186 S. R. DETWILER 



for the simple reason that that is probably just what does take 

 place, not only in the retinal pigment epithelium but in the skin 

 melanophores as well, when stimulated by light. Light causes 

 chemical changes which influence the pigment cells and causes 

 the protoplasm, in which the pigment is carried, to stream. 



SUMMARY 



1. The retinae of the tortoises, Chelopus insculptus, Chelopus 

 guttatus and Chrysemys picta and of the lizard, Sceloporus 

 undulatus contain no rods. 



2. In both the tortoise and lizard double, as well as single, 

 cones occur (figs. 6 and 7). 



3. Light causes a migration of the pigment and a contraction 

 of the cones in both the tortoise and the lizard retina, the extent 

 of migration in the tortoise averaging 3.6/i and in the lizard 3.1/i. 

 The extent of the contraction of the cones in the tortoise averages 

 2.3m. The pigment epithelial cells also flatten in light to the 

 extent of about 2.7^. 



4. Light probably causes the cone nuclei to lengthen and 

 become narrower. Illumination decreases slightly the ability 

 of the outer nuclei to take on stain, but has no effect on the form 

 nor on the ability of the nuclei of the inner granular layer to stain. 



5. Illumination has no effect on the form and volume of the 

 ganglion cells, but it reduces the amount of chromatin and Nissl 

 substance so that the cells stain less darkly and more diffusely 

 (figs. 8 and 9). 



6. Light causes the pigment to migrate and the cones to con- 

 tract in the tortoise retina after the optic nerve is cut (fig. 3). 



7. Stimulation of the enucleated bulbus with an induced cur- 

 rent of moderate strength causes a forward migration as well 

 as a bunching and massing of the pigment. It also causes a 

 slight broadening and contraction of the cones (fig. 10). 



8. Passing a constant current of 15 to 20 M. A. through the 

 eye either centrifugally or centripetally for a period of 15 min- 

 utes brings about a marked migration of the pigment and in 

 addition a broadening and stretching of the cones. When the 



