The Development of the Alligator 309 
ence of short fibers may be made out among the 
cells. The cerebral hemispheres (ch) are well- 
marked structures, their asymmetry being of course 
due to the obliquity of the section. Only one 
eye is cut by the plane of the section, and this one 
shows no connection with the forebrain. The 
outer wall of the optic cup (oc) is so thin that 
under this magnification it can scarcely be seen 
as a dark line surrounding the retinal wall. The 
lens (In) is now a solid mass, of the usual type 
for vertebrate embryos, its front or outer wall 
being a scarcely discernible line. The hindbrain 
(hb) has the usual form for that region and does not 
differ particularly from what was noted in earlier 
stages except in the histological differentiation 
that has already been mentioned. As with the eye, 
it is only on the right side that the auditory vesicle 
(0) is shown. It shows some differentiation, but 
not so much as would be seen were it cut in another 
region. In the center of the section the pharynx 
(ph) forms an irregular cavity connected with the 
exterior on the left by a gill cleft (g) and by an- 
other slit which is simply the anterior margin of the 
stomodeum. On the right neither of these open- 
ings is in the plane of the figure, though the gill 
cleft (hyomandibular), which lies close to the 
auditory vesicle, is almost an open passage. A 
few small blood-vessels are scattered through the 
section; one of these (bv), lying between the noto- 
chord (nf) and the floor of the brain, is noticeable 
