214 HENRY LAURENS AND S. R. DETWILER 
The guanin is light grayish-brown in color (stained sections), 
finely granular and fairly uniformly distributed through the 
cell (fig. 2). In the tapetum (level B, fig. 1), the guanin-con- 
taining protoplasm, although covering over the outer segments 
of the visual cells, does not show the finger-like projections which 
so typify the melanin-containing portions of the epithelium 
(figs. 2, 3, and 4). 
In the lower portion of the retina the epithelial layer is entirely 
devoid of guanin. Here the melanin, in the form of delicate 
brownish-black needles, occupies the entire cell body and finger- 
like processes of the cells which project over and embrace the 
outer segments of the visual cells. The nuclei of the epithelial 
layer are spherical and occupy the choroidal portion of the cell 
body. Light and darkness have no effect on their shape and 
position. 
Visual cells. The retina of Alligator mississippiensis contains 
both rods and cones, differing in this respect from the retinae 
of turtles and lizards (Detwiler, '16). The two kinds of visual 
cells in the alligator are not uniformly distributed, the cone-rod 
ratio changing in different parts of the retina. Histological 
examination of the retina has yielded the significant fact that 
no portion is rod or cone free and that there is no gradual transi- 
tion from the conical elements into rod-like forms, as Garten 
('07) clamis. There are, however, areas which predominate in 
rods as well as areas which contain only a few rods. Viewing 
the retina as a whole, it can be said with justice that it is char- 
acteristically a rod-retina. 
Rods. The structure of the rod is uniform throughout the 
retina. It consists of an inner segment composed of a cylindrical 
myoid and an ellipsoid and a cylindrical outer segment (fig. 5). 
No rods with conical outer segments could be found. The 
rod nuclei are typicall}^ oval in shape and lie just beneath the 
external limiting membrane projecting above it for variable 
distances in both dark and light eyes, and thus form the outer 
part of the external nuclear layer. 
Cones. There are two kinds of cones, of which the predomi- 
nating type is a large thick visual element very similar to that 
