PHOTOMECHANICAL RESPONSES 77 



slight forward migration in the retinas of dogs. Garten 

 (1907), however, who carried out careful experiments on the 

 ape, ox, rabbit, and rat, failed to find any evidence of migra- 

 tion in these forms. In the bat and rat, the author failed to 

 find any expression of migration (Detwiler, 1924). 



It can be said that the most extensive migration takes 

 place in fishes and anuran amphibians. Also in birds, both 

 diurnal and nocturnal, definite movements occur. In rep- 

 tiles, slight migration has been demonstrated in many forms. 

 In mammals and man it is very questionable whether pig- 

 ment migration has been demonstrated definitely in any 

 instance. Despite this lack of evidence for mammals and 

 man, statements can be seen (principally in textbooks) which 

 ascribe to the human eye the photomechanical responses 

 which so typically take place in the eyes of lower vertebrates. 



Following the discovery of pigment migration by Boll and 

 Kiihne, a host of workers investigated the phenomenon in 

 the eyes of all vertebrate classes and the literature became 

 extensive. 



Cones. Not only was it found that light causes a migration 

 of pigment, but that it also brings about positional changes 

 in the rods and cones. The discovery that cones alter their 

 lengths with respect to light and darkness was made by van 

 Genderen Stort, although the earliest announcement was 

 made by Englemann (1885) in whose laboratory the work 

 was done. It was found that light causes a shortening, and 

 darkness a lengthening of the cone. To the contractile 

 portion of the inner member of the cone, Englemann applied 

 the apt term 'myoid.' Subsequent investigations have 

 shown that if the response occurs at all, it is the same in all 

 vertebrates, viz., a shortening when illuminated. 



The most marked positional changes of cones are found 

 in fishes, among which the eel, however, is an exception 

 (Garten, 1907). According to Arey (1928) the myoid may 

 lengthen tenfold. Changes in length of the cone myoids in 

 Ameiurus are shown in Figures 57 and 58. The extensive 

 movement of the rods is shown also. 



