RETINA OF ALLIGATOR MISSISSIPPIENSIS 
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external limiting membrane and the nearest pigment needle, 
show that the pigment in the light eye is about l.Qfx nearer the 
external limiting membrane than in the dark eye. This almost 
insignificant amount, however, when combined with the distance 
the rod myoid has elongated in the light (4/x) gives an effective 
migration which is clearly illustrated in figures 9 and 10, as well 
as in figure 11. An examination of these figures shows that in 
the dark condition the choroidal portion only of the rod ellipsoid 
is covered with pigment, while in the light condition the entire 
Fig. 11 Diagrammatic drawing compiled from 530 measurements showing, 
the effects of light and darkness on the visual cells and on the position of the 
pigment. The left-hand side of the figure represents the dark condition; the 
right side, the light. The drawings were accurately laid out on coordinate 
paper from measurements presented in table A, each millimeter being given a 
value of 0.5;u. 
Fig. 12 Single cones. Animal in diffuse light for seven hours. X 935. 
Fig. 13 Single cones. Animal in darkness for twenty-four hours. X 935. 
ellipsoid and a portion of the myoid is ensheathed. The amount 
of cone contraction in the light, being only slightly more than 
the extent of the pigment migration (table 1), the relation 
between the position of the pigment and the cone ellipsoid is 
practically the same in both dark and light eyes, the choroidal 
portion of the cone ellipsoid being in both covered by pigment 
(figs. 9, 10, and 11). The description of the position of the 
pigment in relation to the ellipsoids of the visual cells in both 
dark and fight eyes pertains only to conditions found in the 
posterior part of the retina (about 2 to 3 mm. from the entrance 
of the optic nerve). Changes in the position of the pigment in. 
