The alligator and its allies

by A. M. Reese

The next stage to be described is shown in surface views in [Figures 3] and [3a]. Of this stage Clarke says:

“The head-fold rapidly increases in depth and prominence, as shown in [Figure 3], which is a ventral view a few hours later [than the preceding stage]. The time cannot be given exactly, as it is found that eggs of the same nest are not equally advanced when laid, and differ in their rate of development. The lighter curve in front of the head-fold is the beginning of the anterior fold of the amnion. The notochord has been rapidly forming, and now shows very distinctly on the ventral side, when viewed by transmitted light. A dorsal view of the same embryo ([Fig. 3]a) shows that the medullary or neural groove is appearing, and that it ends abruptly anteriorly near the large transverse head-fold. Posteriorly it terminates at the thickened area in front of the blastopore, which still remains open.”

[Figures 3b]-[m] are drawn from transsections of an embryo of about this stage of development. For a short distance in front of the beginning of the head-fold, there is a mass of cells of considerable thickness between the ectoderm and entoderm. In [Figure 3b] these cells appear as an irregular thickening of the entoderm, while in [Figure 3c] they form a continuous mass, uniting the upper and lower germ layers. This condition is seen, though in a much less striking degree, in the following stage of development. As to its significance the writer is not prepared to decide.

[Figure 3d] passes through the head-fold, which in this embryo was probably not so far developed as it was in the embryo shown in [Figures 3] and [3a]. Not having seen the embryo, however, before it was sectioned, the writer cannot be certain of this point. The ectoderm and entoderm are here of nearly the same thickness.

[Figure 3e] is a short distance posterior to the preceding. It shows a marked thickening of the ectoderm in the medial region (ec), which is continuous posteriorly with the anterior ends of the medullary folds that are just beginning to differentiate ([Figs. 3f]-[h]).

[Figure 3g] passes through the anterior end of the medullary plate or folds (mf), whichever they may be called. The ectoderm of the folds is thickened and is considerably elevated above the rest of the blastoderm. There is scarcely any sign, in this region, of a medullary groove. The entoderm (en) is considerably thickened in the medial region, this thickening being continuous posteriorly, as in the preceding stage, with the mesoderm.

In [Figure 3h], cut in a plane at some distance posterior to the preceding, the medullary groove (mg) is well marked; its bordering folds gradually thin out laterally to the thickness of the ordinary ectoderm. The medial thickening of the entoderm is very marked, but it has not in this region separated into a distinct mesoblastic layer.

Immediately under the medullary groove is a dense mass of cells (nt), apparently the anterior end of the notochord in process of formation.

[Figure 3i], still farther toward the blastopore, shows the medullary groove wider and shallower than in the more anterior sections. The mesoderm (mes) is here a layer laterally distinct from the entoderm. In the middle line it is still continuous with the entoderm, and at this place it is the more dense mass of cells that may be recognized as the notochord (nt). It is evidently difficult to decide whether this group of cells (nt), which will later become a distinct body, the notochord, is derived directly from the entoderm or from the mesoderm, which is itself a derivative of the entoderm. There is here absolutely no line of demarcation between the cells of the notochord and those of the mesoderm and entoderm.

In [Figure 3]j the ectoderm (ec) is nearly flat, scarcely a sign of the medullary groove appearing. The mesoderm (mes) is here a distinct layer, entirely separate from both notochord (nt) and entoderm (en). The notochord is a clearly defined mass of cells, distinct, as has been said, from the mesoderm, but still closely united with the underlying entoderm, which is much thinner than the ectoderm. This condition of the notochord, which is found throughout about one third of the length of the embryo, would give the impression that the notochord is of a distinctly entodermal origin.