The Development of the Alligator 275 
yolk (y). The great size of this and the following 
figure is due partly to the increase in size men- 
tioned above and partly to the fact that the sections 
pass through the region of cranial flexure. The 
present figure (12a) represents the brain cavity as 
large and dumbbell-shaped, with comparatively 
thick walls of compactly arranged cells. The 
ventral end of this cavity (fb) is cut anterior to 
the region of the optic vesicles, while the dorsal 
end (mb) may perhaps be called the midbrain. In 
the sections that follow this one the two cavities 
are distinct from each other. The medullary 
canal, as was stated above, is now completely 
enclosed, except for the ventral opening of the 
neurenteric canal, to be presently noticed. Sur- 
rounding the brain is a considerable mass of meso- 
blast (mes). It is composed of the typical stellate 
cells. The ectoderm (ec) is made up of the same 
irregularly and loosely arranged cells that have 
been seen in earlier stages; it is of unequal thick- 
ness in different regions, the thicker parts being at 
the sides. The amnion (a) has the usual appear- 
ance, and in this region of course completely sur- 
rounds the embryo. 
Figure 12) is ten sections posterior to the 
section just described. The width of the embryo 
is greater in this region, but the dorso-ventral 
diameter is about the same as in the more anterior 
section. 
The overlying yolk and blastoderm are not 
