MOVEMENTS IN THE VISUAL CELLS 145 



Hence we may conclude that the condition found in the frog 

 is unhke that found in the fishes. It should be noted, however, 

 that between certain limits the two animals show similar ten- 

 dencies in their pigment responses. These hmits are approxi- 

 mately 0° to 19°C. in the dark for the frog, and 0° to 28°C. in 

 either darkness or light, for the fishes. 



Since the tendency of pigment migration under the influence 

 of temperature agrees between the limits of 0° to 19°C. for the 

 dark adapted frog and 0° to 28°C. for the fishes either in light 

 or in darkness, the query may be raised — is it not possible that 

 if the fishes were subjected to higher temperatures a reversal 

 of the temperature effect would occur in which an expansion of 

 the pigment would again be found as in the frog? The fact that 

 Herzog found but slight differences at 18°C. and 24°r. increases 

 this suspicion. I am convinced, however, that a tendency to- 

 ward such a response does not exist in the retinal pigment of 

 fishes even to the slightest extent. In the first place, as stated 

 before, prolonged heating of the frog's retina at 24*^C. would 

 be likely to produce more striking changes than Herzog obtained. 

 Furthermore, a few experiments in which the temperature of 

 fishes was raised to 30°C. and over failed to show, in either 

 light or darkness, anything beyond the characteristic response 

 of less complete expansion than at the lower temperature. 



The extent to which the retinal pigment of the frog and of 

 fishes moves under the influence of temperature differs in a high 

 degree. Among the fishes the differences are small and amount 

 to little more than a redistribution of pigment in the dark and 

 light phases respectively, whereas in the dark-adapted frog 

 varying temperature induces the whole range of pigment re- 

 sponse usually occasioned by light or darkness. This further 

 indicates that the nature of the response in the two kinds of pig- 

 ment cells differs fundamentally. In the fishes probably the 

 response is through the direct action of temperature on the cell 

 protoplasm, while in the frog the pigment migration may be 

 produced indirectly through the intervention of the nervous 

 system. 



