PHOTOMECHANICAL RESPONSES 



75 



to the external limiting membrane, whereas in the dark, 

 it contracted back into the body of the epithelial cells so as to 

 form a compact band close to the choroid. Examples of the 

 two conditions are shown in Figures 55 and 56. Klihne 

 predicted that a phototropic action on retinal pigment would 

 be found in all vertebrates. His prediction has not been fully 



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Fig. 55 (Left). Photomicrograph of Hght-adapted frog retina, showing 

 expanded (migrated) epithehal pigment. X 267. 



Fig. 56 (Right). Photomicrograph of dark-adapted frog retina showing con- 

 tracted epithehal pigment. X 267. 



substantiated, for these changes in the eyes of mammals and 

 man have never been convincingly demonstrated. In the 

 reptiles the results have been contradictory. 



Among teleost fishes and amphibians a definite response 

 of the pigment to Hght is general (van Genderen Stort, 1886; 

 Pergens, 1896; Exner and Januschke, 1906; Garten, 1907; 

 Arey, 1916a, 19166). This has been demonstrated in am- 

 phibians by Boll (1877) and Kuhne (1877), Arcoleo (1890), 

 Angelucci (1878), van Genderen Stort (1886), Arey (1919), 

 Detwiler and Lewis (1926), and others. 



Angelucci (1878) was not able to say with definiteness from 

 the results of a few experiments on the turtle, Testudo graeca 

 and on the rodless lizards (L. agilis, L. muralis, and L. viridis) 

 whether pigment migration took place or not. Boll (1881), 

 who regarded the migration of pigment as concerned with the 



