76 VERTEBRATE PHOTORECEPTORS 



using up and regeneration of visual purple in the rods, con- 

 cluded that pigment migration should not take place in a 

 rodless retina, such as in the lizards. He did not say, how- 

 ever, whether it does or not. Angelucci (1894) described 

 migration in the eye of the tortoise, Testudo marina. Chia- 

 rini (1906) demonstrated migration in Lacerta agilis. Garten 

 (1907) was unable to obtain preparations of the retina of 

 Emys, Chameleon, or of Lacerta, which showed consistent 

 differences in the position of the pigment depending on 

 whether the animal had been kept in darkness or in bright 

 light. Hess (1910) also was no more successful than Garten 

 in his observations on Emys europaea. 



The author (Detwiler, 1916) reinvestigated these matters 

 in the eyes of a number of reptiles (mostly upon the tortoise, 

 Chrysemys picta, and the lizard, Sceloporus undulatus). 

 It was concluded that in both forms, there was slight but 

 definite pigment migration in the light (3.6 ijl in the tortoise 

 and 3 jLt in the lizard) . It was found also that stimulation of 

 the enucleated bulb with an induced current of moderate 

 strength caused a migration. A still more marked migration 

 was brought about by passing a constant current (15 to 20 

 M.A.) through the enucleated eye, either centrifugally or 

 centripetally for a period of fifteen minutes. 



Pigment migration was demonstrated also in the eye of 

 the Chinese lizard, Eremias argus (Detwiler, 1923a), in the 

 gecko, Gecko Swinhonis Giienther (Detwiler, 1923c), and 

 in AUigator mississippiensis (Laurens and Detwiler, 1921). 



In birds possessing both rods and cones, migration is 

 usually much more extensive than in reptiles (Chiarini, 1906). 

 Van Genderen Stort (1887) showed this to be true in both 

 areas of the pigeon retina, where rods are present, and to a 

 less extent in the region where they are absent. Krause 

 (1894) also described migration in the diurnal hen, as did 

 Angelucci (1878) for the owl. 



As regards the mammals, evidence for pigment migration 

 is very insecure. Angelucci (op. cit.) claimed to have demon- 

 strated migration in the rabbit. Chiarini (1906) described 



