MOVEMENTS IN THE VISUAL CELLS 169 



from the same animal were used simultaneously, one at each 

 temperature extreme. 



1. Effect of temperature upon retinal pigment. After many 

 trials it was found that sharp differentiation of the retinal pig- 

 ment of fishes was hard to secure at the extreme temperatures 

 when the initial temperature had been intermediate. Accord- 

 ingly, the expedient was employed of subjecting the Uving ani- 

 mals to a preliminary treatment either at 3°C. or at 25°C., and 

 as a result uniformly satisfactory differentiation was obtained. 



The retinal pigment of Ameiurus behaved precisely as in 

 hving animals. At a low temperature, both in darkness and 

 in light (figs. 3, 1), the degree of distal migration was greater 

 than that at a high temperature (figs. 4, 2). Particularly in 

 experiments conducted in the light was this strikingly appar- 

 ent, since in many preparations at 3°r. the pigment migrated 

 so far distally that the more proximal portions of the cells were 

 free of granules, while a sharp line of demarcation existed be- 

 tween the pigmented and non-pigmented zones. 



A few experiments were made upon the dark-adapted eyes 

 of Abramis. In this case also, a greater pigment expansion 

 was found at 3°C\ than at 25°(\ (figs. 11, 12). 



Reference has been made to the contention of many investi- 

 gators that there is an apparent nervous control over the move- 

 ments of the frog's retinal elements. It has been shown that at 

 a medium temperature in the dark the pigment is maximally 

 contracted (fig. 18), whereas at higher and lower temperatures 

 (figs. 17, 19) a considerable degree of expansion is effected. Not 

 only is the amount of migration occasioned by temperature 

 much more extensive than that in fishes, but also the similaritj^ 

 between the effects of the two temperature extremes as con- 

 trasted with an intermediate temperature, has no parallel among 

 other pigment cells or even in melanophores. 



Since it is at least agreed that the pigment of excised eyes of 

 the frog expands in the light, it ought to be possible to observe 

 the effect of temperature, if this agent acts directly upon the 

 pigment cells. Excised eyes from animals that previously had 

 been at an intermediate temperature in the dark, were subjected 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 26, NO. 2 



