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 (ni) 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 135, about forty sections posterior to 
Figure 13a, 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 
