RETINA OF ALLIGATOR MISSISSIPPIENSIS 223 
In the eye of the alHgator the migration of the pigment and the 
change in position of the visual cells seem to be correlated with 
the relative distribution of rods and cones. The rods show the 
greatest difference in position in light and dark eyes, in the regions 
designated by C in figure 1 (rod-cone ratio 60 to 40), and by D 
(where the rods and cones are about equal in number), much 
less in the region B (rod-cone ratio 85 to 15), and not demon- 
strably at all in the region designated by A, or in the region 
(E and F) below the optic nerve, where the rods represent only 
about 5 per cent of the total number of visual cells. The pig- 
ment can be demonstrated to move forward only in the posterior 
portion of the retina (regions C and D), thus corresponding to 
the regions where the maximum change in position of the rods 
takes place. The cones throughout the retina show the same 
(shght) degree of shortening in the Ught, except that the double 
cones, which are most numerous in the regions B and C, show a 
slightly greater amount of change in length. 
Garten ('07, p. 38) weakened the general application of the 
suggestion put forth by Herzog ('05) and Exner and Januschke 
('06) by observations which seemed to show an extremely high 
sensitivity to stimulation by weak hght, so that the light con- 
dition of the visual cells was considered as assumed in dim Ught. 
Arey ('19) has recently brought forward evidence indicating that 
the sensitivity of the retinal pigment and of the rods and cones 
is nowhere nearly so high as is generally believed, and the con- 
ception that the changes observed in those eyes where marked 
effects of hght are obtained are adaptive has been thereby placed 
on much surer ground. 
We should not, however, a priori, deny that light effects similar, 
if less marked, changes in retinae so constituted that there can 
be no, or little, question of any advantage to be gained by a 
correlative shifting of the position of the visual elements. Garten 
cites the facts that in the selachians, which presumably have 
pure-rod retinae, although the literature on the subject is not 
without disagreement,^ there is little, if any, pigment, and that 
1 Schultze, '66; Krause, 76 and '95; Neumayer, '97; Schaper, '99; Hesse, '04; 
Franz, '05; Retzius, '05; Garten, '07; Cajal, '11, and Putter, '12. 
