The Development of the Alligator 277 
about the same appearance as in the preceding 
figure, though the former seems somewhat thinner. 
Figure 12c¢ is just back of the bent-under fore- 
brain represented in the preceding figure and in 
front of the main body of the heart. The plane of 
the section not being at right angles to the long 
axis of the body (as was mentioned above), the 
figure is not bilaterally symmetrical. The neural 
canal, since the section passes through the auditory 
vesicles, may here be called the hindbrain (b). It 
has an almond-shaped cavity, surrounded by a wall 
of medium thickness. In close contact with the 
wall of the hindbrain, on each side, is the inner 
side of the auditory vesicle (0), which is seen as a 
deep, wide-mouthed pit in the superficial ectoderm. 
On the right side of the section the auditory pit 
is cut through its middle region; it is simply a 
thickened and condensed area of the ectoderm 
which has been invaginated in the usual way. 
Directly beneath the hindbrain is the notochord 
(nt), on each side of which, in the mesoblast, is 
the dorsal aorta (ao), or rather the continuation of 
the aorta into the head. Beneath these structures 
and extending from one side of the section to the 
other is the pharynx (ph); its lining wall is fused 
on each side with the ectoderm, but there is no 
actual opening to the exterior. These points of 
contact (g) between entoderm and ectoderm are of 
course the gill clefts; they are not yet visible from 
the outside. The roof of the pharynx is flat and 
