MOVEMENTS IN THE VISUAL CELLS 183 



value, and, moreover, as in the case of retinal pigment, variable 

 temperature can play no part in the normal movements of these 

 cells in homothermous animals. 



It maj' be asked how it happens that high temperature in 

 the dark produces diametrically opposed results upon the cone 

 myoids of fishes and of the frog, when the responses to light 

 are identical. The nervous system, presumably, is not involved 

 in these reactions since temperature has a similar effect upon 

 excised eyes. Between the minimal and optimal limits the 

 movement of undifferentiated protoplasm is uniformly acceler- 

 ated with increased temperature, and as has been shown in 

 another place (p. 135) the responses in length of the cones of 

 Abramis are markedly similar in this respect. Whatever the 

 details of the process may be, it seems evident that among the 

 fishes tem])erature acts purely as a physical agent in controlling 

 the velocity of the reactions leading to positional changes of the 

 cones. In frogs temperature apparently acts in an entirely 

 different manner. The fact that only high temperature is 

 effective in producing a change, a shortening of the myoid, is 

 best explained on the basis of Dittler's theory of chemical stimu- 

 lation, whereby increased tem])erature can be conceived of as 

 favoring the formation of catabolic wastes which chemically 

 stimulate the cones to shorten, while low temperature probably 

 acts both in retarding catabolism, and by reducing the sensitivity 

 of the myoid toward such products as are fonned even under 

 these conditions. 



A comparative study of the responses of the visual cells, 

 throughout the various classes, to light and temperature re- 

 veals difficulties in explaining the mechanism by which their 

 positional changes are accomplished. Why do the rod cells 

 of some animals shorten in the light whereas others lengthen? 

 Why do the rods of fishes and of birds lengthen through the 

 action of light, whereas the cones shorten? 



From the physiology of simple protoplasm, two alternatives 

 are open. Either the myoids of the visual cells have become 

 specialized to respond to certain stimulating agents in different 

 ways, or, in various retinas the morphological structure of these 



