92 VERTEBRATE PHOTORECEPTORS 



Most nocturnal animals have either a vertical or hori- 

 zontal slit pupil which probably allows for more extensive 

 closing than a round pupil — hence a greater protection 

 against bright light. The gecko which is a typical nocturnal 

 animal has a vertical slit and a rapidity of closure surpassing 

 that of man. It is also said that the spotted night snake 

 (Hypsiglena), a nocturnal form, exhibits an extremely rapid 

 pupillary response. 



Despite all of the theories, none of which seem to satisfy 

 all the conditions, it would seem most logical to regard the 

 pigment processes as useful to some extent in optically 

 isolating the visual elements. In 1916a, Arey summed up 

 his observations by saying, '^Although the movements of 

 the visual cells and retinal pigment, when present, may have 

 a certain unknown significance in connection with the 

 mechanism of light perception, such movements can be 

 interpreted at present only in terms of protoplasmic responses 

 to definite stimulating agents." In this sense the pigment 

 cells respond to light as do skin melanophores, viz., an 

 expansion and streaming of the pigment. The adaptive 

 significance of these phototropic responses in the retina still 

 remains an intriguing problem, with no all-inclusive solution. 



