The alligator and its allies

by A. M. Reese

[Figure 7g] passes through the blastopore and shows essentially the same structure as was described in connection with the corresponding section of Stage I ([Fig. 2f]).

[Figure 7h] 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 8-8j ([Plates XII.], [XIII.], [XIV.])

On opening the egg this embryo ([Figs. 8 and 8a]) 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. 8a]) 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 8b] and [8c]. 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 8b] 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 8c] 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 8b]; 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 ([8b]). 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 8a] is represented in [Figures 8d]-[j].

[Figure 8d] 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.