The alligator and its allies

by A. M. Reese

“This is so unexpected a method of formation for the anterior part of the medullary folds that I have made use of both [Figures 4 and 5]. They were made from very perfect specimens, and the sections of both of them, and of the specimen from which [Figure 6] was drawn, proves that the structure is what it is indicated to be in surface appearance. That is, the transverse sections posterior to the V, in the embryos shown in [Figures 4 and 5], show the medullary groove and the medullary folds; the several sections passing through the apex of the V show neither groove nor folds, but only a median thickening; and in front of the point or apex of the V the successive sections discover a gradually widening groove between the arms, which is also much deeper than the shallow groove found posterior to the V. While I have not seen, and from the nature of the conditions one cannot see, the change actually proceeding from the form of [Figure 5] to that of [Figure 6], still the explanation given appears to be the only one possible” ([17]).

A somewhat extended series of transverse sections of an embryo of about this age is represented in [figures 6a-i].

[Figure 6a] is a section through the head-fold; it passes through the extreme anterior end of the secondary folds (sf) that were described, in surface view, above ([Figs. 5] and [6]). The section was not quite at right angles to the long axis of the embryo, so that the fold on the right was cut farther toward its anterior end than was the fold on the left. The pushing under of the head causes a forward projection of the secondary folds, so that the fold to the right appears as a rounded mass of cells with a small cavity near its center. On the left the plane of the section passes through the posterior limit of the head-fold, and shows the cells of the secondary fold continuous with the dorsal side of the ectoderm (ec). As pointed out above by Clarke, the secondary folds are here some distance apart, and gradually approach each other as we proceed toward the posterior. The entoderm (en) is here flat and takes no part in the secondary folds.

In [Figure 6b], a short distance back of the one just described, the secondary folds (sf) are much larger and are closer together. On the right the section passes through the extreme limit of the head-fold, so that the secondary fold of that side is still a closed circle, with a few scattered cells enclosed. On the left the section is posterior to the head-fold; on this side the secondary fold is seen as a high arch of ectoderm, with a thick mass of entoderm beneath it.

[Figure 6c] represents a section which passes back of the head-fold on both sides. The secondary folds (sf) are seen as a pair of ectodermal arches continuous with each other in the middle line of the embryo. The ectoderm of the folds is much thickened and gradually becomes thinner distally. On the right the entoderm shows the same thickening (en) that was shown on the left side of the preceding figure. This thickened appearance of the entoderm is due to the fact that the section passes through the anterior limit of a tall fold of that layer, which underlies the similar fold of the ectoderm that has already been described. This secondary fold of the entoderm is seen on the left side of the section. It may be traced through several sections, but soon flattens out posteriorly.

[Figure 6d] is a short distance posterior to the preceding. The secondary folds are here much less pronouncedly arched and the deep groove between them is reduced to a line (l). The entoderm (en) is no longer markedly arched and is closely adherent, along the median plane, to the ectoderm, where there is seen the thickening (th) that has been mentioned by Clarke (see [above]). Springing from the entoderm on each side of this thickening is a small mass of mesoderm (mes).

The section immediately posterior to the one just described is represented in [Figure 6e]. The line (l) which separated the two secondary folds in the preceding section is no longer present, so that the ectoderm (ec) is continuous from side to side, with only a shallow depression (mg), which may be considered as the extreme anterior end of the medullary groove. The median thickening (th) is cut near its posterior limit and still shows a close fusion of the germ layers. There is no line of demarcation between the gradually flattening secondary folds of the anterior end of the embryo and the just forming medullary folds of the posterior end, so that it is impossible to say whether the thickening of ectoblast in this figure should be called secondary folds or medullary folds. As a matter of fact, the secondary folds become, of course, the anterior ends of the medullary folds. The mesoblast (mes) is here of considerable extent, and its entodermal origin is beyond doubt, though not well shown in the figure.

[Figure 6f] is about one sixth of the length of the embryo posterior to the preceding. The medullary thickening of the ectoderm (ec) is still marked and the shallow medullary groove (mg) is fairly distinct. The entoderm (en) is medially continuous with both mesoderm (mes) and notochord (nt), though these two tissues are otherwise distinct from each other.

[Figure 6g] is nearly one third the length of the embryo posterior to the preceding and passes through the posterior third of the embryo. The medullary thickening of the ectoderm (ec) is marked, but shows no sign of a medullary groove; in fact, the median line is the most elevated region of the ectoderm. The notochord (nt) is larger in cross-section than in the more anterior regions. It is still continuous with the entoderm (en) and is fairly closely attached to, though apparently not continuous with, the mesoderm (mes) on each side.

[Figure 6h] passes through the blastopore (blp). The appearance of the section is almost identical with that of [Figure 2f], already described.