The alligator and its allies

by A. M. Reese

[Figure 8e] represents a section a short distance posterior to the one just described, and passes through the short region where the dorsal edges of the medullary folds have fused with each other. The ventral side of the medullary groove (mg) is, as in the preceding section, still unclosed. An epidermal layer of ectoblast (ep) is now distinct from the nervous layer (nl).

[Figure 8f] is through a region still farther toward the posterior end. Here the medullary groove is again open above, and is still open below. A well-marked space is seen between the epidermal (ep) and nervous (nl) layers of the ectoderm, but no mesoblast is yet to be seen.

[Figure 8g] passes through the middle part of the head-fold, and shows that the medullary folds in this region are fused below, but are widely 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 8d], while sections farther caudad pass through the short region where the folds are fused, and we have the appearance represented in [Figure 8e]. In [Figure 8f] is shown a section passing posterior to the short, fused region of the folds, and we again have the medullary canal open both above and below. [Figure 8g] represents a section through the tip of the bent-under region of the medullary folds, which are here fused below and open above.

[Figure 8h] 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 8i] 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 8j] 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 9a-9m ([Plates XIV.], [XV.])