614 



THE EYE IN EVOLUTION 



Xenojyus 



Amjuilla 



epithelium can be swamped and obscured by the migration of fuscin 

 from the outer halves of the cells. 



(c) Adaptation both to scotopic and photopic vision is also to a 

 considerable extent facilitated both by d3mamic changes in the retina 

 and in its static organization. Into the first category come the 

 RETINAL PHOTO-MECHANICAL CHANGES, more marked in the lower 

 Vertebrates than in the higher. These comprise a migration of the 

 fuscin from the bodies of the cells of the pigmentary epithelium into 

 their processes which dip inwards between the rods and cones thus 

 enveloping them in a dark sheath of pigment in bright illumination, 

 and its return back to the cell-bodies in dim illumination so that the 

 visual elements are freely exposed to any light there may be available 

 (Figs. 761 to 764). A corresponding movement may involve the 

 visual cells themselves, the myoid element of which is sometimes 

 strongly contractile.^ The rods are usually relatively static and in 

 the few species wherein they migrate they elongate towards the pigment 

 of the epithelial cells to take refuge from bright light. The cones, on 

 the other hand, may remain stationary or contract inwards, away from 

 the pigmentary processes. 



The migration of the pigment was first noted in the frog by Czerny (1867), 

 Boll (1877) and Angelucci (1878), and was exhaustively studied by Klihne (1878), 

 Engelmann (1855), v. Hess (1910), and a host of others (see Arey, 1916) ; in Rana 

 temporaria the migration occurs even in the excised eye, a reaction which demon- 

 strates that a local control exists (Weale, 1956). In other species of Anurans 

 (Xenopus) no demonstrable migration of pigment occurs (Saxen, 1953 ; Weale, 

 1956). The movement, however, is relatively slow, being fully evident within 

 half a minute but is not complete for 50 minutes, while its return in darkness is 

 slower still (1 to 2 hours, Arey, 1916). Migration is inost evident in the lower 

 Vertebrates ; it is absent in Selachians for in the greater part of their retina there 

 is no jjigment, but is marked and extensive in many Teleosteans, less extensive 

 and somewhat slower in Anvirans, still less and slower in Urodeles, slight and 

 slow in most Reptiles (turtles, Crocodilians, and less in lizards), more marked 

 and raj^id in Birds, but has never been adequately demonstrated in Mammals 

 although in this class the retina has been said to cling more tenaciously to the 

 pigment epithelium after ilkimination (see v. Hess, 1912 ; Detwiler, 1916-23 ; 

 Laurens and Detwiler, 1921 ; Holm, 1922 ; Bayliss et al., 1936). 



The migration of the rods and cones on illumination is more rare and less 

 dramatic, but takes place more rapidly (about 2 minutes) and with less intensities 

 of illumination than the migration of pigment (Arey, 1919) ; these movements 

 are said to be associated with swelling on illumination and subsequent shrinkage 

 of the rod and cone nuclei (Pviff, 1951-53). It is most marked in Ariiia and 

 teleostean Fishes (apart from the flat-fishes), but the eel, Anguilla, is unique in 

 that only the rods participate in the imovement. Among Teleosteans these 

 movements have received a considerable amount of study. ^ The rods retract 

 and the cones elongate to enter the guanine layer of the retinal tapetum in dark 

 adaptation (the pike-perch, Lucioperca — Wunder, 1930 ; the guppy, Lebistes — 



^ Such movements may occur in Invertebrates, cf., Notonecta, p. 170. 

 ^ For the diurnal variation in the migration of visual cells, see p. 19. 



