EFFECT OF LIGHT ON THE RETINA 



173 



could find no fovea in the retinae of any of the Chelonians which 

 he examined, although in several there was a small papilla. 

 Chievitz ('89, p. 143) describes an area for Emys but no fovea, 

 while in Lacerta viridis (p. 147) he finds a fovea. 



EXPERIMENTAL 



When sections of eyes taken from animals which had been 

 placed in sunlight are compared with sections of eyes from ani- 

 mals kept in darkness it is seen, in the first place, that pigment 

 migration does take place (figs. 1 and 2). In the dark eye (fig. 

 1) the pigment occupies more of the body of the epithelial cell, 

 so that the nuclei are for the most part covered, and extends 

 forward just beyond the oil drop, which in a few cases can be 

 seen through the pigment. In the light eye (fig. 2) it is seen that 

 the pigment has migrated forward so that the pigment cell 

 nuclei are almost entirely uncovered while the pigment extends 

 further toward the external limiting membrane, in many cases 

 as far as the paraboloid. It was also noted that in the light 

 eye the pigment epithelium adheres more closely than in the 

 dark eye and is not so easily torn away. 



The results of a series of measurements to determine the 

 extent of migration are shown in table 1 (column 3). It will 

 be seen that the average distance from the external limiting mem- 

 brane to the nearest pigment needle in the dark eye is 8.8/1 

 while that for the light eye is 5.2/x. The difference of 3.6m repre- 

 sents the extent of the migration. 



TABLE H 



'■ Based on 10 measurements taken about 1 mm. from the entrance of the optic 

 nerve. 



