104 LESLIE B. AREY 



a shortening of the cones in localized areas through the action 

 of Hght but found darkness to be ineffectual. The spread of 

 the response to portions of the retina unstimulated by light, led 

 Dittlcr to investigate further the cause of cone retraction. He 

 was able to furnish experimental proof that weak acids, resulting 

 from catabolic processes in the retina, caused the cone myoid 

 to shorten; hence he concluded that the cone myoid was not of 

 itself 'UchtempfindHch,' as Englemann ('85) had believed, but 

 was stimulated to movement through chemical agents result- 

 ing from the action of light on the retina. Fujita ('11), as a 

 result of very limited experimentation, stated that the pigment 

 of the excised eye of a frog expanded in the light but did not con- 

 tract in the dark. 



Ringer's solution, normal saline solution, and tap water were 

 used by me for the immersion of excised eyes. When the first 

 two media were emploj^ed the movements of the rods and cones 

 of Ameiurus, through the action of light, were never clearly 

 demonstrated; possibly such results are to be interpreted as 

 evidence of a chemical control somewhat comparable to that de- 

 scribed by Spaeth ('13) for the melanophores of Fundulus. Tap 

 water did not inhibit the movements of any of the retinal ele- 

 ments of Ameiurus and consequently it was used in all subse- 

 quent experimentation. 



The pigment of Ameiurus, Abramis and Fundulus did not 

 contract when excised eyes from light-adapted fishes were sub- 

 jected to darkness for periods of 4 hours or less. At most there 

 was only evidence of a retraction of the distal accumulation of 

 pigment, which is characteristic of hght-adapted eyes, to form 

 a more homogeneously pigmented zone (figs. 2, 10, 6). When 

 the reverse experiment (subjection to light) was performed, 

 the pigment of Ameiurus became maximally expanded in 2 hours 

 (figs. 1, 3). Only the sHghtest tendency toward expansion, 

 however, could be found after similar experimentation on the 

 two other fishes. It thus appears that light acts directly on 

 the pigment of Ameiurus only, while darkness is totally ineffec- 

 tive on all three animals. 



