274 The Alligator and Its Allies 



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 (i'') a small crescentic thicken- 

 ing 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 re- 

 gions there is not very much difference in size. 



Figure 12a passes through the region of the fore- 

 brain. This end of the embryo lies on its side, as 

 was noted above and as may be recognized from 

 the relative positions of the head and the overlying 



