The alligator and its allies

by A. M. Reese

[Figure 9i] is through a region nearly one hundred sections posterior to the preceding, and cuts the embryo, therefore, through the posterior one fourth of its length. The chief difference between this and the preceding section is in the medullary canal, which is here open and is in the form of a wide groove with an irregular, rounded bottom and vertical sides. The size of the section is considerably greater than in the preceding, the increase being especially noticeable in the notochord (nt), which is cut near its posterior end. There is little or no sign of mesoblastic cleavage.

[Figure 9j] is about twenty sections posterior to [Figure 9i]. The medullary groove (mg) is considerably larger than in the more anterior regions, and its folds are somewhat inclined toward each other, though still wide apart. The notochord and entoderm are fused to form a large, compact mass of tissue close under the ventral wall of the medullary groove. On the ventral side of this mass of cells a groove (blp) marks the anterior and ventral opening of the blastopore shown in the next figure. The mesoblast shows no sign of cleavage.

[Figure 9k] shows the medullary groove (mg) in about the same position as in the preceding section. The blastopore (blp) is here seen as a small cavity in the center of the large mass of cells that was noted in the last figure. The entoderm (en) is continuous from side to side, but is not so sharply differentiated from the other germ layers as is represented in the figure.

[Figure 9l] is four sections back of the preceding; the wide, dorsal opening (blp) of the blastopore or neurenteric canal into the medullary groove (mg) is shown. The blastopore or neurenteric canal, then, is still at this stage a passage that leads entirely through the embryo, the medullary canal being in this region unenclosed above. Ventrally it is seen as a narrow opening through the entoderm; it then passes upward and backward, behind the end of the notochord, as a small but very distinct canal, which may be traced through about ten sections. The enclosed portion of the canal lies, as has been stated ([Figure 9k], blp), in the center of the mass of cells that is fused with or is a part of the floor of the medullary groove.

The above-described neurenteric canal is essentially like that described by Balfour in the Lacertilia. He does not say, however, and it is not possible to tell from his figures, whether there is a long, gradually diminishing groove posterior to the dorsal opening of the canal, as in the alligator. He says that the medullary folds fuse posteriorly until the medullary canal is enclosed over the opening of the neurenteric canal; also that “the neurenteric canal persists but a very short time after the complete closure of the medullary canal.”

In [Figure 9m], for about thirty sections (one tenth the entire length of the embryo), behind the section represented in the last figure, there is a very gradual change in the embryo, converting the deep groove, mg in [Figure 9l], into the shallow slit, pg, [Figure 9m].

There is no line of demarcation between the typical medullary groove region of [Figure 9l] and the equally typical primitive groove region represented in [Figure 9m]. As was noted in the preceding stage, the medullary folds are quite continuous with the folds of the primitive streak, and the medullary groove with the primitive groove; so that, unless we take the dorsal opening of the neurenteric canal as the point of separation, there is no line of division between these structures. The entoderm (en) and the lateral regions of the ectoderm (ec) and mesoderm (mes) in [Figure 9m] are about as they were in [Figure 9l], but in the middle line is seen a compact mass of cells forming the primitive streak (ps), with the shallow primitive groove (pg) on the dorsal side. The cells on each side of the primitive groove and for a short distance below it are compact, as is shown in the figure, but as we pass ventrally and laterally they become looser and more angular to form the lateral sheets of mesoblast (mes), very much as is the case in the chick and other forms. For a few sections posterior to the one shown in [Figure 9m] the primitive streak may be seen, then it disappears, and only the ectoderm and entoderm remain as thin sheets of tissue above the yolk.

STAGE VII

Figures 10 AND 10a ([Plates XV.], [XVI.])

Although of practically the same size as the preceding, this stage has advanced sufficiently in development to warrant a description.