In [Figure 3k] there is no sign of the medullary groove, though the ectoderm (ec) is still much thickened in the middle line. The section passes, posterior to the notochord, through the anterior edge of the ventral opening of the blastopore (blp). The mesoderm (mes) is here again continuous with the entoderm, around the edge of the blastopore, but is distinct from the ectoderm.
[Figure 3l] represents the third section posterior to the preceding. The blastopore, which passes upward and backward through the blastoderm, is seen as an enclosed slit (blp). It is surrounded by a distinct layer of compactly arranged cells continuous with the thickened ectoderm (ec) above, with the thin entoderm (en) below, and laterally with the gradually thinning and scattering mesoderm (mes).
[Figure 3m] is the next section posterior to the one just described. It passes through the dorsal opening of the blastopore (blp), which appears as a deep, narrow cleft with thick ectodermal borders. The three germ layers are still continuous with each other, though the connection of the entoderm with the other two is slight. The sections posterior to this one will be described in the next stage, where they have essentially the same structure and are better preserved.
[Figures 3n and 3o] are sagittal sections of an embryo of about the stage under discussion. In both figures the head-fold is seen as a deep loop of ectoderm and entoderm, while the head-fold of the amnion is seen at a.
The beginning of the foregut is seen in [Figure 3n] (fg), which is the more nearly median of the two sections, [Figure 3o] being a short distance to the side of the middle line.
In [Figure 3o] the thin entoderm (en) is separated from the much thicker ectoderm (ec) by a considerable layer of rather loose mesoderm (mes). In [Figure 3n], 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, 4a, 5, 5a AND 6-6i ([Plates X.], [XI.])
“[Figures 4 and 4a] are of an embryo removed, on June 18th, from an egg which had been taken out of an alligator two days before. [Figure 4], a dorsal view, is of special interest in that it shows a 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 4a], made from the same embryo as [Figure 4], grows most rapidly on the mid-line, and also becomes thicker at that place. The medullary folds now begin to form on either side of the medullary groove, 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. 5a]) represents the thickened process on the mid-line at its greatest development. For some reason the notochord did not show in this embryo, possibly owing to particles of the yolk material adhering about the mid-line.
“In an embryo a day or two older, the V-shaped fold of the head-fold is seen to have broken through at the apex, and each of the arms thus separated from one another unites with the medullary fold of its respective side. This can be seen in [Figure 6], which is a dorsal view of part of an embryo a day or two older than the one represented by [Figures 5 and 5a].