RETINA OF ALLIGATOR MISSISSIPPIENSIS 
213 
ANATOMICAL 
Epithelial pigment layer. A retinal tapetum occurs 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. The relative amount 
and distribution of the guanin and the ordinary melanin is shown 
in figure 1. In the region designated by B the epithelium is 
relatively devoid of melanin, which forms a narrow border of a 
Fig. 2 A portion of the tapetum designated by the letter B (fig. 1), showing 
broad zone of guanin, a narrow vitreal border of melanin, and a few scattered 
needles of pigment near the choroidal margin. X 665. 
Fig. 3 A portion of the epithelial layer designated by letter C (fig. 1), show- 
ing choroidal guanin-containing portion and vitreal melanin-containing portion. 
X 665. 
Fig. 4 A portion of the epithelial pigment layer corresponding to the region 
designated by letter D (fig. 1). X 665. 
few needles along the vitreal margin and occurs also as scattered 
needles here and there in the choroidal portion of the cells (fig. 2). 
As the optic nerve is approached, the amount of melanin grad- 
ually increases as the guanin decreases, until within about 1.5 
mm. above the entrance of the optic nerve guanin is no longer 
found. The gradual increase in the amount of melanin as the 
optic nerve is approached is seen in figures 2, 3, and 4, which 
show in detail the condition as found at the levels B, C, and D 
in figure 1. Above the level B in figure 1 the guanin again 
gradually decreases in amount and the melanin shows a corre- 
sponding increase (level A, fig. 1). 
THE JOUHNAL OP EXPERIMENTAL ZOOLOGY, VOL. 32, NO. 2 
