2 8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 51 



sections posterior to the point at which the neurenteric canal empties 

 into it. The mesoblast (vies) is so closely attached to the lower 

 wall of the neurenteric canal that it seems to be actually continuous 

 with it. 



For a considerable distance posterior to the end of the medullary 

 canal we find the structure similar to that shown in figure 11k, 

 which is about the twentieth section posterior to figure 117. The 

 break (cc') 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, how- 

 ever, shows that the two groups of cells are distinct. As the sec- 

 tions 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 i2-i2g (Plates XIII, XIV) 



The entire length of the embryo proper is 6.5 mm. from the ex- 

 treme posterior end to the region of the midbrain (v 2 ), 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 (5) 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 fore- 

 brain (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 sisrn of a tail-fold. 



