RETINA OF ALLIGATOR MISSISSIPPIENSIS 
217 
The cone nuclei are easily distinguished from the rod nuclei; 
the former being somewhat pear shaped, the latter more or less 
oval or elliptical. Furthermore, the cone nuclei occupy a deeper 
level than the rod nuclei and constitute a second row. 
EXPERIMENTAL 
Effects of light: Rods and copies. When sections of eyes of 
animals which have been exposed to diffuse light are compared 
with sections of eyes of animals kept in darkness, it is seen that 
there is an average difference in the lengths of the rod myoid of 
4m (table 1). The relative lengths of the rod myoids in the dark 
and light conditions are shown in figures 9 and 10, as well as 
TABLE 
2 
DISTANCE FROM ROD NUCLEUS TO ELLIPSOID 
(myoid) in fj. 
Region 
3 mm from optic 
nerve 
Tapetum (region B, 
fig 1) 
Light 
11.5 
7.5 
8.9 
Dark 
6.9 
Difference 
4.0 
2.0 
diagrammatically in figure 1 1 . The change in the length of the 
rods is found to be less extensive in the tapetal area (fig. 1, B), 
where rods predominate, than in the region close to the optic 
nerve, where the cones are considerably more numerous (table 2). 
The effect of light on the cones is not so easily demonstrable. 
The results, however, of several series of measurements (table 1) 
show that the cone myoids of light eyes are slightly shorter 
(2.1/z) than those of dark eyes (figs. 11, 12, 13). The contraction 
is found to occur in the double as well as in the single cones. 
The measurements of the double cones show that the myoid of 
the smaller member has shortened more than that of the larger 
(figs. 7 and 8) . Further evidence of changes in the length of the 
visual cells in light and darkness is afforded by a study of the 
relative positions of the cone and rod ellipsoids. In the dark 
