The alligator and its allies

by A. M. Reese

In this stage the curvature of the body and tail is less marked than was seen in the last surface view. The body has increased greatly in size, so that the size of the head is relatively not so great. The size of the eye in relation to that of the head is much diminished also. The five anterior and four posterior digits are well formed, and their claws are of considerable size, though of course not present on all the digits. The outlines of scales may be traced from the tip of the tail to the skull; they are especially prominent along the dorsal profile. The skin is just beginning to show traces of pigment, which is, however, not shown in the photograph. The umbilical stalk is seen projecting with a loop of the intestine from the abdominal wall; this is shown more clearly in the next stage. The embryo now begins to exhibit some of the external characteristics of the adult alligator.

STAGE XXII

Figure 25 ([Plate XXVIII.])

This embryo needs no particular description. It has reached in its external appearance practically the adult condition, although there is still considerable yolk (not shown in the figure) to be absorbed, and the embryo would not have hatched for many days. Pigmentation, begun in the last stage, is now complete. The umbilical stalk is clearly seen projecting from a large opening in the body wall. The long loop of the intestine that extends down into the yolk sac is here evident, and it is hard to understand how it can all be drawn up into the body cavity when the umbilical stalk is withdrawn. No sharp shell-tooth at the tip of the snout, such as is described by Voeltzkow ([78]) in the crocodile, is here seen.

STAGE XXIII

Figure 26 ([Plate XXVIII.])

This figure shows the relative sizes of the just-hatched alligator and the egg from which it came. It also shows the position of the young alligator in the egg, half of the shell having been removed for that purpose. The blotchy appearance of the unopened egg is due chiefly to stains produced by the decayed vegetation of the nest. At hatching the young alligator is about 20 cm. long, nearly three times the length of the egg; but the tail is so compressed that, though it makes up about half of the length of the animal, it takes up very little room in the egg.

SUMMARY

Owing to the fact that the embryo may undergo considerable development before the egg is laid, and also to the unusual difficulty of removing the very young embryos, the earlier stages of development are very difficult to obtain.

The mesoderm seems to be derived chiefly by proliferation from the entoderm, in which way all of that anterior to the blastopore arises. Posterior to the blastopore the mesoderm is proliferated from the lower side of the ectoderm in the usual way. No distinction can be made between the mesoderm derived from the ectoderm and that derived from the entoderm.