The Development of the Alligator 321 
except that the tip of the wall of one of the cerebral 
hemispheres (ch) is cut. The left nasal chamber 
(n) is shown: it will be noted again in the following 
section. The eye on the right side shows no 
remarkable features; its lens (Jn) is large and lies 
well back of the lips of the optic cup, which may 
now be called the iris (ir). A thin layer of meso- 
blast has pushed in between the lens and the 
superficial ectoderm to form the cornea, and the 
outer wall of the optic cup is now distinctly pig- 
mented. The inner wall of the optic cup is be- 
ginning to differentiate into the retinal elements. 
The eye on the left side is cut farther from its 
central region and has a very different appearance 
from the eye just described. This unusual appear- 
ance is due to the fact that the section passed 
through the choroid fissure, which is very large and 
seems to be formed by the pushing in of the walls 
of the cup and not by a mere cleft in these walls. 
This fissure is hardly noticeable in the stage pre- 
ceding the present, andin a stage slightly older it has 
disappeared; so that it would seem to bea very tran- 
sient structure. It apparently is formed at about 
the time that the optic stalk, as such, disappears. 
Itisintheregion of the choroid fissure, if not through 
it, that the opticnerve (om) enterstheeye. Through 
the fissure also enters a vascular tuft of mesoblast 
(pt) which may be seen projecting into the optic 
cup after the disappearance of the fissure. This 
loop of blood-vessels is doubtless the pecten. 
2r 
