184 S. R. DETWILER 



DISCUSSION AND CONCLUSIONS 



In the retina of the tortoises (Chelopus insculptus, C'helopus 

 guttatus and C'hrysemys picta) and of the lizard (Sceloporus 

 undulatus) pigment migration and cone contraction undoubt- 

 edly take place, however slight they may be as compared with 

 the conditions observed in other groups of animals. Garten 

 ('07 a and '07 b) has advanced a theory to explain the migration 

 of the pigment and the contraction of the cones. His theory 

 necessitates the assumption that there is no migration of pig- 

 ment in the eyes of animals possessing a purely rod retina (skates 

 and sharks) or a purely cone one (reptiles in general). Garten's 

 theory in brief is as follows: one must assume that, were it not 

 for the optical isolation of the visual elements by the pigment 

 which migrates forward in the eyes of most of the lower verte- 

 brates, a great deal of light would be scattered in all directions, 

 on account of the large ellipsoids (fishes) and the strongly re- 

 fractive oil-drops (many Amphibians, Reptiles and Birds) which 

 would stimulate the neighboring rods and cones. In the narrow 

 slender rods, however, total internal reflection prevents this 

 dispersion of light, which is borne out by the fact that in pure 

 rod retinae the pigment is entirely lacking from the pigment 

 epithelium. On the other hand in a pure cone retina with 

 strongly refractive oil drops, the pigment in both light and dark 

 eyes must be forward in position (that is, covering the outer 

 segments and the oil drops). By means of the stretching of the 

 cones in darkness or in faint light, which in many of the lower 

 animals is accompanied by a contraction of the inner segments 

 of the rods, there is presented for faint light stimulation, a cone- 

 free purely rod layer, which does not need the pigment, which 

 therefore moves backward thus enabling oblique light to enter 

 the rods. On the other hand the contraction of the cones pro- 

 duced by bright light, which is accompanied by stretching of 

 the rods, similarly leads to a forward migration of the pigment 

 which surrounds the cone outer segments. 



This is the part of Garten's theory which is of interest to 

 us. It cannot be denied that the tortoise and lizard retina con- 



