For a considerable distance posterior to the end of the medullary canal we find the structure similar to that shown in Figure 11_k_, which is about the twentieth section posterior to Figure 11_j_. The break (_ec′_) in the ectoderm is here seen as a compact group of cells which at first glance seem to be continuous with a rounded mass of cells below (_ps_). Examination under greater magnification, however, shows that the two groups of cells are distinct. As the sections are followed back of this region, the upper mass of cells (_ec′_) gradually disappears, and after its disappearance the lower mass (_ps_), which is already continuous with the mesoderm (_mes_) on either side, becomes continuous with the under side of the ectoderm. The mass of cells (_ps_) is apparently the primitive streak, though it is distinct from the ectoderm for a considerable distance posterior to the neurenteric canal. Just what may be the meaning of the thickened ridge of ectoderm (_ec_) it is difficult to determine. STAGE IX FIGURES 12-12_g_ (PLATES XVIII., XIX.) The entire length of the embryo proper is 6.5 mm. from the extreme posterior end to the region of the midbrain (_v²_), which now, on account of the cranial flexure, forms the most anterior part of the body. Besides being slightly longer than the preceding stage, the embryo has increased in thickness, especially in the anterior region, where the enlargement of the cerebral cavity is considerable. Body torsion has begun (Fig. 12), so that the anterior third of the embryo now lies on its right side, while the rest of the body is still dorsal side up. The direction of body torsion does not seem to be as definite as it is in the chick, some alligator embryos turning to the right side, others to the left. Clarke has illustrated this fact in his alligator figures. He says (17) that embryos lie “more frequently on the left, but often on the right side.” The head is distinctly retort-shaped, and at the side of the forebrain (_v′_) a small crescentic thickening is the optic vesicle (_e_). The auditory vesicle, though of considerable size, does not show in this surface view. The head-fold (_h_) extends for about one third the length of the entire embryo, though its exact limit is difficult to determine in surface view. There is no sign of a tail-fold. About seventeen pairs of somites are present. The amnion extends over the anterior two thirds of the embryo. The above-mentioned increase in the diameter of this embryo over that of the preceding is evident when the first two transverse sections of this stage are compared with the corresponding sections of the earlier stage; in the middle and posterior regions there is not very much difference in size.