The Development of the Alligator 285 



are thin on the dorsal and ventral sides. The 

 dorsal wall is reduced to a mere membrane, which, 

 with the overlying ectoderm, has been pushed into 

 the brain cavity, as is generally the case with such 

 embryos. Close to the ventral wall of the hind- 

 brain the notochord (nf) is seen. The character of 

 the notochord has already begun to change; the 

 cells are becoming rounded and vacuolated, with 

 but few visible nuclei except around the periphery 

 of the notochord. Near the center of the section, 

 close to the ventral end of the forebrain, is the 

 pharynx (ph) , cut near its anterior limit ; it is here a 

 small, irregularly rectangular cavity with a com- 

 paratively thin wall. On the left side of the 

 pharynx the first gill cleft (g) is indicated as a 

 narrow diverticulum reaching toward the ectoderm. 

 A few sections posterior to this one the first gill 

 cleft is widely open to the exterior. As has been 

 said, in the surface view of this stage above de- 

 scribed none of the gill clefts showed ; so that in this 

 respect at least the sectioned embryo was more 

 nearly of the state of development of the embryo 

 represented in Figure 14, to be described later. 



Figure 136, about forty sections posterior to 

 Figure 130, passes through the hindbrain in the re- 

 gion of the ears. Being back of the region affected 

 by cranial flexure, this section is of course of much 

 less area than the preceding. The ectoderm shows 

 no unusual features; it is of uniform thickness 

 except where it becomes continuous with the 



