The alligator and its allies

by A. M. Reese

For about one third of the length of the embryo posterior to [Figure 11f] there is a gradual flattening, in a dorso-ventral direction, with loss of the amnion, until the condition represented in [Figure 11g] 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 11h] is about ten sections posterior to [Figure 11g], 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 11i], 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 11j] 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 11k], which is about the twentieth section posterior to [Figure 11j]. 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 12-12g ([Plates XVIII.], [XIX.])

The entire length of the embryo proper is 6.5 mm. from the extreme posterior end to the region of the midbrain (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.