep furrow, the primitive groove (_pg_).
The ectoblast (_ec_) bordering this groove is much thickened and may be
called the primitive streak. The lower side of this primitive streak
is continuous with the mesoblast (_mes_), while the entoderm (_en_)
is here entirely distinct from the mesoderm. It is evident that the
mesoderm posterior to the blastopore is proliferated from the lower
side of the ectoblast and not from the upper side of the entoblast, as
is the case anterior to the blastopore. The primitive groove gradually
becomes more and more shallow, as it is followed toward the posterior,
until it is no longer discernible; back of this point the primitive
streak may be traced for a considerable distance, becoming thinner and
thinner until it too disappears, and there remains only the slightly
thickened ectoblast underlaid by the thin and irregular layers of
mesoblast and entoblast. The primitive streak may be traced for a
distance equal to about one third the distance between the head-fold
and the blastopore.
STAGE IV
FIGURES 7_a_-7_h_ (PLATES XI., XII.)
No surface view of this stage was seen by the writer, and hence is not
figured. The figures were drawn from one of the series of sections
obtained through the courtesy of Prof. S. F. Clarke. This series was
marked “3 Urwirbeln,” so that the embryo was apparently slightly
younger than the youngest stage obtained by myself and represented in
Figures 8 and 8_a_.
Figure 7_a_ represents a section that passed through the head-fold of
the amnion (_a_) just in front of the head-fold of the embryo; the
amniotic cavity here appears as a large empty space.
Figure 7_b_ is several sections posterior to the preceding; it passes
through the head-fold of the embryo, but is just back of the head-fold
of the amnion. The deep depression of the ectoderm (_ec_) and entoderm
(_en_) caused by the head-fold is plainly seen. In this depression lie
the ends of the medullary folds, distinct from each other both dorsally
and ventrally. Each medullary fold is made up of two parts--a medial,
more dense nervous layer (_nl_), and a distal, less dense epidermal
layer (_ep_). The section corresponding to this one will be more fully
described in connection with the following stage.
Figure 7_c_ is some distance posterior to the preceding, though
the exact distance could not be determined because of a break in
the series at this point. The section passes through the posterior
limit of the head-fold. The medullary groove (_mg_) is very deep and
comparatively wide; around its sides the germ layers are so closely
associated that they may scarcely be distinguished. Ventral to the
medullary groove the foregut (_fg_) is seen as a crescentic slit.
Figure 7_d_ is about a dozen sections posterior to the one just
described and is about one seventh the length of the embryo from the
anterior end. The embryo is much more compressed, in a dorso-ventral
direction, and the medullary groove (_mg_) is correspondingly more
shallow. Where the ectoderm (_ec_) curves over to form the medullary
folds it becomes much more compact and somewhat thicker. The notochord
(_nt_) is large and distinct, but is still fused with the entoderm
(_en_). The mesoderm (_mes_) forms a well-defined layer, entirely
distinct from both the notochord and the entoderm. From this region,
as we pass caudad, the size of the embryo in cross-section gradually
decreases and the medullary groove becomes more shallow.
Figure 7_e_ is about one third of the length of the embryo from the
posterior end, and is only a few sections from the caudal end of the
medullary groove. The ectoderm (_ec_) is much thinner than in the
preceding figure and the medullary groove (_mg_) is much more shallow.
The notochord (_nt_) is of about the same diameter as before, but
is here quite distinct from the entoderm (_en_) as well as from the
mesoderm (_mes_).
Figure 7_f_ is seven sections posterior to Figure 7_e_. The medullary
groove has disappeared and the medullary folds have flattened to form
what might be called a medullary plate (at the end of the reference
line _ec_), which continues to the anterior border of the blastopore.
The notochord (_nt_) is larger in cross-section than in the more
anterior regions; it is still distinct from the entoderm.
Figure 7_g_ passes through the blastopore and shows essentially the
same structure as was described in connection with the corresponding
section of Stage I (Fig. 2_f_).
Figure 7_h_ represents the region of the primitive groove (_pg_) and
primitive streak (_ps_). The section shows the typical structure for
this region--a thick mass of cells is proliferating from the ventral
side of the ectoderm (_ec_) and is spreading laterally to form a
distinct mesoderm (_mes_). The entoderm (_en_) is entirely distinct
from the other layers.
STAGE V
FIGURES 8-8_j_ (PLATES XII., XIII., XIV.)
On opening the egg this embryo (Figs. 8 and 8_a_) was found to lie
on the end of the yolk, near the center of the irregular, lighter
area which was mentioned in connection with the description of the
egg. The length of the embryo proper is 3 mm. This was the youngest
stage found in 1905, and approximates quite closely the condition of
the chick embryo after 24 hours’ incubation. The dorsal aspect of
this embryo, viewed by transmitted light, is shown in Figure 8. The
medullary folds (_mf_) have bent over until they are in contact, though
apparently not fused for a short distance near their anterior ends. As
will be described in connection with the sections of this stage, the
medullary folds are actually fused for a short distance at this time,
though in surface views they appear to be separated from each other.
In the Madagascar crocodile (78) the first point of fusion of the
medullary folds is in the middle region of the embryo, or perhaps even
nearer the posterior than the anterior end of the medullary groove.
Throughout the greater part of their length the medullary folds are
still widely separated; posteriorly they are merged with the sides of
the very distinct primitive streak (_ps_), which seems, owing to its
opacity, to extend as a sharp point toward the head. Extending for the
greater part of the length of the primitive streak is the primitive
groove (_pg_), which, when the embryo is viewed by transmitted light,
is a very striking feature at this stage of development and resembles,
in a marked way, the same structure in the embryo chick. Clarke (17)
figures the blastopore at this stage as a small opening in front of the
primitive streak, but does not mention any such condition as above
described at any stage of development. Five pairs of somites (_s_)
have been formed and may be seen, though but faintly outlined, in
both dorsal and ventral views of the embryo; they lie about half-way
between the extreme ends of the embryo. The head-fold (_h_, Fig. 8_a_)
shows plainly in a ventral view as a darker, more opaque anterior
region, extending for about one fourth the length of the embryo. The
still unfused region of the medullary folds may be seen also in the
ventral view at _mg_. The head-fold of the amnion (_a_) forms a very
thin, transparent hood over the extreme anterior end of the embryo.
The tail fold of the amnion has not made its appearance, and in fact
is not apparent at any stage in the development. This is true also of
the Madagascar crocodile. The notochord (_nt_) may be seen in a ventral
view as a faint, linear opacity extending along the middle line from
the head-fold to the primitive streak.
Two sagittal sections of this stage are shown in Figures 8_b_ and 8_c_.
The embryo from which the sections were made was apparently somewhat
crooked, so that it was not possible to get perfect longitudinal
sections. For example, in Figure 8_b_ the plane of the section is
almost exactly median in the extreme posterior and middle regions,
but is on one side of the middle line elsewhere. This explains the
enormous thickening of the ectoblast in the region of the head,
where the section passes through one of the medullary folds (_mf_) at
its thickest part; and also explains the fact that the ectoblast is
thinner in the middle region (_ec_), where the section passes through
the medullary groove, than it is farther toward the blastopore where
the section cuts the edge of the medullary folds. The outlines of the
middle and extreme posterior regions of the ectoblast are much more
irregular and ragged than is shown in the figure. The plane of the
section passes through the notochord (_nt_) in the posterior region,
but not in the anterior end of the embryo, where a layer of mesoblast
(_mes_) is seen. The great size of the blastopore (_blp_) is well
shown, as is the beginning of the foregut (_fg_). Comparison of this
figure with the more anterior transverse sections and with the dorsal
surface view of this stage will make the rather unusual conditions
comprehensible.
Figure 8_c_ is cut to one side of the median plane, distal to the
medullary folds. Being outside of the medullary folds, the ectoderm
(_ec_) is thinner and less dense than in Figure 8_b_; anteriorly it is
pushed down and back as the head-fold, and posteriorly it becomes thin
where it forms the dorsal boundary of the primitive streak (_ps_).
The foregut (_fg_), as would be expected, is not so deep as in the
median section (8_b_). The most striking feature of the section is the
presence of five mesoblastic somites (_s_). Each somite, especially
the second, third, and fourth, is made up of a mass of mesoblast whose
cells are compactly arranged peripherally, but are scattered in the
center, where a small myocœl may be seen.
A series of transverse sections of the embryo shown in Figures 8 and
8_a_ is represented in Figures 8_d_-_j_.
Figure 8_d_ is through the anterior end of the embryo; the posterior
edge of the amnion is cut only on one side (_a_). The medullary folds
(_mf_) are shown as two distinct masses of tissue, separated by a
considerable space from each other, both dorsally and ventrally;
they are underlaid by the ectoderm of the head-fold, through which
the section passes. A mass of yolk (_y_) is shown at one side of the
section.
Figure 8_e_ represents a section a short distance posterior to the one
just described, and passes through the short region where the dorsal
edges of the medullary folds have fused with each other. The ventral
side of the medullary groove (_mg_) is, as in the preceding section,
still unclosed. An epidermal layer of ectoblast (_ep_) is now distinct
from the nervous layer (_nl_).
Figure 8_f_ is through a region still farther toward the posterior end.
Here the medullary groove is _again_ open above, and is still open
below. A well-marked space is seen between the epidermal (_ep_) and
nervous (_nl_) layers of the ectoderm, but no mesoblast is yet to be
seen.
Figure 8_g_ passes through the middle part of the head-fold, and shows
that the medullary folds in this region are fused below, but are widely
separated above, where their margins are markedly bent away from the
mid-line. Between the epidermal and nervous layers of the ectoderm
a considerable mass of mesoderm cells (_mes_) is seen. The curious
appearance of the preceding four figures, as well as the first three
figures of the next stage, was at first quite puzzling, until a model
of the embryo was made from a series of sections. It was then plain,
as would have been the case before, except for the unusual depth
dorso-ventrally of the head of the embryo, why the medullary canal
should at the extreme anterior end be open both dorsally and ventrally,
while a few sections caudad it is open only ventrally, and still
farther toward the tail it is again open both above and below. These
conditions are produced by the bending under of the anterior region of
the medullary folds, probably by the formation of the head-fold. It is
apparently a process distinct from the ordinary cranial flexure, which
occurs later. The fusion of the medullary folds to form a canal begins,
as has been already mentioned, near the anterior end, whence it extends
both forward and backward. Hence, if the anterior ends of the medullary
folds be bent downward and backward, their unfused dorsal edges will
come to face ventrally instead of dorsally and sections through the
anterior part of this bent-under region will show the medullary canal
open both above and below, as in Figure 8_d_, while sections farther
caudad pass through the short region where the folds are fused, and we
have the appearance represented in Figure 8_e_. In Figure 8_f_ is shown
a section passing posterior to the short, fused region of the folds,
and we again have the medullary canal open both above and below. Figure
8_g_ represents a section through the tip of the bent-under region of
the medullary folds, which are here fused below and open above.
Figure 8_h_ passes through the posterior part of the head-fold, between
the limits of the fold of the ectoderm and that of the entoderm. The
medullary groove (_mg_) is here very wide and comparatively shallow;
its walls are continued laterally as the gradually thinning ectoderm
(_ec_). The enteron (_ent_) is completely enclosed, and forms a large,
somewhat compressed, thick-walled cavity. Between the dorsal wall
of the enteron and the lower side of the medullary canal lies the
notochord (_nt_), a small, cylindrical rod of closely packed cells
derived, in this region at least, from the entoderm. In the posterior
region of the embryo it is not possible to determine with certainty
the origin of the notochord, owing to the close fusion of all three
germ layers. Between the wall of the enteron and the lower side of
the ectoderm is a considerable mass of mesoderm (_mes_), which here
consists of more scattered and angular cells than in the preceding
section.
Figure 8_i_ shows the appearance of a section through the mesoblastic
somites, in one of which a small myocœl (_myc_) is seen. As is seen by
the size of the figure, which is drawn under the same magnification as
were all the sections of the series, the embryo in this region is much
smaller in section than it is toward either end, especially toward the
anterior end. The medullary groove (_mg_) is still more shallow than
in the more anterior sections, and the ectoderm (_ec_), with which
its folds are continuous laterally, is here nearly horizontal. The
mesoblast (_mes_) is of a more compact nature than in the preceding
section and shows little or no sign of cleavage, although a distinct
myocœl may be seen and cleavage is well marked in sections between this
one and the preceding.
The notochord (_nt_) has about the same appearance as in the preceding
section, but is more distinctly separated from the surrounding cells.
Figure 8_j_ is through the posterior end of the embryo; it shows the
relation of parts in the region of the primitive streak. Although
not visible in surface views, and hence not represented in Figure 8,
the medullary groove is continued without any line of demarcation
into the primitive groove, and the medullary folds into the edges of
the primitive streak, so that it is impossible to set any definite
boundaries between these structures unless the dorsal opening of the
blastopore be taken as the point of division. The medullary groove
(_mg_), if it be here so called, is proportionately more shallow than
in the preceding figure and is actually much wider. The section passes
behind the posterior end of the notochord, so that structure is not
seen. Though not so well indicated as might be desired in the figure,
the three germ layers are here indistinguishable in the middle line,
and in the center of this mass of cells the blastopore (_blp_) or
neurenteric canal may be seen as a small vertical slit. As will be more
fully described in the following stage, this canal opens dorsally a
few sections posterior to the one under discussion and ventrally a few
sections farther toward the head.
In all the sections of this stage the ectoderm and entoderm are fairly
thick in the region of the embryo proper, but become thinner until
reduced to a mere membrane as we pass to more distal regions. Both
layers are composed of loosely arranged cells, with scattered nuclei.
Where the ectoderm becomes thickened to form the medullary folds, the
cells are much more compactly arranged; hence this region stands out in
strong contrast to the rest of the ectoderm.
STAGE VI
FIGURES 9_a_-9_m_ (PLATES XIV., XV.)
The embryo represented by this series of transverse sections is
intermediate in development between those represented in surface views
by Figures 8 and 10. The amnion and head-fold are nearly the same as
in Figure 8; the medullary folds are intermediate in development, the
anterior end not showing so marked an enlargement as shown in Figure
10, _v′_. There are six or seven faintly distinguishable somites.
Figure 9_a_ represents a section through the anterior part of the
head-fold; it shows one unusual condition: the head lies entirely
beneath the surface of the yolk. This condition is quite confusing when
the section is studied for the first time. The pushing of the head
under the yolk is shown at its commencement in Figure 11. The process
continues until nearly the entire anterior half of the embryo is
covered; but when the embryo attains a considerable size it is seen to
lie entirely above the yolk, as in the chick. According to Voeltzkow’s
figures (78) this same condition is found in the crocodile, and Balfour
also mentions it in connection with the lizard. The fusion of the
medullary folds has made considerable progress, so that the entire
anterior end of the canal is enclosed, except in the region where the
folds are bent down and back, as in the preceding stage; here the folds
are still distinct from each other, leaving the medullary canal open
on the ventral side, as shown in Figures 9_a_ and 9_b_. In the section
under discussion the ectoderm (_ec_) is a very thin membrane on top of
a considerable mass of yolk, while no entoderm can be distinguished.
The amnion (_a_) completely surrounds the embryo as an irregular
membrane of some thickness in which no arrangement into layers can
be seen. The epidermal ectoderm (_ep_) is composed of the usual
loosely arranged cells, so that it is clearly distinguishable from the
compactly arranged cells of the nervous layer (_nl_), from which it is
separated by only a line.
In Figure 9_b_, which shows a section a short distance posterior to the
preceding, the medullary canal (_mc_) is somewhat deeper and is still
open ventrally. There is a distinct space between the nervous (_nl_)
and epidermal (_ep_) layers of the ectoderm, in which space a few
mesoblast cells (_mes_) may be seen. The section is cut just posterior
to the edge of the amnion, so that there is now neither amnion nor yolk
above the embryo.
Figure 9_c_ is about ten sections posterior to Figure 9_b_. The
section passes through the anterior wall of the bent-under part of the
medullary canal (_mc′_), so that the actual canal is shown only on the
dorsal side (_mc_), where it is completely closed and begins to assume
the shape of the typical embryonic spinal cord. The space between the
superficial (_ep_) and nervous (_nl_) layers of the ectoderm is quite
extensive and is largely filled by a fairly compact mass of mesoderm
(_mes_).
Figure 9_d_, although only five sections posterior to the preceding,
shows a marked change in structure. The medullary canal (_mc_) is here
of the typical outline for embryos of this age. A large, compact mass
of cells (_ent_) appears at first glance to be the same that was noted
in the preceding stage at the tip end of the turned-under medullary
canal; it is, however, the extreme anterior wall of the enteron, which
is in close contact with the above-mentioned tip of the medullary
canal. Between this anterior wall of the enteron, of which wall it is
really a part, and the medullary canal is the notochord (_nt_). The
space surrounding the notochord and enteron is filled with a fairly
compact mass of typical, stellate mesoblast cells. The depression of
the ectoderm (_ec_) and entoderm (_en_) of the blastoderm caused by the
formation of the head-fold is here less marked, and the dorsal side of
the embryo in this region is slightly elevated above the level of the
blastoderm.
Figure 9_e_ represents a section passing through the posterior edge
of the head-fold. The epidermal ectoderm is here continuous with the
thin layer of superficial ectoderm (_ec_) of the blastoderm, while
the entoderm (_en_) of the blastoderm is still continuous beneath the
embryo. The thick ectoderm of the embryo is sharply differentiated from
the thin layer of ectoderm that extends laterally over the yolk. The
pharynx (_ent_) is a large cavity whose wall is thick except at the
dorsal side, where it is thin and somewhat depressed, apparently to
make room between it and the medullary canal for the notochord (_nt_).
Figure 9_f_ is about twenty sections posterior to the preceding
section, and passes through the point of separation of the folds of
the entoderm (_en_). From this point the entoderm gradually flattens
out, leaving the enteron unenclosed. The medullary canal (_mc_) and
notochord (_nt_) are about as in the preceding section, but the
ectoderm (_ep_) is somewhat thinner and more flattened. The mesoderm
(_mes_) on the right side exhibits a distinct cleavage, the resulting
body cavity (_bc_) being a large, triangular space.
Figure 9_g_, the twenty-fifth section posterior to that represented in
Figure 9_f_, shows a marked change in the form of the embryo. While
of about the same lateral dimensions, the dorso-ventral diameter of
the embryo in this region is less than one half what it was in the
head region. The epidermal ectoderm (_ep_) is now nearly horizontal
in position and is not so abruptly separated laterally from the thin
lateral sheets of ectoblast. The medullary groove (_mg_) is here a
very narrow vertical slit. At this stage the fusion of the medullary
folds has taken place over the anterior third of the embryo. For a
short distance, represented in about thirty-five sections, the canal
is open as in the figure under discussion; for the next one hundred
sections (about one third the length of the embryo) in the region of
the mesoblastic somites the canal is again closed, while throughout the
last one third of its length the canal is widely open dorsally. The
enteron is here entirely open ventrally, the entoderm being almost
flat and horizontal. The notochord (_nt_) is distinctly outlined and is
somewhat flattened in a dorso-ventral direction. The body cavity (_bc_)
is well marked, but is separated by a considerable mass of uncleft
mesoblast from the notochord and the walls of the medullary groove.
A space of about one hundred sections, or one third the length of the
embryo, intervenes between Figures 9_g_ and 9_i_. This is the region of
the mesoblastic somites, and in this region, as has been above stated,
the medullary canal is completely enclosed. It is evident then that the
entire anterior two thirds of the medullary canal is enclosed except
for the short region represented in Figure 8_g_. Whether or not this
short open region between the two longer enclosed regions is a normal
condition the material at hand does not show.
Figure 9_h_ represents a typical section in the region of the
mesoblastic somites just described. It shows the enclosed medullary
canal (_mc_), the body cavity (_bc_) on the right, and a mesoblastic
somite with its small cavity (_myc_) on the left. The entire section
is smaller than the sections anterior or posterior to this region, and
seems to be compressed in a dorso-ventral direction, this compression
being especially marked in the case of the notochord.
Figure 9_i_ is through a region nearly one hundred sections posterior
to the preceding, and cuts the embryo, therefore, through the
posterior one fourth of its length. The chief difference between this
and the preceding section is in the medullary canal, which is here open
and is in the form of a wide groove with an irregular, rounded bottom
and vertical sides. The size of the section is considerably greater
than in the preceding, the increase being especially noticeable in the
notochord (_nt_), which is cut near its posterior end. There is little
or no sign of mesoblastic cleavage.
Figure 9_j_ is about twenty sections posterior to Figure 9_i_. The
medullary groove (_mg_) is considerably larger than in the more
anterior regions, and its folds are somewhat inclined toward each
other, though still wide apart. The notochord and entoderm are fused
to form a large, compact mass of tissue close under the ventral wall
of the medullary groove. On the ventral side of this mass of cells a
groove (_blp_) marks the anterior and ventral opening of the blastopore
shown in the next figure. The mesoblast shows no sign of cleavage.
Figure 9_k_ shows the medullary groove (_mg_) in about the same
position as in the preceding section. The blastopore (_blp_) is here
seen as a small cavity in the center of the large mass of cells that
was noted in the last figure. The entoderm (_en_) is continuous from
side to side, but is not so sharply differentiated from the other germ
layers as is represented in the figure.
Figure 9_l_ is four sections back of the preceding; the wide, dorsal
opening (_blp_) of the blastopore or neurenteric canal into the
medullary groove (_mg_) is shown. The blastopore or neurenteric canal,
then, is still at this stage a passage that leads entirely through the
embryo, the medullary canal being in this region unenclosed above.
Ventrally it is seen as a narrow opening through the entoderm; it then
passes upward and backward, behind the end of the notochord, as a
small but very distinct canal, which may be traced through about ten
sections. The enclosed portion of the canal lies, as has been stated
(Figure 9_k_, _blp_), in the center of the mass of cells that is fused
with or is a part of the floor of the medullary groove.
The above-described neurenteric canal is essentially like that
described by Balfour in the _Lacertilia_. He does not say, however,
and it is not possible to tell from his figures, whether there is a
long, gradually diminishing groove posterior to the dorsal opening of
the canal, as in the alligator. He says that the medullary folds fuse
posteriorly until the medullary canal is enclosed over the opening of
the neurenteric canal; also that “the neurenteric canal persists but a
very short time after the complete closure of the medullary canal.”
In Figure 9_m_, for about thirty sections (one tenth the entire length
of the embryo), behind the section represented in the last figure,
there is a very gradual change in the embryo, converting the deep
groove, _mg_ in Figure 9_l_, into the shallow slit, _pg_, Figure 9_m_.
There is no line of demarcation between the typical medullary groove
region of Figure 9_l_ and the equally typical primitive groove region
represented in Figure 9_m_. As was noted in the preceding stage, the
medullary folds are quite continuous with the folds of the primitive
streak, and the medullary groove with the primitive groove; so that,
unless we take the dorsal opening of the neurenteric canal as the point
of separation, there is no line of division between these structures.
The entoderm (_en_) and the lateral regions of the ectoderm (_ec_) and
mesoderm (_mes_) in Figure 9_m_ are about as they were in Figure 9_l_,
but in the middle line is seen a compact mass of cells forming the
primitive streak (_ps_), with the shallow primitive groove (_pg_) on
the dorsal side. The cells on each side of the primitive groove and for
a short distance below it are compact, as is shown in the figure, but
as we pass ventrally and laterally they become looser and more angular
to form the lateral sheets of mesoblast (_mes_), very much as is the
case in the chick and other forms. For a few sections posterior to the
one shown in Figure 9_m_ the primitive streak may be seen, then it
disappears, and only the ectoderm and entoderm remain as thin sheets of
tissue above the yolk.
STAGE VII
FIGURES 10 AND 10_a_ (PLATES XV., XVI.)
Although of practically the same size as the preceding, this stage has
advanced sufficiently in development to warrant a description.
The medullary folds are apparently still slightly open for the greater
part of their length, though they are evidently fused together in the
head region, except at the extreme end. Transverse sections, however,
of Figure 12, in which the medullary folds, from the dorsal aspect,
seemed open (_mg_) as in Figure 10, have shown that these folds are
fused throughout their length.
The first cerebral vesicle (_v′_) is clearly indicated as an
enlargement of the anterior end of the nervous system, and a slight
enlargement (_v″_) posterior to the first probably represents the
second cerebral vesicle.
There are now eight pairs of somites (_s_).
The head-fold (_h_) now shows in both dorsal and ventral views,
appearing in the former, when viewed by transmitted light, as a
lighter, circular area on either side of the body, just posterior to
the hinder edge of the amnion.
The head-fold of the amnion (_a_) has extended about twice as far
backward as it did in the preceding stage.
Owing to the opacity caused by the medullary folds being in contact
along the middle line, the notochord is no longer visible in surface
views.
The head at this stage begins to push down into the yolk in a strange
way that will be described later.
STAGE VIII
FIGURES 11-11_k_ (PLATES XVI., XVII., XVIII.)
This stage is about one fourth longer than the preceding. The medullary
canal is enclosed throughout its entire length, though it appears in
surface view (Fig. 11) to be open in the posterior half (_mc_) of the
embryo. An enlargement of this apparently open region at the extreme
posterior end (_pg_) is probably caused by the remains of the primitive
groove or the neurenteric canal, and a slight opacity at the same point
may be caused by the primitive streak. The anterior end of the neural
tube is bent in a ventral direction (_v_), as in the preceding stage.
The somites (_s_) now number fifteen pairs; they are somewhat irregular
in size and shape.
The head-fold is not so striking a feature as in the preceding stage.
The head-fold of the amnion (_a_) now covers nearly two thirds of the
embryo. The heart (_ht_) is seen as a dark, rounded object projecting
to the right side of the neural canal, just anterior to the first
somite. The vitelline blood-vessels are just beginning to form, but are
not shown in the figure.
The depression of the anterior region that was noted in the preceding
stage has advanced so far that a considerable part of the embryo now
projects forward under the blastoderm. In some cases it is almost
concealed in a dorsal view; in other cases it may easily be seen
through the transparent membranes, especially after clearing.
In opening eggs of this stage one is at first apt to underestimate the
size of the embryos, since the anterior part of the embryos cannot be
seen until after they are removed from the yolk and are viewed from the
ventral side.
The embryo from which the series of transverse sections of this stage
was made, while of the same state of development as that shown in
Figure 11, was more fully covered by the blastoderm than is shown in
the surface view in question.
Figure 11_a_ passes through the tip of the head. Dorsal to the embryo
is the ectoderm and a thick mass of yolk (_y_). The amnion (_a_) is
seen as an irregular membrane which entirely surrounds the head.
The medullary canal (_mc_) is entirely closed except at the extreme
anterior end, which is bent downward so that the opening is on the
ventral side. The nervous (_nl_) and epidermal (_ep_) layers of the
ectoderm are in contact throughout, but are clearly distinguishable
because of the difference in the compactness of their cells.
In Figure 11_b_ is represented a section, behind the preceding, which
passes through the posterior tip of the turned-under anterior end
(_mc′_). Here the medullary canal is closed both above (_mc_) and
below (_mc′_). The amnion (_a_) has about the same appearance as in the
more anterior section, but there is here a considerable space, filled
with mesoblast (_mes_), between the nervous (_nl_) and epidermal (_ep_)
layers of ectoderm.
Figure 11_c_ is twenty sections, about one tenth the length of the
embryo, posterior to the one last described. The large mass of
overhanging yolk (_y_) is still present, as is also the amnion (_a_),
though the latter no longer passes entirely around the embryo; only the
true amnion could be made out. The thickened walls of the medullary
canal have reduced that cavity to a narrow, Y-shaped slit (_mc_).
The notochord (_nt_) is very slender in this region, compared to its
diameter farther toward the posterior end. The enteron (_ent_) is a
large cavity, whose wall is made up of loosely arranged cells except
around a median, ventral depression where the cells are more compact.
This depression may be traced through ten or fifteen sections and may
represent the beginning of the thyroid gland, though this point was
not worked out with certainty. Surrounding the notochord and enteron
is a loose mass of typical, stellate mesoblast cells (_mes_), which
are cleft on either side to form the anterior limit of the body cavity
(_bc_). Between the body cavity below and the enteron above, on each
side, is a small blood-vessel (_bv_) which when followed caudad is
found to open ventrally and medially into the anterior end of the heart.
Figure 11_d_ is about a dozen sections posterior to the preceding. The
appearance of the overhanging yolk (_y_), of the amnion (_a_), and of
the notochord (_nt_) is about as in the more anterior section. The
medullary canal (_mc_) is a straight, vertical slit, and the depression
in the floor of the pharynx (_ent_) is much more shallow. The body
cavity (_bc_) is much larger and extends across the mid-ventral line
beneath the heart (_ht_), which is cut through its middle region. The
heart may be traced through about twenty sections (one tenth the length
of the embryo); its mesoblastic wall (_mes′_) is thin and irregular,
and is lined by a distinct endothelium (_en′_) whose exact origin has
not yet been worked out.
Figure 11_e_ is just back of the heart, and shows in its place the
two vitelline veins (_vv_). The depression in the floor of the
enteron (_ent_) is entirely distinct from the one that has been
mentioned above, and is simply the posterior limit of the head-fold
of the entoderm; the fifth section posterior to this shows where this
depression opens ventrally to the yolk sac. The other structures shown
in the figure are not markedly different from what was seen in Figure
11_d_.
Figure 11_f_ is about one tenth the length of the embryo posterior to
Figure 11_e_. The chief differences here noticed are in the enteric and
cœlomic cavities. The former is no longer enclosed, a dorsal fold in
the entoderm being all that remains of the cavity that was seen in the
more anterior figures, while the latter is here reduced to a narrow
cleft between the somatic and splanchnic mesoblast. A thickening of
the mesoblast on either side of the notochord, especially on the left,
represents a mesoblastic somite. The medullary canal (_mc_) is more
open than in the more anterior sections.
For about one third of the length of the embryo posterior to Figure
11_f_ there is a gradual flattening, in a dorso-ventral direction,
with loss of the amnion, until the condition represented in Figure
11_g_ is reached. The most striking feature of this region is the great
thickness of the ectoderm (_ec_), which is still made up of scattered,
irregular cells. In the middle line, directly over the medullary canal
(here a nearly cylindrical tube), is a sort of break in the ectoderm,
as though there had not been a complete fusion of the epidermal layer
when the nervous layer came together on the closure of the medullary
groove. This break in the ectoderm may be followed back to the region
of the primitive streak, and will be mentioned again. As has been
noted, the medullary canal (_mc_) is nearly circular in cross-section,
and is closely underlaid by the notochord (_nt_), which is several
times the diameter that it was in more anterior sections. The mesoblast
(_mes_) is a comparatively thin layer, intermediate in thickness
between the ectoderm and entoderm. It shows laterally a slight
separation to form the body cavity.
Figure 11_h_ is about ten sections posterior to Figure 11_g_, and
differs from it chiefly in that the notochord (_nt_) is continuous with
the lower side of the medullary canal (_mc_), though still distinct
from the underlying entoderm (_en_).
Figure 11_i_, four sections farther from the head, shows the same
greatly thickened ectoderm (_ec_) with the same break (_ec′_) in the
middle line. The section is posterior to the notochord and passes
through the anterior edge of the blastopore or, as it may now perhaps
better be called, the neurenteric canal. The cells of the medullary
wall are continuous with those of the entoderm. The mesoderm (_mes_) is
still distinct from the other germ layers.
Figure 11_j_ is the next section posterior to the one just described
and differs from it only in showing the actual opening of the
neurenteric canal (_nc_) into the medullary canal (_mc_). The medullary
canal extends, with gradually diminishing caliber, for about fifteen
sections posterior to the point at which the neurenteric canal empties
into it. The mesoblast (_mes_) is so closely attached to the lower wall
of the neurenteric canal that it seems to be actually continuous with
it.
For a considerable distance posterior to the end of the medullary
canal we find the structure similar to that shown in Figure 11_k_,
which is about the twentieth section posterior to Figure 11_j_. The
break (_ec′_) in the ectoderm is here seen as a compact group of cells
which at first glance seem to be continuous with a rounded mass of
cells below (_ps_). Examination under greater magnification, however,
shows that the two groups of cells are distinct. As the sections are
followed back of this region, the upper mass of cells (_ec′_) gradually
disappears, and after its disappearance the lower mass (_ps_), which is
already continuous with the mesoderm (_mes_) on either side, becomes
continuous with the under side of the ectoderm. The mass of cells
(_ps_) is apparently the primitive streak, though it is distinct from
the ectoderm for a considerable distance posterior to the neurenteric
canal. Just what may be the meaning of the thickened ridge of ectoderm
(_ec_) it is difficult to determine.
STAGE IX
FIGURES 12-12_g_ (PLATES XVIII., XIX.)
The entire length of the embryo proper is 6.5 mm. from the extreme
posterior end to the region of the midbrain (_v²_), which now, on
account of the cranial flexure, forms the most anterior part of the
body. Besides being slightly longer than the preceding stage, the
embryo has increased in thickness, especially in the anterior region,
where the enlargement of the cerebral cavity is considerable.
Body torsion has begun (Fig. 12), so that the anterior third of the
embryo now lies on its right side, while the rest of the body is still
dorsal side up. The direction of body torsion does not seem to be as
definite as it is in the chick, some alligator embryos turning to the
right side, others to the left. Clarke has illustrated this fact in his
alligator figures. He says (17) that embryos lie “more frequently on
the left, but often on the right side.”
The head is distinctly retort-shaped, and at the side of the forebrain
(_v′_) a small crescentic thickening is the optic vesicle (_e_). The
auditory vesicle, though of considerable size, does not show in this
surface view. The head-fold (_h_) extends for about one third the
length of the entire embryo, though its exact limit is difficult to
determine in surface view. There is no sign of a tail-fold.
About seventeen pairs of somites are present.
The amnion extends over the anterior two thirds of the embryo.
The above-mentioned increase in the diameter of this embryo over that
of the preceding is evident when the first two transverse sections of
this stage are compared with the corresponding sections of the earlier
stage; in the middle and posterior regions there is not very much
difference in size.
Figure 12_a_ passes through the region of the forebrain. This end
of the embryo lies on its side, as was noted above and as may be
recognized from the relative positions of the head and the overlying
yolk (_y_). The great size of this and the following figure is due
partly to the increase in size mentioned above and partly to the
fact that the sections pass through the region of cranial flexure.
The present figure (12_a_) represents the brain cavity as large and
dumbbell-shaped, with comparatively thick walls of compactly arranged
cells. The ventral end of this cavity (_fb_) is cut anterior to the
region of the optic vesicles, while the dorsal end (_mb_) may perhaps
be called the midbrain. In the sections that follow this one the two
cavities are distinct from each other. The medullary canal, as was
stated above, is now completely enclosed, except for the ventral
opening of the neurenteric canal, to be presently noticed. Surrounding
the brain is a considerable mass of mesoblast (_mes_). It is composed
of the typical stellate cells. The ectoderm (_ec_) is made up of
the same irregularly and loosely arranged cells that have been seen
in earlier stages; it is of unequal thickness in different regions,
the thicker parts being at the sides. The amnion (_a_) has the usual
appearance, and in this region of course completely surrounds the
embryo.
Figure 12_b_ is ten sections posterior to the section just
described. The width of the embryo is greater in this region, but
the dorso-ventral diameter is about the same as in the more anterior
section.
The overlying yolk and blastoderm are not shown in any figure of
the series except the first. In this figure the forebrain (_fb_) and
midbrain (_mb_) are widely separated instead of being connected, as
in the preceding figure, where the section passed through the actual
bend of the cranial flexure. The anterior and ventral part of the
cranial cavity, the forebrain (_fb_), is nearly circular in outline.
It exhibits on one side a well-marked optic vesicle (_ov_), which
is sufficiently advanced in development to show a rudimentary optic
stalk. The outer wall of the optic vesicle is in close contact with
the superficial ectoderm, which shows as yet no sign of the formation
of a lens vesicle. The plane of the section being probably not quite
at right angles to the long axis of the embryo, the optic vesicle of
one side only was cut. The wall of this part of the forebrain is of
about the same thickness and appearance as in the preceding stage.
The other cerebral cavity (_mb_) of this section is probably the
hinder part of the midbrain, though it may be the anterior part of the
hindbrain; there is no sharp line of demarcation between these regions
of the brain. This cavity (_mb_) is much smaller in section than the
forebrain; its walls are of about the same thickness.
Ventral to the midbrain is the anterior end of the notochord (_nt_),
surrounded by the mesoblast. At various places throughout the mesoblast
irregular open spaces may be seen; these are blood-vessels. The
ectoderm (_ec_) and amnion (_a_) have about the same appearance as in
the preceding figure, though the former seems somewhat thinner.
Figure 12_c_ is just back of the bent-under forebrain represented
in the preceding figure and in front of the main body of the heart.
The plane of the section not being at right angles to the long axis
of the body (as was mentioned above), the figure is not bilaterally
symmetrical. The neural canal, since the section passes through the
auditory vesicles, may here be called the hindbrain (_hb_). It has an
almond-shaped cavity, surrounded by a wall of medium thickness. In
close contact with the wall of the hindbrain, on each side, is the
inner side of the auditory vesicle (_o_), which is seen as a deep,
wide-mouthed pit in the superficial ectoderm. On the right side of
the section the auditory pit is cut through its middle region; it is
simply a thickened and condensed area of the ectoderm which has been
invaginated in the usual way. Directly beneath the hindbrain is the
notochord (_nt_), on each side of which, in the mesoblast, is the
dorsal aorta (_ao_), or rather the continuation of the aorta into
the head. Beneath these structures and extending from one side of
the section to the other is the pharynx (_ph_); its lining wall is
fused on each side with the ectoderm, but there is no actual opening
to the exterior. These points of contact (_g_) between entoderm and
ectoderm are of course the gill clefts; they are not yet visible from
the outside. The roof of the pharynx is flat and comparatively thin,
while the floor is thickened and depressed to form a deep, wide pit,
traceable through six or eight sections. This pit may be the thyroid
gland already noticed in the preceding stage. Below the main cavity
of the pharynx and close to each side of the thyroid rudiment just
mentioned is a large blood-vessel (_tr_). These two vessels when traced
posteriorly are found to be continuous with the anterior end of the
heart, and hence may be called the truncus. They were noticed in Figure
11_c_, _bv_. The ectoderm surrounding the lower side of the embryo was
so thin and indistinct that it could scarcely be distinguished from
the mesoderm of that region. The amnion (_a_) is still a continuous
envelope entirely surrounding the embryo.
Figure 12_d_, about twenty sections posterior to Figure 12_c_, is
in the posterior heart region. The spinal cord (_sc_), as might be
expected, is smaller than in the more anterior region, but is otherwise
not markedly different from what was there seen. The notochord (_nt_)
also has the same appearance as before. The enteron (_ent_) shows
of course in this region no gill clefts; it is a small, irregular
cavity with thicker walls than in the figure just described. The
ventro-lateral depression is entirely distinct from the depression
that was called the thyroid rudiment in the preceding figure. Dorsal
to the enteron are the two dorsal aortæ (_ao_), now smaller and more
ventral to the notochord than in the preceding figure. Ventral to the
enteron is the large heart (_ht_), projecting below the body cavity,
which is no longer enclosed. The mesodermic wall (_mes′_) of the heart
is still comparatively thin and is separated by a considerable space
from the membranous endocardium (_en′_). The extent and shape of the
heart are shown in the surface view of this stage. On the right side
of the section the body cavity extends to a point nearly opposite the
middle of the spinal cord, considerably dorsal to the notochord, while
on the left side the dorsal limit of the body cavity is scarcely level
with the lower side of the notochord. Between the dorsal end of the
body cavity and the side of the spinal cord, on the left, is a dense
mass of mesoblast (_s_), one of the mesoblastic somites. A few sections
either anterior or posterior to the one under discussion will show the
condition of the two sides reversed--that is, the body cavity will
extend to the greater distance on the left and will be interrupted by
a mesoblastic somite on the right. It is evident, then, that the upper
angle of the body cavity is extended dorsally as a series of narrow
pouches between the somites. The mesoblast that lines the body cavity,
the splanchnopleure (_sm_) and somatopleure (_so_), is somewhat denser
than the general mass of mesoblast, so that these layers are quite
distinct, the former (_sm_) extending around the enteron (_ent_) and
heart (_ht_), and the latter (_so_) being carried dorsalward as the
mesoblastic part of the amnion (_a_). The amnion may be traced through
about 130 of the 200 sections into which this embryo was cut.
Figure 12_e_ is nearly one fourth the length of the embryo posterior to
Figure 12_d_; it is approximately in the middle region. The diameter
of the embryo has been gradually decreasing until now it is very much
less than in the head region. The section being behind the head-fold
the entoderm (_en_) is nearly flat and the enteron is quite unenclosed.
The canal of the spinal cord (_sc_) is smaller in proportion to the
thickness of its walls, and the notochord (_nt_) is somewhat larger
than in the preceding sections. In proportion to its extent, the
ectoderm is very thick. Under the notochord the dorsal aortæ (_ao_) are
seen as two large, round openings in the mesoblast. On the left side
the section passes through the center of a somite and shows a small,
round myocœl (_myc_). The mesoblastic layer of the amnion (_so_) is
distinct throughout from the ectoblastic layer (_a_).
The most important structures to be here noted are the first rudiments
of the Wolffian ducts (_wd_). They are seen in the present section
as lateral ridges of mesoblast projecting outward and upward toward
the ectoblast, which suddenly becomes thin as it passes over them.
These ridges or cords of mesoblast are as yet quite solid. They arise
suddenly at about the eightieth section of the series of two hundred
and may be traced through about forty sections, or one fifth of the
length of the embryo. Their exact length is difficult to determine
because, while their anterior ends are blunt and sharply defined, they
taper so gradually posteriorly that it is hard to tell just where they
end. They apparently originate anteriorly and gradually extend toward
the tail. In a slightly younger embryo the rudimentary Wolffian duct
could be seen as a still smaller rod of cells extending posteriorly for
a few sections, from the seventy-fifth section of a series of about two
hundred. In the particular series under discussion the left rudimentary
Wolffian duct was about one fifth longer than the right one.
Figure 12_f_ is just posterior to the head-fold of the amnion, passing,
in fact, on the left side through the extreme edge of its lateral fold,
which is shown as a upward bend in the ectoblast and somatopleure.
The ectoblast (_ec_) shows the same remarkable thickening that was
noted in the corresponding region of the preceding stage. The spinal
cord (_sc_), notochord (_nt_), aortæ (_ao_), and entoderm (_en_) need
no special mention. The mesoderm seems to be separated by unusually
wide spaces from both ectoderm and entoderm, and is made up of rather
closely packed cells except around the aortæ, where there seems
scarcely enough tissue to hold these vessels in place. The body cavity
(_bc_) is large, and a small myocœl (_myc_) is seen on the left.
Figure 12_g_ is through the neurenteric canal (_nc_), a distinct
opening through the floor of the spinal canal. The section is of course
just back of the posterior end of the notochord. The entoderm (_en_)
along the margin of the neurenteric canal is naturally continuous with
the wall of the spinal cord (_sc_). The ectoderm (_ec_) is thicker than
ever, except in the median plane, where it passes over the spinal cord.
The mesoblast is more abundant than in the preceding figure, and shows
on the left what appears to be a distinct myocœl (_myc_), though in
surface view the mesoblastic somites do not extend this far toward the
tail.
STAGE X
FIGURES 13-13_g_ (PLATES XIX., XX., XXI.)
This embryo (Fig. 13) is about 5 mm. in length, and hence is slightly
smaller than the preceding stage, though somewhat more advanced in
development. The medullary canal is still _apparently_ unclosed for
a short distance at the extreme posterior end; this appearance is
probably due to the neurenteric canal (_nc_) and to the thinness of
the roof of the medullary canal rather than to any lack of fusion of
the medullary folds. The optic vesicle is more distinct than in the
preceding stage; a somewhat similar, though smaller, opacity (_o_)
marks the position of the ear. There are now about twenty pairs of
somites, though it is difficult to determine their exact number on
account of the torsion of the body. The amnion is at about the same
stage of development as in Stage IX. The heart (_ht_) is a large double
mass, whose outlines may be dimly seen when the embryo is viewed by
transmitted light. The vitelline vessels (_vv_) are still but faintly
outlined in the vascular area; the veins and arteries cannot yet be
distinguished from each other. The gill clefts, though not visible
externally in the embryo drawn, may be seen in sections of this stage
as evaginations of the wall of the pharynx.
The transverse sections of this stage are slightly more advanced in
development than was the embryo that has just been described in surface
view. Only those sections have been figured which show a decided
advance in the development of some special structures over their
condition in the preceding stage. The sections of the preceding stages
were drawn under a magnification of eighty-seven diameters; those of
this and the following stage were drawn under a magnification of only
forty-one diameters. All of the figures have been reduced one half in
reproduction.
Figure 13_a_ is the most anterior section of this series to be
described. On account of the cranial flexure, which causes the long
axis of the forebrain to lie at right angles to that of the spinal
cord, this section cuts the head region longitudinally. The ectoderm
(_ec_) is of varying thickness, the thickest areas being on each
side of the forebrain; it is more compact than in the earlier stages,
and, owing to the low magnification under which it is drawn, it is
represented here by a single heavy line. Under this magnification only
the nuclei of the mesoderm cells (_mes_) can be seen, so that this
tissue is best represented by dots, more closely set in some places
than in others. The forebrain is an elongated cavity (_fb_) with thick,
dense walls. Attached to each side of the forebrain is an optic vesicle
(_ov_), which is considerably larger than in the preceding stage. The
connection between the cavity of the forebrain and that of the optic
vesicle is not seen in this section; it is a wide passage that may be
seen in several sections posterior to the one under discussion. The
beginning of the invagination of the optic vesicle to form the optic
cup may be seen on both sides, but more plainly on the right. On the
right side also is noticed a marked thickening of the ectoderm, which
is invaginated to form a small pit, the lens vesicle (_lv_); on the
left side the section is just behind the lens vesicle. Above the optic
stalk on each side, in the angle between the optic vesicle and the
side of the forebrain, is a small blood-vessel (_bv_). Several other
blood-vessels may be seen at various places in the mesoblast, four of
them near the pharynx being especially noticeable. The hindbrain (_hb_)
is wider than, but not so deep as, the forebrain; its walls are very
thick laterally, but are thin on the dorsal and ventral sides. The
dorsal wall is reduced to a mere membrane, which, with the overlying
ectoderm, has been pushed into the brain cavity, as is generally the
case with such embryos. Close to the ventral wall of the hindbrain the
notochord (_nt_) is seen. The character of the notochord has already
begun to change; the cells are becoming rounded and vacuolated, with
but few visible nuclei except around the periphery of the notochord.
Near the center of the section, close to the ventral end of the
forebrain, is the pharynx (_ph_), cut near its anterior limit; it is
here a small, irregularly rectangular cavity with a comparatively thin
wall. On the left side of the pharynx the first gill cleft (_g_) is
indicated as a narrow diverticulum reaching toward the ectoderm. A few
sections posterior to this one the first gill cleft is widely open to
the exterior. As has been said, in the surface view of this stage above
described none of the gill clefts showed; so that in this respect at
least the sectioned embryo was more nearly of the state of development
of the embryo represented in Figure 14, to be described later.
Figure 13_b_, about forty sections posterior to Figure 13_a_, passes
through the hindbrain in the region of the ears. Being back of the
region affected by cranial flexure, this section is of course of much
less area than the preceding. The ectoderm shows no unusual features;
it is of uniform thickness except where it becomes continuous with
the entoderm around the mandibular folds (_md_); there it is somewhat
thickened. The most striking feature of the section is the presence
of two large auditory vesicles (_o_). The section being not quite at
right angles to this part of the embryo, the vesicles are not cut in
exactly the same plane; the one on the left is cut through its opening
to the exterior, while the one on the right appears as a completely
enclosed cavity. In a section a short distance posterior to this one
the appearance of the vesicles would be the reverse of what it is here.
As may be seen in the figure, the vesicles are large, thick-walled
cavities lying close to the lateral walls of the hindbrain. The
hindbrain itself has the usual triangular cross-section, with thick
lateral walls and a thin, wrinkled dorsal wall. Close to the ventral
side of the hindbrain lies the notochord (_nt_), on each side of
which, in the angle between the brain and the auditory vesicles, is
a small blood-vessel (_bv_). Ventral to these structures and close
to the dorsal wall of the pharynx (_ph_) are the two large dorsal
aortæ (_ao_). The ventral side of the section passes through the open
anterior end of the pharynx (_ph_). On the left is seen the widely open
hyomandibular cleft (_g′_), between the main body of the section and
the mandibular arch (_md_). On the right side the plane of the section
was such that the hyomandibular cleft was not cut through its external
opening. In each mandibular fold a large aortic arch (_ar_) is seen,
and also a slight condensation of mesoblast, the latter probably being
the forerunner of cartilage.
Figure 13_c_ passes through the anterior part of the heart about
seventy-five sections posterior to Figure 13_b_. The embryo in this
region is narrow but deep (dorso-ventrally), the depth being largely
due to the size of the heart. The ectoderm (_ec_) is considerably
thickened on each side of the pharynx (_ph_); this thickened area
may be traced for some distance both anteriorly and posteriorly from
this point; its significance could not be determined. The spinal cord
(_sc_) and notochord (_nt_) need no special description; the former is
smaller and the latter larger than in the more anterior sections. The
two large blood-vessels (_ac_) near the spinal cord and notochord are
probably the anterior cardinal veins. The aortæ are cut by the plane
of this section just anterior to their point of fusion into a single
vessel. A few blood corpuscles are seen in the right aorta. The enteron
(_ent_), cut posterior to the region of the gill clefts, is a large
elliptical cavity, with its long axis in a transverse position. Its
entodermal wall is comparatively thin and smooth, with the cell nuclei
arranged chiefly on the outer side, _i. e._, away from the cavity of
the enteron. The body cavity (_bc_) is here still unenclosed, and its
walls, the somatic stalk, are cut off close to the body of the embryo.
The heart (_ht_), the most conspicuous feature of this section, is
nearly as large in cross-section as all the rest of the embryo. As seen
in such a section it is entirely detached from the body of the embryo,
and in this particular case has about the shape of the human stomach.
The mesoblastic portion of its wall (_mes′_) is of very irregular
thickness; it forms a dense layer entirely around the outside, except
for the pointed dorsal region, and is especially thick along the
ventral margin, where it is thrown into well marked folds, the heavy
muscle columns. Lining the cavity of the heart is the membranous
endothelium (_en′_), and between this and the dense outer wall just
described is a loose reticular tissue with but few nuclei.
As the series is followed toward the tail the sections diminish in size
until, at a point about one third the embryo length from the posterior
end, they are of scarcely one fourth the area of the sections through
the region of the hindbrain.
Figure 13_d_ is about one hundred and twenty-five sections posterior
to Figure 13_c_. Although not so small as the sections that follow
it, this section is considerably smaller in area than the one last
described. The amnion (_a_), which was not represented in the last
three figures, is very evident here. The spinal cord (_sc_) is
considerably smaller here than in the preceding figure, while the
notochord (_nt_) is not only relatively but actually larger than in
the more anterior regions. Beneath the notochord is the aorta (_ao_),
now a single large vessel. The mesoblast on each side of the body is
here differentiated into a distinct muscle plate (_mp_). These muscle
plates have very much the appearance of the thickened ectoderm seen in
the younger stages of development. At about its middle region (_i. e._,
at the end of the reference line _ec_) each muscle plate is separated
from the overlying ectoderm by an empty space; this space is still more
marked in some other series. Ventral to the aorta, and supported by a
well marked though still thick mesentery (_ms_), is the intestine. It
is a small, nearly cylindrical tube with thick walls; the splanchnic
mesoblast which surrounds it is more dense than the general mass of
mesoblast; it was somewhat torn in the section and is so represented in
the figure. The urinary organs have made considerable progress since
the last stage. In the figure under discussion they are seen as a group
of tubules on either side of the aorta. The tubule most distant from
the middle line, on each side, is the Wolffian duct (_wd_). It extends
through the posterior two thirds of the embryo and varies in diameter
at different points; it is usually lined with a single layer of cubical
cells which contain large nuclei. The Wolffian bodies (_wt_) are a
mass of slightly convoluted tubules that may be traced throughout the
greater part of the region through which the Wolffian duct extends.
These tubules also vary somewhat in diameter, but they are usually
of greater caliber than the duct. No actual nephrostomes are to be
seen, though the occasional fusion of a tubule with the peritoneal
epithelium, as is seen on the left side of the present figure, may
represent such an opening.
Figure 13_e_ is about one hundred and forty sections posterior to the
section just described. The embryo is here very slender, so that the
contrast between this and the first figure (13_a_) of this stage is
remarkable. Except in size, this section does not differ greatly from
the preceding. The spinal cord, notochord, etc., are smaller than
before, but are of about the same relative size. The mesentery (_ms_)
in the section drawn was torn across, so that the intestine is not
represented. Medial to the Wolffian duct is a tubule (_wt_), which
seems to be the same as those which were called Wolffian tubules in the
preceding stage, but which may be the beginning of the ureter.
Figure 13_f_, about two hundred and fifty sections posterior to the
last, passes through the extreme posterior end of the embryo. The
section is nearly circular in outline and is somewhat larger than
the preceding. The amnion (_a_) completely encircles the embryo. The
ectoderm (_ec_) is of fairly even thickness, and the mesoblast which it
encloses is of the usual character. The spinal cord (_sc_) is nearly
circular in outline, as is its central canal. The digestive tract
(_ent_) is larger in section than it was in more anterior regions;
it is nearly circular in cross-section and its walls are made up of
several layers of cells, so that it resembles to a considerable degree
the spinal cord of the same region. In the narrow space between the
spinal cord and the hindgut is seen the notochord (_nt_), somewhat
flattened and relatively and actually smaller than in the preceding
figure. A few scattered blood-vessels may be seen in the mesoblast at
various places.
A sagittal section of an embryo of this stage, drawn under the same
magnification as were the transverse sections, is shown in Figure
13_g_. The embryo being bent laterally could not be cut by any one
plane throughout its entire length, so that only the anterior end is
represented in the figure. The amnion (_a_) may be clearly seen except
at certain places where it is closely adherent to the superficial
ectoderm. Under the low magnification used the superficial ectoderm
cannot be distinguished from the ectoderm of the nervous system.
The plane of the section being in the anterior end almost exactly
median, this part of the central nervous system is seen as the usual
retort-shaped cavity, while in the region back of the brain, where
the neural canal is narrow, the section passes through the wall of
the spinal cord (_sc_) and does not show the neural canal at all.
The wall of the forebrain (_fb_) is quite thick, especially at the
extreme anterior end; the wall of the midbrain (_mb_), where the marked
cranial flexure takes place, is somewhat thinner, and it gradually
becomes still thinner as it is followed posteriorly over the hindbrain
(_hb_). Between the floors of the fore- and hindbrains, in the acute
angle caused by the cranial flexure, is the anterior end of the
notochord (_nt_), the only part of that structure that lies in the
plane of the section. Ventral and posterior to the notochord is a large
cavity, the pharynx (_ph_), whose entoblastic lining can scarcely be
distinguished under this magnification from the surrounding tissues.
The stomodeal opening being as yet unformed, the pharynx is closed
anteriorly; posteriorly also, owing to the plane of the section, the
pharynx appears to be closed, since its connection with the yolk stalk
is not shown. In the floor of the pharynx, almost under the reference
line _ph_, a slight depression marks the position of the first gill
cleft. In the mesoblast ventral to the pharynx and near the gill cleft
just mentioned, a couple of irregular openings represent the anterior
end of the bulbus arteriosus, posterior and ventral to which is the
heart (_ht_), a large, irregular cavity. The dorsal aorta (_ao_) may
be seen as an elongated opening in the mesoblast, extending in this
section from the middle region of the pharynx to the posterior end of
the figure where it is somewhat torn. Two of the eighteen or twenty
pairs of mesoblastic somites possessed by this embryo are shown at the
posterior end of the figure (_s_), where the plane of the section was
far enough from the median line to cut them.
STAGE XI
FIGURE 14 (PLATE XXI.)
Only the anterior region of this embryo is shown in the figure, which
is a ventro-lateral view. While there is some change in the general
shape and in parts of the head, the reason for figuring this stage is
to show the first gill cleft (_g′_), which lies at an acute angle to
the long axis of the neck behind the eye (_e_). The cleft is narrow
but sharp and distinct in outline; it shows neither in this nor in the
following stages the branched, Y-shaped outline mentioned by Clarke.
STAGE XII
FIGURES 15-15_f_ (PLATES XXI., XXII.)
In this stage, also, only the anterior region of the embryo is figured
in surface view. The shape of the head is about the same as in the
preceding stage, but it is drawn in exact profile. Three gill clefts
(_g¹⁻³_) are now present, and are wide and distinct. The first cleft,
as in the preceding stage, lies at an acute angle to the long axis of
the pharynx and nearly at right angles to the second cleft. The third
cleft sends a wide branch (_g⁴_) toward the posterior, as has been
described by Clarke, from which, or in connection with which according
to Clarke, the fourth cleft will develop. All three clefts may be
distinctly seen to open entirely through the pharyngeal wall. The
outlines of the visceral folds, especially of the mandibular, begin to
be apparent. The nasal pit (_n_) now shows as a round depression in
front of the more definitely outlined eye (_e_). The auditory vesicle
(_o_) is so deep beneath the surface that it may be seen only by
transmitted light.
Figures 15_a_-_e_ represent transverse sections of an embryo of about
this general state of development, except that the gill clefts are not
so definitely open as in the surface view.
Figure 15_a_, the most anterior section of the series, passes through
the forebrain (_fb_) in the region of the eyes, and through the
hindbrain (_hb_) anterior to the auditory vesicles. The forebrain
is here a large cavity with a dense wall of a comparatively even
thickness. Owing probably to the section not being exactly in the
transverse plane, the eyes are cut in different regions, that on the
left (_ov_) being cut through its stalk, while that on the right
(_oc_) is cut near its middle region and hence does not show any
connection with the forebrain. The almost complete obliteration of the
cavity of the optic vesicle to form the optic cup by the invagination
of the outer wall of the vesicle is shown on the right side of the
section (_oc_). The lens vesicle (_lv_) is completely cut off from the
superficial ectoderm (_ec_), which is comparatively thin. The hindbrain
(_hb_) has the usual shape for that structure. Its ventral wall is
dense and thick, while its roof is reduced to the usual thin, wrinkled
membrane. Close to the floor of the hindbrain lies the notochord
(_nt_), which is large and is distinctly vacuolated. To the right of
the hindbrain a large mass of darkly stained cells (_cn_) is one of the
cranial nerves, which is connected with the hindbrain a few sections
anterior to the one under consideration. The pharynx (_ph_), which is
cut near its extreme anterior end, is represented by three irregular
cavities near the base of the forebrain. Scattered throughout the
mesoblast, which makes up the greater part of the section, are numerous
blood-vessels (_bv_).
Figure 15_b_ is twenty sections posterior to Figure 15_a_ and passes
through the tip of the bent-under forebrain (_fb_). On the left the
section is anterior to the optic vesicle and barely touches the side of
the optic stalk, which is seen as a small lump on the ventro-lateral
wall of the brain. On the right the connection of the optic vesicle
(_ov_) with the forebrain is shown. Dorsal to the optic vesicle just
mentioned is a markedly thickened and slightly invaginated region of
the ectoderm (_n_); this is the nasal pit; on the left side of the
figure the thickening is shown, but the section did not pass through
the invagination. The hindbrain (_hb_) is somewhat narrower than in
the preceding figure, but is otherwise about the same; the origin of
a cranial nerve is seen on its left side (_cn_). The notochord (_nt_)
has the same appearance as in the preceding section. A number of
blood-vessels may be seen, the pair lying nearest the notochord being
the aortæ (_ao_), while the two other pairs, on either side of the
fore- and hindbrains, are the anterior cardinals (_ac_). The first
aortic arches are shown at _ar_. On the left the section passes through
the exterior opening of the first gill cleft (_g′_), so that the
mandibular fold (_md_) on that side is a distinct circular structure,
made of a dense mass of mesoderm surrounded by a rather thick ectoderm.
The mesoderm of this fold is especially dense near the center, probably
the beginning of the visceral bar. Near the center is also seen the
aortic arch that has already been mentioned. On the right the section
does not pass through the external opening of the first gill cleft
(_g′_) so that the tissue of the mandibular fold is continuous with
the rest of the head. It is of course the slight obliquity of the
section that causes the pharynx (_ph_) to be completely enclosed on
the right, while on the left it is open to the exterior both through
the gill cleft and between the mandibular fold and the tip of the
head. The superficial ectoderm shown here as a heavy black line varies
considerably in thickness, being thickest in the region of the nasal
pit already mentioned and thinnest over the roof of the hindbrain. The
amnion (_a_) in this, as in the other sections of the series, has the
appearance of a thin, very irregular line.
Figure 15_c_ is posterior to the region affected by cranial flexure
and so shows only one region of the embryo, that of the hindbrain
(_hb_), which is here of essentially the same structure as above
described. On each side of the hindbrain is a large auditory vesicle
(_o_); that on the left is cut through its center and shows the
beginning of differentiation, its lower end being thick-walled and
rounded, while its upper end is more pointed and has a thin, somewhat
wrinkled wall. The notochord (_nt_) is slightly larger than in the more
anterior sections. Numerous blood-vessels (_bv_, _ar_) are seen in the
mesoblast. The pharynx (_ph_) is here open ventrally and also through
the gill cleft of the left side; on the right side the plane of the
section did not pass through the external opening of the cleft. The
mesoblast of the visceral folds is much more dense than that of the
dorsal region of the section.
Figure 15_d_, as is evident, is a section through the region of the
heart, which appears as three irregular cavities (_ht_) with fairly
thick mesoblastic walls (_mes′_) lined with endothelium (_en′_). The
body wall, though consisting of but little besides the ectoderm (_ec_),
completely surrounds the heart, and the pericardial or body cavity
thus formed extends dorsally as a narrow space on either side of the
foregut, giving the appearance of a rudimentary mesentery, though no
especial development of such a structure would naturally be expected in
this region of the embryo. The foregut (_ent_) is a moderately large
cavity lined with a very distinct entoderm of even thickness. Dorsal
to the foregut are three large blood-vessels, a median, and now single,
dorsal aorta (_ao_), and a pair of cardinal veins (_cv_). The notochord
(nt) is small and is flattened against the ventral side of the spinal
cord (_sc_), which latter structure needs no special mention. The
muscle plates (_mp_) are considerably elongated, so that they now
extend ventrally to a point slightly below the upper angles of the body
cavity.
Figure 15_e_ is through the middle region of the embryo, and, owing to
the curvature of the body, is not an exact dorso-ventral section; this
accounts, in part at least, for the unusual diameter in a dorso-ventral
direction of the aorta (_ao_), which is very large in proportion to
the other structures. The posterior cardinal vein is shown on the
left, but not on the right. The relative sizes of the spinal cord
(_sc_) and notochord (_nt_) are very different from what was seen in
the preceding figure. In this section the spinal cord is considerably
smaller than in the preceding, while the notochord is very much larger;
in fact the notochord here seems abnormally large when compared to
corresponding sections of other series. It is true, however, that while
the spinal cord has been diminishing in diameter the notochord has
been increasing. The spinal cord, notochord, and dorsal aorta are all
so large that they are flattened against each other, the pushing in of
the ventral side of the spinal cord being even more marked than is
shown in the figure. On either side of the spinal cord a large spinal
ganglion (_sg_) is seen, closely wedged in between the spinal cord and
the adjacent muscle plate (_mp_). As in the preceding stage, there is a
marked space between the muscle plate and the adjacent ectoderm (_ec_).
The somatic mesoblast at the upper angle of the unenclosed body cavity
is thickened on each side and somewhat bulged out by the Wolffian
body to form what might be termed a Wolffian ridge (_wr_). In the
mid-ventral line is the considerably developed mesentery (_ms_), from
which the intestine has been torn. The Wolffian bodies now consist, on
each side, of a group of five or six tubules (_wt_) of various sizes,
near which in a more ventro-lateral position, close to the upper angle
of the body cavity, is the more distinct Wolffian duct (_wd_). The
allantois is fairly large by this time, and may be seen in the most
posterior sections as an irregular, thick-walled outgrowth from the
hindgut.
A horizontal section through the anterior end of an embryo of this
age is shown in Figure 15_f_. While enclosed of course in the same
membranous amnion (_a_), the pharyngeal region of the section is
separated by a considerable space from the more anterior region where
the section passes through the forebrain (_fb_) and eyes. The spinal
cord (_sc_), notochord (_nt_), muscle plates (_mp_), aortæ (_ao_),
and anterior cardinal veins (_ac_) need no special description. The
appearance of the pharynx (_ph_), with its gill clefts and folds, is
quite similar to that of the corresponding structures in the chick.
None of the four clefts (_g¹⁻⁴_) show, in the plane at which the
section was cut, any connection with the exterior; in fact the fourth
cleft (_g⁴_) would scarcely be recognized as a cleft if seen in this
section alone. One or two of the more anterior clefts are open to the
exterior. Three pairs of aortic arches are seen, and each visceral fold
has a central condensation of mesoblast.
STAGE XIII
FIGURES 16-16_g_ (PLATES XXII., XXIII.)
The embryo (Fig. 16) now lies on one side, body torsion being complete.
The curvature of the body is so marked that the exact length is
difficult to determine. The eye (_e_) and ear (_o_) have about the
same superficial appearance as in the preceding stage. The nose is
not shown in this figure. About thirty somites are present; the exact
number cannot be determined in surface view. The amnion is complete,
though not shown in the figure, and the tail (_t_) is well formed. The
umbilical stalk was torn in the removal of the embryo, so that it is
not shown in the figure. The dim outline of the now convoluted heart
may be seen if the “cleared” embryo be viewed by transmitted light;
it is not shown in the figure. The allantois (_al_) is a rounded sac
of considerable size just anterior to the tail. Four gill clefts
(_g¹⁻⁴_) are now present; the most posterior one is more faint than is
represented in the figure, and it could not be definitely determined
from a surface view whether or not it opened to the exterior. The
mandibular fold (_md_) is now fairly well outlined, but there is as yet
no sign of the maxillary process.
Figure 16_a_ is the most anterior of a series of transverse sections
made of an embryo of the approximate age of the surface view just
described; it passes through the tip of the forebrain (_fb_) and shows
the nasal pit (_n_) of the right side. The great thickening of ectoderm
in the region of the nasal invagination is represented by a solid
line. Owing to the obliquity of the section, the left nasal pit was
not cut. The mesoblast is quite dense and contains two or three small
blood-vessels near the roof of the brain. The plane of this section,
owing to the cranial and body flexure, cut the embryo also in the
region of the pharynx; this part of the section was, as a matter of
convenience, omitted from the drawing.
Figure 16_b_ is in reality more anterior in position, considering
the entire embryo, than the preceding; but the region of the embryo
represented is more posterior, so that it is described at this point.
The greatly elongated outline of the brain is due to its being cut
through the region of flexure, so that the forebrain (_fb_), or,
perhaps, midbrain, is shown at one end, and the hindbrain (_hb_) at
the other. The walls of these cavities are somewhat wrinkled and
irregular and their constituent cells are beginning to show slight
differentiation, though this is not shown in the figure. On the left
side are seen a couple of darkly stained masses; one is the origin of
a cranial nerve (_cn_); and the other is one of the auditory vesicles
(_o_), which is still more irregular in outline than it was in the
preceding stage. The only blood-vessels to be seen are a few very small
ones that lie close to the wall of the brain. The ectoderm is quite
thin at all points.
Figure 16_c_, the largest section of this series, passes through the
forebrain in the region of the eyes and through the gill clefts. The
forebrain (_fb_) exhibits on the left a marked thickening of its wall
(_ch_), the edge of the cerebral hemisphere of that side, which is just
beginning to develop; on its right side the lower part of the forebrain
is connected by a well marked optic stalk (_os_) with the optic cup
(_oc_), in whose opening lies the lens vesicle (_lv_), now reduced to a
crescentic slit by the thickening of its posterior wall. The mesoblast
is more dense in those parts of the section adjacent to the pharynx
than in the more distant regions, and the ectoderm thickens in a marked
way as it approaches the borders of the pharynx and gill clefts. Only
a few small blood-vessels (_bv_) are to be seen in the region of the
forebrain.
Parts of three pairs of clefts (_g_) are shown in the figure: one
pair opens widely on either side, so that there is a large area of
the section that is distinct from the two still larger portions and
contains a small, thick-walled cavity (_g_) on the right side; this
cavity is a gill cleft that is cut through neither its outer nor its
pharyngeal opening.
No structures other than this small portion of a gill cleft and a few
blood-vessels are to be seen in this middle region of the section. In
the more posterior part of the section, in which the notochord (_nt_)
is located, a pair of curved clefts may be seen, opening entirely
through the wall on the left, but closed on the right (_g_). One
distinct pair of aortic arches is shown (_ar_), and also the dorsal
aortæ (_ao_), which are of very unequal size. The spinal cord (_sc_)
and muscle plates need no special description.
Figure 16_d_ is in the region of the heart (_ht_) and lungs (_lu_). The
former is an irregular cavity whose walls, especially on the ventral
side (_mes′_), are becoming very thick and much folded. Although thin,
the body wall completely surrounds the heart, as would be expected,
since this was true of the preceding stage. The lung rudiments (_lu_)
and the foregut from which they have arisen have the same appearance
as in the chick; they consist of three small, thick-walled tubes so
arranged as to form a nearly equilateral triangle. They are surrounded
by a swollen, rounded mass of mesoblast which almost completely fills
the surrounding portion of the body cavity (_bc_). The pleural sides
of these crescentic portions of the body (or pleural) cavity--that
is, the boundary of the mass of mesoblast just mentioned--are lined
with a thickened layer of cells, shown by the solid black lines in the
figure. The lung rudiments may be traced through about fifty sections
of this series, or about one twelfth of the entire series. At the
dorsal angle of the part of the body cavity (_bc_) just described, near
the dorsal aorta (_ao_), are two dark, granular masses (_ge_), which,
under a higher magnification than is here used, are seen to consist of
a small group of blood-vessels filled with corpuscles; although several
sections in front of the anterior limits of the kidneys these are
evidently glomeruli. They may be traced, though diminishing in size,
far toward the tail, in close connection with the Wolffian bodies. At
intervals they are connected by narrow channels with the dorsal aorta;
no such connection was present in the section drawn. The notochord
(_nt_), spinal cord (_sc_), muscle plates (_mp_), and spinal ganglia
(_sg_) need no special mention. The mesoblast is beginning to condense
in the neighborhood of the notochord, and the ectoderm is slightly
thickened laterally and dorsally.
Figure 16_e_ is in the region of the liver and the Wolffian bodies;
it also shows the tip of the ventricular end of the heart. The liver
(_li_) is a large irregular mass, of a blotchy appearance under this
magnification, lying between the heart (_vn_) and the intestine (_i_).
Under greater magnification it is seen to be made up of indefinite
strings of cells; and its still wide opening into the intestine may be
seen in more posterior sections. The intestine (_i_), which in this
section might be called the stomach, is a fairly large cavity with
the usual thick entodermic walls; it is supported by a comparatively
narrow mesentery. The body cavity on the side next this mesentery has
the same thick lining that was noted in the region of the lungs. The
convolutions of the thick peritoneal lining may easily be mistaken in
places for parts of the enteron. The Wolffian bodies may be seen as
two groups of tubules (_wt_) in their usual location. The heart is
cut through the ventricle (_vn_), as has been said. The section being
at right angles to the long axes of the villi-like growths of the
myocardium, the depressions between these mesoblastic cords are seen as
a number of small irregular areas, each one lined with its endocardium.
The incompleteness of the body wall below the heart is apparently due
to an artificial break and not to a lack of fusion. The only point that
need be mentioned in connection with the structures of the dorsal part
of the section is that the distinctness of the myocœl (_myc_) on the
right side is somewhat exaggerated.
Figure 16_f_ is in the middle region of the embryo where both
splanchnopleure and somatopleure are unfused. Owing chiefly to the
unclosed condition of the midgut (_i_) and to the increase in length
of the mesentery (_ms_), the section is quite deep dorso-ventrally.
The continuation of the amnion (_a_) with the somatopleure is of course
here evident.
The most striking feature of the section is the marked projection of
the Wolffian ridges, though no local enlargements of these ridges
indicate the rudiments of the limbs. A large mass of Wolffian tubules
(_wt_) is seen projecting into the upper part of the body cavity on
each side; close to each of these masses is the posterior cardinal
vein (_pc_), and between them is the large aorta (_ao_). The other
structures are about as in the preceding section.
Figure 16_g_ represents a sagittal section of the anterior half of
the body of an embryo of this or possibly a slightly younger stage of
development. The three regions of the brain are clearly indicated, as
well as the cavity of the spinal cord (_sc_). The roof of the hindbrain
has been made too thick in the figure; it should be represented by a
mere line. A little mesoblast is to be seen at places between the roof
of the brain and the superficial ectoderm. A slight invagination of the
epithelium (_p_), between the floor of the brain and the anterior end
of the notochord, probably represents the beginning of the hypophysis.
No indication of the paraphysis is yet to be seen. Extending from the
region of the hypophysis to the posterior end of the section is the
notochord (_nt_); it is much vacuolated and gradually increases in
thickness toward the posterior, though its outline is quite irregular;
except at the extreme anterior end and at one or two other places,
it lies in close contact with the floor of the neural tube. Directly
under the notochord lies, in the posterior half of the figure, the
large dorsal aorta (_ao_). The pharynx (_ph_), opening between the end
of the forebrain and the thick mandibular fold (across which opening
the amnion, _a_, of course extends), is a funnel-shaped space which
passes out of the plane of the section toward the posterior end of the
figure. Its thick endodermal lining extends to the mandibular fold
on the ventral side, while on the dorsal side it gradually thins out
and becomes continuous with the thin ectoderm that extends over the
forebrain. Just back of the mandibular fold is the bulbus (_b_), and
back of that is the edge of the ventricle (_vn_). Posterior and dorsal
to the ventricle the liver (_li_) is seen as an irregular mass of
cells, and dorsal to the liver one of the Wolffian bodies (_wt_) is cut
through its extreme edge.
STAGE XIV
FIGURES 17-17_g_ (PLATES XXIII., XXIV.)
Body flexure has increased until now the forebrain and tail are almost
in contact (Fig. 17). The eye has developed somewhat; the ear vesicle,
which is not shown in the figure, is small and seems to lie nearer the
ventral side; the nasal pit is much larger and is crescentic in shape.
The hyomandibular cleft (_g′_) still persists as a small crescentic
slit, while the next three clefts are now represented merely by
superficial grooves separated by distinct ridges, the visceral folds.
No indication of a fifth cleft is seen. The maxillary process (_mx_)
grows ventralward under the forebrain and is already longer than the
mandibular arch (_md_).
The chief advance in development over the preceding stage, besides
the formation of the maxillary process, is in the appearance of the
appendages (_aa_ and _pa_); they have the characteristic shape of the
rudimentary vertebrate appendage, though the anterior pair seem to
point in an unusual direction at this stage and to be slightly more
developed than the posterior. The curious, anteriorly directed heart
(_ht_) is, perhaps, somewhat abnormal. The umbilical stalk (_u_) is
comparatively narrow and, like the allantois, was cut off close to the
body.
Transverse sections of an embryo of this stage are represented in
Figures 17_a_-_g_, drawn under a lower magnification than were any of
the preceding figures.
Figure 17_a_ is in the region of the pharynx, and passes through the
forebrain (_fb_) and posterior part of the hindbrain (_hb_). In the
thick walls of both of these structures histological differentiation
has begun, so that even under low power an inner granular and an outer
clear zone may be distinguished. Under greater magnification the
presence of short fibers may be made out among the cells. The cerebral
hemispheres (_ch_) are well-marked structures, their asymmetry being
of course due to the obliquity of the section. Only one eye is cut by
the plane of the section, and this one shows no connection with the
forebrain. The outer wall of the optic cup (_oc_) is so thin that under
this magnification it can scarcely be seen as a dark line surrounding
the retinal wall. The lens (_ln_) is now a solid mass, of the usual
type for vertebrate embryos, its front or outer wall being a scarcely
discernible line. The hindbrain (_hb_) has the usual form for that
region and does not differ particularly from what was noted in earlier
stages except in the histological differentiation that has already
been mentioned. As with the eye, it is only on the right side that the
auditory vesicle (_o_) is shown. It shows some differentiation, but not
so much as would be seen were it cut in another region. In the center
of the section the pharynx (_ph_) forms an irregular cavity connected
with the exterior on the left by a gill cleft (_g_) and by another slit
which is simply the anterior margin of the stomodæum. On the right
neither of these openings is in the plane of the figure, though the
gill cleft (hyomandibular), which lies close to the auditory vesicle,
is almost an open passage. A few small blood-vessels are scattered
through the section; one of these (_bv_), lying between the notochord
(_nt_) and the floor of the brain, is noticeable from its being very
closely packed with corpuscles, so that at first glance, under low
magnification, it looks more like a nerve than a blood-vessel.
Figure 17_b_ is also through the pharyngeal region, a short distance
behind the preceding section. The growth of the cerebral hemispheres
(_ch_) is better shown than in the preceding figure, as is also the
general form of the optic cup (_oc_). On the left the nasal cavity
(_n_) is seen as an elongated slit with thick walls; it is cut near,
but not through, its opening to the exterior. The same gill cleft
(_g_) that was seen in the preceding figure is seen here as a narrow,
transverse cleft, open at both ends. Between the notochord (_nt_) and
the spinal cord (_sc_) is the same, though now double, blood-filled
vessel (_bv_) that was seen in the preceding section. The other
blood-vessels are larger here than in the more anterior region. There
is a faint condensation of mesoblast in the neighborhood of the
notochord, and a more marked condensation (_mp_) farther toward each
side is the curiously shaped muscle plate.
Figure 17_c_ is through the heart region, and that organ is cut through
the opening from the lower or ventricular into the upper or auricular
chamber. The thickening of the wall of the ventricle, which was noticed
in the preceding stage, has increased to such an extent that there
is now a marked difference in the thickness of the ventricular and
auricular walls. As in the preceding stage, the body wall is torn,
probably in handling, so that it appears to be incomplete around the
ventral side of the heart. Dorsal to the heart two small circular
holes (_ent_) with thick walls are the œsophagus and trachea, cut
anterior to the point of bifurcation of the latter into the bronchial
or lung rudiments. On either side of these structures is an elongated
blood-vessel (_ac_), the anterior cardinal vein, its elongation being
due to the fact that it is cut at the place where it turns downward to
empty into the heart. Dorsal to the œsophagus are the aortæ (_ao_),
which are here cut just at the point where the two vessels unite to
form one; the next section, posterior to the one under discussion,
shows an unpaired aorta. The notochord (_nt_) and spinal cord (_sc_)
need no description, except to note that the latter shows active
histological differentiation, numerous mitotic figures being seen under
higher magnification, especially in the cells that line the spinal
canal. On the right of the cord the edge of a spinal ganglion (_sg_) is
seen, in connection with which in other sections are seen the clearly
defined nerve roots. The condensation of mesoblast around the notochord
is quite evident, and in close contact with this medial condensation
are two very characteristic, S-shaped muscle plates (_mp_), which
extend from the level of the dorsal side of the spinal cord to the
upper limits of the cardinal veins. In some sections the muscle plates
even yet show slight remains of the myocœl at the dorsal end.
Figure 17_d_ is in the region of the posterior end of the heart (_ht_),
which is cut through the tip of the ventricle, and the anterior end
of the liver (_li_), which has the appearance of a mass of darkly
stained cords or strands of cells surrounding a large blood-vessel
(_mv_). This blood-vessel may be called the _meatus venosus_, though
it is not separated by any line of demarcation from the auricle. A few
sections anterior to this region the _meatus venosus_ opens dorsally
into a large vessel on each side (_dc_), which at first glance seems a
part of the body cavity, but which is in reality the _ductus Cuvieri_,
formed by the union of the anterior and posterior cardinal veins. An
irregular, crescentic cleft (_bc_), lying medial and parallel to each
of the Cuvierian vessels, is the body cavity. In the upper angle of
this cavity is a granular mass, the glomerulus, that of the left side
being accompanied by the extreme anterior end of the Wolffian duct.
In the rounded mass of mesoblast, between the cleft-like regions of
the body cavity, the lung rudiments (_lu_), and the œsophagus (_oe_)
are seen as three small, circular openings; that of the œsophagus is
somewhat smaller than the other two. The notochord (_nt_), spinal cord
(_sc_), and muscle plates (_mp_) have almost the same appearance as in
the preceding section. A spinal ganglion (_sg_) is seen on each side of
the spinal cord; the one on the left shows a well-defined spinal nerve
(_sn_), which may be traced ventrally as far as the end of the muscle
plate, along whose medial side it courses. The ventral nerve root is
plainly seen; the dorsal root, in this section, less plainly. The
amnion (_a_) and abdominal wall are, as in the preceding figure, torn
in the region of the ventricle.
Figure 17_e_ is a short distance posterior to the figure just
described. The liver is cut through its middle region and forms a
large, darkly staining, reticular mass on the left side of the figure.
The digestive tract is seen at two places to the right of the liver;
the smaller and more ventral of these openings (_i_) may be called
the intestine, while the larger is evidently the stomach (_i′_). The
body wall is here unfused and becomes suddenly thinner as it passes
upward into the amnion (_a_). The Wolffian tubules (_wt_) form a very
conspicuous mass on either side of the mesentery, in close connection
with the posterior cardinal veins (_pc_). In the mesoblast between the
dorsal aorta (_ao_) and the notochord are two small, irregular, darkly
stained masses (_sy_). These are shown in the preceding two figures,
but were not mentioned in the description. They may be traced through
a great part of the length of the embryo back of the head region; at
intervals corresponding in length to the distance between the spinal
ganglia they are enlarged, while between these enlargements they are
very small in cross-section. At certain points a small blood-vessel is
given off by the dorsal aorta to the immediate neighborhood of each of
these small areas. Although they show no connection with the central
nervous system, these structures appear to be the rudiments of the
sympathetic nerves.
Figure 17_f_ is in the region just in front of the hind legs. The
abdominal walls are here unfused, and into the unenclosed body cavity
projects the intestine (_i_), supported by a narrow mesentery and
surrounded by a comparatively thick mass of mesoblast. The Wolffian
body and duct form a mass of considerable size on each side of the
mesentery. The Wolffian body is cut near its posterior end and consists
of smaller tubules than in the more anterior regions. The Wolffian
ducts (_wd_), on the other hand, are very large and are much more
clearly distinguishable from the Wolffian tubules than in the preceding
sections. The Wolffian ridges (_wr_) are very marked projections on the
sides of the body, and in a region farther caudad become especially
developed as the posterior appendages, to be described in connection
with the following section. Both spinal ganglia are shown in this
figure (_sg_), and in connection with the left ganglion the spinal
nerve (_sn_), extending ventrally as far as the level of the Wolffian
duct. The sympathetic nerve rudiments do not extend so far caudad as
the plane of this section. The dorsal end of each muscle plate (_mp_)
is seen, in this and other sections, to be slightly enlarged to form
a round knob; this knob contains a distinct cavity (not shown in the
figure), the myocœl.
In Figure 17_g_, owing to the curvature of the body, the plane of the
section passes through the body at three places: through the region of
the heart and lungs (Fig. 17_d_), through the region of the posterior
appendages, and through the tail. In fact, the plane of the section
represented by each of the preceding figures cut the embryo in more
than one region, but for the sake of simplicity only one region was
represented in each figure. In the figure under discussion only the leg
and tail regions have been drawn, though the latter region (_t_), being
cut through one of its curves, is seen as an elongated body with a
section of the spinal cord, notochord, etc., at each end. Both regions
shown in the figure are enclosed in the same fold (_a_) of the amnion.
Of the structures in the dorsal side of the larger or more anterior
part of this figure nothing need be said. The most striking feature
of the section is the presence of the large posterior leg rudiments
(_pa_). As was noted in the preceding figure, they are, as usual,
merely local enlargements or projections of the mesoblast (covered, of
course, with ectoblast) of the Wolffian ridge. They are, as shown in
this section and in the surface view of this stage (Fig. 17), bluntly
pointed projections from the sides of the body. The anterior appendage
seems to be slightly more developed than the posterior, as was noted
in describing the surface view of the embryo. The digestive tract is
cut through its extreme posterior end, in the region that may be termed
the cloaca (_cl_), for into it at this point the Wolffian ducts open
(_wdo_). As the narrow cloacal chamber is followed toward the tail,
it becomes still smaller in diameter, and the ventral depression or
cleft seen in this figure gradually becomes deeper until its walls
are continuous with the ectoderm that covers the ventral projection
of mesoderm between the hind legs; no actual opening to the exterior
is present, however. There is a space of about twenty-five or thirty
sections (in a series of eight hundred) between the posterior ends of
the Wolffian bodies and the cloacal openings of the Wolffian ducts. The
body cavity (_bc_) and the posterior cardinal veins (_pc_) are very
small in this region, as might be expected.
STAGE XV
FIGURE 18 (PLATE XXIV.)
Only the head of this embryo is represented, as the general state of
development is about the same as in the preceding stage.
The chief object in making the figure is to show the five gill clefts
(_g¹⁻⁵_). The fifth cleft, though small and probably not open to the
exterior, is quite distinct in this embryo. The writer would feel more
doubt of its being a true, though rudimentary, gill cleft had not
Clarke (17) found a fifth pair of clefts in his material. The nasal pit
has advanced in development somewhat and shows the beginning of the
groove that connects it with the mouth. In this figure the crescentic
hyomandibular cleft shows its connection with the groove between the
mandibular and the hyoid folds.
STAGE XVI
FIGURE 19 (PLATE XXIV.)
This embryo is only slightly more developed than the preceding. Body
flexure is so great that the forebrain and tail nearly touch. Only
the anterior three gill clefts are visible. The maxillary process
(_mx_) is longer and more narrow; the mandibular fold has not changed
appreciably. The nasal pit (_n_) is now connected by a distinct
groove with the stomodæum. The appendages have increased in size, the
posterior (_pa_) being the longer. The anterior appendage (_aa_) is
distinctly broadened to form the manus, while no sign of the pes is to
be seen at the extremity of the posterior appendage. The heart (_ht_)
is still very prominent. The stalk of the umbilicus (_u_), which is
quite narrow, projects from the ventral wall in the region between the
heart and the hind legs. The tail is of considerable length and is
closely coiled.
STAGE XVII
FIGURES 20-20_j_ (PLATES XXV., XXVI.)
The superficial changes noted in this stage chiefly concern the head,
which has increased considerably in length (Fig. 20). The curvature
of the body is slightly more marked, and the tail is more tightly
coiled at the end. There are still signs of three gill clefts. The
maxillary process (_mx_) is long and narrow, while the mandibular arch
(_md_) is still short and broad. The fronto-nasal region has greatly
increased and has the aquiline profile noted by Clarke. The nasal
groove has disappeared, and there remains the small opening (_n_) at
the side of the fronto-nasal region, near the end of the still separate
maxillary process. The umbilicus is in about the same condition as in
the preceding stage, but the heart is less prominent. The outline of
the manus (_ma_) is more definite, and the extremity of the posterior
appendage is distinctly flattened out to form the rudimentary pes
(_pe_). The position of the elbow-joint in the anterior appendage is
seen at the end of the reference line _aa_.
Typical transverse sections of this stage are shown in Figures
20_a_-_j_.
Figure 20_a_ is a section through the middle region of the head,
cutting the hindbrain on one side and the forebrain on the other. The
walls of the brain show rather more histological differentiation than
was seen in the preceding sections, though this cannot be shown under
the low magnification used. The hindbrain (_hb_), which is cut near its
anterior border, exhibits the usual membranous dorsal and thick ventral
walls. The forebrain is here seen as three distinct cavities--a median
third ventricle (_tv_), with a thick ventral wall, and a thin dorsal
wall extended to form a large paraphysis (_epi_), and two lateral
ventricles (_ch_), the cavities of the cerebral hemispheres, whose
walls are quite thick except on the side next the third ventricle.
The sections of this series being slightly oblique, the eye is here
cut on the right side only, where it is seen as a large, semicircular
cavity (_e_) with thick, dense walls. The mesoblast, in which several
blood-vessels (_bv_) are seen, exhibits three distinct areas--a median,
lighter zone, with a more dense area on either side. The significance
of this variation in the density of the mesoblast is not apparent.
Figure 20_b_ is only a few sections posterior to the section just
described. It is drawn chiefly to show the appearance of the forebrain,
the other structures being about as in the preceding figure, except
that both eyes (_e_) are here represented. The section passes through
the wide opening between the third (_tv_) and the lateral ventricles
(_ch_) and cuts the anterior edge of the pineal body[8] (_epi_). The
paraphysis is very large and is directed backward instead of forward,
as is usually the case among the lower vertebrates (if the alligator
may be so classed). It is shown in Figure 17_a_ of a preceding stage
and will be again shown in a sagittal section to be described later.
The same areas of more dense and less dense mesoblast noted in the
preceding figure are seen here.
[8] Subsequent investigation showed that the structure here described
as the pineal body is, in reality, the paraphysis; the pineal body is
absent in _A. mississippiensis_.
Figure 20_c_, though still in the head region, shows several features
that were not seen in the preceding figures. On the left of the
hindbrain (_hb_) the auditory vesicle (_o_), which is now considerably
more advanced than in earlier figures, is seen as a larger,
flask-shaped cavity and a smaller, round one. Between the larger cavity
and the hindbrain is the root of a cranial nerve (_cn_), apparently the
eighth, since in another section it comes in close contact with the
wall of the larger part of the auditory vesicle just mentioned. On the
right side, ventral to the hindbrain, another and much larger nerve
(_cn_) is seen. Nearly in the center of the figure is seen a small,
irregular, thick-walled cavity (_p_); this is the pituitary body,
and its connection with the roof of the pharynx may easily be made
out in another section. The mesoblast in this region of the sections
contains numerous large and small blood-vessels and exhibits certain
denser areas which probably represent the beginnings of the cranial
cartilages. No sign of the forebrain is seen (the plane of the section
passing in front of that region), except that the tip of the wall of
one of the cerebral hemispheres (_ch_) is cut. The left nasal chamber
(_n_) is shown: it will be noted again in the following section. The
eye on the right side shows no remarkable features; its lens (_ln_) is
large and lies well back of the lips of the optic cup, which may now be
called the iris (_ir_). A thin layer of mesoblast has pushed in between
the lens and the superficial ectoderm to form the cornea, and the outer
wall of the optic cup is now distinctly pigmented. The inner wall of
the optic cup is beginning to differentiate into the retinal elements.
The eye on the left side is cut farther from its central region and
has a very different appearance from the eye just described. This
unusual appearance is due to the fact that the section passed through
the choroid fissure, which is very large and seems to be formed by the
pushing in of the walls of the cup and not by a mere cleft in these
walls. This fissure is hardly noticeable in the stage preceding the
present, and in a stage slightly older it has disappeared; so that it
would seem to be a very transient structure. It apparently is formed at
about the time that the optic stalk, as such, disappears. It is in the
region of the choroid fissure, if not through it, that the optic nerve
(_on_) enters the eye. Through the fissure also enters a vascular tuft
of mesoblast (_pt_) which may be seen projecting into the optic cup
after the disappearance of the fissure. This loop of blood-vessels is
doubtless the pecten.
Figure 20_d_ represents a section through the hindbrain (_hb_), pharynx
(_ph_), and tip of the snout. On either side of the hindbrain are
a convoluted auditory vesicle (_o_), and several blood-vessels and
nerves, while ventral to it is seen the anterior end of the notochord
(_nt_), around which the mesoblast is somewhat more dense than
elsewhere. The pharynx (_ph_) sends out toward the surface a narrow
gill cleft (_g′_) in the neighborhood of each auditory vesicle. These
clefts connect with the exterior by very narrow slits, not seen in
the plane of this section. The opposite end of the pharynx, as seen
in this figure, opens on the left (_pn_) into the nasal chamber. The
nasal cavity on the right is cut in such a plane that it shows neither
its external nor its pharyngeal opening. The nasal passages are here
fairly long and nearly straight chambers; their lining epithelium is
quite thick in the middle region, but becomes thinner where it merges
into the epithelium of the pharynx at one end, and into the superficial
epithelium at the other end. The unusual appearance of the eye (_e_),
on the right side of the figure, is due to the fact that the plane of
the section cut tangentially through the extreme edge of the eye in the
region of the choroid fissure.
Figure 20_e_ is only a short distance posterior to the preceding. On
the left side the pharynx (_ph_) is connected with the exterior through
the stomodæum, and on the right the hyomandibular cleft (_g′_) is cut
almost through its opening to the exterior. The auditory vesicle (_o_)
on the right is cut near its middle region, while that on the left
is barely touched by the plane of the section. The notochord (_nt_),
with its condensed area of mesoblast, is somewhat larger than in the
preceding section. The nasal canal on the right (_n_) is cut through
neither anterior nor posterior opening, while on the left side the
canal shows the anterior opening (_an_).
Figure 20_f_, which is in the region of the posterior part of the
pharynx and the anterior part of the heart, shows some rather unusual
conditions.
The spinal cord (_sc_) and notochord (_nt_), with the faintly
outlined condensations of mesoblast in their region, need no special
description. The pharynx (_ph_) is here reduced to an irregular,
transversely elongated cavity, the lateral angles of which are
connected on each side with the exterior through a tortuous and almost
closed gill cleft (_g_), which must be followed through many sections
before its inner and outer openings may be determined. Dorsal to the
pharynx numerous blood-vessels (_bv_), both large and small, may be
seen, while ventral to it is noticed a faint condensation of mesoblast
(_la_), in the form of an inverted T, the anlage of the laryngeal
structures. The ventral portion of the figure is made up of a nearly
circular thin-walled cavity, the pericardium (_pr_). Most of the
pericardial cavity is occupied in this section by the thick-walled
ventricle (_vn_), above which is the bulbus (_b_) and the tip of the
auricle (_au_). The bulbus is nearly circular in outline, though its
cavity is very irregular. A few sections anterior to this, the opening
of the bulbus into the ventricle is seen.
In Figure 20_g_ the section represented is only a short distance
posterior to the one represented by Figure 20_f_. The mesoblastic
structures in the neighborhood of the spinal cord (_sc_) and notochord
(_nt_) will be described in connection with the next figure, where they
are more clearly defined. The œsophagus (_oe_)--or posterior end of the
pharynx, whichever it may be called--is here a crescentic slit, with
its convex side upward; ventrally it opens by a narrow glottis into the
trachea (_ta_). The trachea is surrounded by the same condensed area of
mesoblast (_la_) that was mentioned in connection with the preceding
figure, but the condensation is here more marked. From the bulbus
(_b_) an aortic arch (_ar_) extends upward for a short distance on the
right side, while to the left of the œsophagus an aortic arch (_ar_)
is cut through the upper part of its course. Ventral to the bulbus the
ventricle (_vn_) and two auricles (_au_) are seen surrounded by the
pericardial wall.
Figure 20_h_ is in the region of the liver (_li_), which has about the
same position in relation to the auricles (_au_) that was occupied by
the ventricle in the last figure. The auricles are connected with each
other by a wide passage. The trachea (_ta_) and the œsophagus (_oe_)
are entirely distinct from each other; the former is a small, nearly
circular hole, while the lumen of the latter is obliterated and its
walls form a solid, bow-shaped mass of cells. Since there is a narrow
space between this mass of cells and the surrounding mesoblast, it
might be thought that the lumen of the œsophagus had been closed by
the simple shrinkage of its walls; higher magnification, however,
fails to show any sign of a collapsed lumen. It is doubtless the
problematic and temporary closure of the œsophagus that is noticed in
other forms. On each side of the œsophagus, in close relation with the
anterior cardinal vein (_ac_), is noticed a nerve (_cn_) cut through a
ganglionic enlargement. When traced forward these nerves are seen to
arise from the region of the medulla, and when followed caudad they
are found to be distributed chiefly to the tissues surrounding the
newly formed bronchi; they are doubtless the tenth cranial nerves. On
the right side of the figure the close connection of this nerve with
the near-by gill cleft is seen. Above the paired aortæ (_ao_) the
sympathetic nerves (_sy_) will be noticed. The mesoblast surrounding
the spinal cord (_sc_) and notochord (_nt_) is distinctly condensed
(more so than the figure shows) to form what may be called the centrum
(_c_) and neural arch (_na_) of the vertebræ. The arch, owing to the
slight obliquity of the section, shows here only on one side. The
spinal cord is not yet completely enclosed by the neural arches. The
muscle plates (_mp_) are in close connection with the rudiments of the
vertebræ just mentioned. The spinal cord (_sc_) is here differentiated
into three areas--a dense, deeply stained area immediately around the
neurocœl; a less dense area of cells surrounding the inner area and
extending ventralward as a rounded projection on each side; and an
outer layer, with few or on nuclei, surrounding the inner two layers
except on the dorsal side.
In Figure 20_i_ the size and complexity of the figure are due, it will
be easily understood, to the fact that the plane of the section passed
through the curve of the body, thus practically cutting the embryo in
two regions--an anterior, where the lungs (_lu_) and liver (_li_) are
seen, and a posterior, where the Wolffian bodies (_wt_) are present.
The spinal cord and the surrounding structures have almost the same
characteristics at both ends of the figure, except that the primitive
spinal column is rather more distinct in the posterior end of the
section. The posterior cardinal veins (_pc_), Wolffian ducts (_wd_),
and Wolffian bodies (_wt_) are also prominent structures of this end
of the figure, the last being made up of a great number of tubules.
The extreme anterior ends of the Wolffian bodies are seen in the other
half of the section in the upper angles of the body cavity, dorsal
to the lung rudiments (_lu_). Filling most of the body cavity (_bc_)
and making up the greater part of the middle of the figure are the
liver (_li_), now a very large organ; the stomach (_i′_), also quite
large; the pancreas (_pan_), a small body lying near the stomach; and
the lungs (_lu_), which here consist of several thick-walled tubes,
surrounded by lobes of mesoblast. The other features of the figure need
no special mention.
Figure 20_j_ is through the base of the posterior appendages (_pa_),
in which the cartilages are already being outlined by condensations
of mesoblast. The intestine (_i_) is cut in two regions--at a more
anterior point, where it is seen as a small, circular hole surrounded
by mesoblast and hung by a narrow mesentery, and through the cloacal
region, the larger and more ventral cavity, into which the Wolffian
ducts (_wd_) open a short distance caudad to this section. The
blood-vessels present a rather curious appearance. A short distance
anterior to this point the aorta has divided into three, or it might
be said that it has given off two, large branches. These two branches,
one on either side near the posterior cardinal vein, pass toward the
ventral side of the embryo on each side of the cloaca and end at
about the region represented by the present figure. The small portion
of the aorta that remains after the giving off of the two branches
just described continues, as the caudal artery (_ca_), into the tail;
it is a small vessel just under the notochord, and gives off small,
paired branches at regular intervals toward the vertebral region. The
posterior cardinal veins (_pc_), posterior to the openings of the
Wolffian ducts into the cloaca, unite to form a large caudal vein lying
just ventral to the caudal artery.
STAGE XVIII
FIGURE 21 (PLATE XXVII.)
This embryo, as may be seen, for example, by the form of the
appendages, is slightly further developed than the one represented in
Figure 20. The figure is from a photograph of a living embryo as it
lay in the egg, a portion of the shell and shell membranes having been
removed. The embryo, which lies on its left side, is rather faintly
outlined because of the overlying allantois. The allantois has been
increasing rapidly in size, and is here so large that it extends
beneath the cut edges of the shell at all points except in the region
in front of the head of the embryo, where its border may be seen. Its
blood-vessels, especially the one that crosses the head just back of
the eye, are clearly shown in the figure, and in the living specimen,
when filled with the bright red blood, they form a most beautiful
demonstration. As in the chick, the allantois lies close beneath the
shell membranes and is easily torn in removing them.
STAGE XIX
FIGURE 22 (PLATE XXVII.)
Figure 22 is a photograph of a somewhat older embryo, removed from the
egg and freed of the fetal membranes. The appendages show the position
of both elbow and knee joints, and in the paddle-shaped manus and pes
the digits may be faintly seen. The tail is very long and is spirally
coiled, the outer spiral being in contact with the frontal region of
the head. The jaws are completely formed, the upper projecting far
beyond the lower. The elliptical outline of the eyes is noticeable, but
the lids are still too little developed to be seen in this figure. The
surface of the embryo is still smooth and white.
STAGE XX
FIGURES 23-23_b_ (PLATE XXVII.)
In this surface view (Fig. 23) several changes are seen, though no
very great advance in development has taken place. The outlines of
the digits (five in the manus and four in the pes) are now well de
fined; they even project slightly beyond the general outline of the
paddle-shaped part. The tail has begun to straighten out, and it now
extends across the front of the face. The lower jaw has increased in
length, but is still shorter than the upper. The eyelids, especially
the upper, are beginning to be discernible in surface view. Though
still without pigment, the surface of the body is beginning to show by
faint transverse lines the development of scales; these lines are most
evident in this figure in the middle region of the tail, just before it
crosses the nose.