RETINA OF ALLIGATOR MISSISSIPPIENSIS 231 
this type of pupil, we find that many of them are essentially 
nocturnal in habits — the gecko, the alhgator, the cat — animals 
in which the rod functions are predominant over those of the 
cones. At night or in dim light the pupil of the cat is wide open 
and round. Furthermore, in the daytime, when the pupil is a 
vertical sht or oval, cats hunt and catch a great deal on the ground, 
for example, birds and squirrels, as well as chase the rapidly 
swirUng leaves. The cat is furthermore said to have very defec- 
tive daylight vision and to be colorblind (DeVoss and Ganson, 
'15). The alligator hunts along horizontal contours, and yet 
one finds that the shape of the pupil is a vertical slit. 
SUMMARY 
1. The eye of the alligator possesses a well-developed retinal 
tapetum in the dorsal and posterior portions of the retina to 
within 1.5 mm. of the entrance of the optic nerve. It is formed 
by the inclusion of guanin in the cells of the epithelial layer 
(figs. 1, 2, 3, 4). The pecten consists of a slightly raised pig- 
mented cap covering the entrance of the optic nerve. 
2. Typical rods and cones occur throughout the retina, the 
cone-rod ratio being different for different regions, but character- 
istic for particular regions (p. 216). 
3. The rods are all of one type (fig. 5), the cones of two, thick 
and thin, of which the first is by far the more numerous, occurring 
particularly in the posterior and ventral portions of the retina. 
Those of the second type are found only in the ventral portion 
and are not numerous (fig. 6). Double cones also occur (figs. 
7 and 8). None of the cones contain oil drops. The rod nuclei 
are oval or elliptical in shape, and the majority of them project 
through the external limiting membrane for a variable extent, 
the rest of them being just under it. The cone nuclei are pear 
shaped and, occupying a deeper level than the rod nuclei, con- 
stitute a second row (figs. 5, 6, 9, 10). 
4. The- rods show a change in length of their myoids averaging 
4/x, being longer in the hght and shorter in the dark (figs. 9, 10, 
11 and table 1). The single cones (thick and thin) show an 
average change of 2.1^t (figs. 11, 12, 13 and table 1). The 
