MOVEMENTS IN THE VISUAL CELLS 135 



1. Ameiurus. At 25°C. in the light (fig. 2), the characteristic 

 position of the expanded pigment is in a broad band about 95 m 

 wide, which extends nearly to the external limiting membrane. 

 The pigment granules are evenly distributed and show no ten- 

 dency to aggregate distally. 



At 15°C, the condition is very .similar to that just described. 

 The pigment, on the whole, tends to be homogeneously dis- 

 tributed, although in many retinas at this temperature there is 

 a slight distal accumulation. 



The disposition of pigment at 5°C. is markedly dilTerent 

 (fig. 1) for it migrates to an extreme dstal situation and forms 

 a dense zone, approximately 30 ii wide, close to the external 

 limiting membrane, although the pigment of fishes and amphil)- 

 ians, under normal conditions, never actually touches this 

 membrane. Between this heavy pigment-mass and the bases 

 of the cells lie scattered granules, but the intervening space, 

 nevertheless, appears relatively devoid of pigment. This ex- 

 treme condition is best produced on the brightest days, and it is 

 impossible to obtain as complete a migration on cloudy days, 

 regardless of the temperature. On the other hand, the uniform 

 distribution characteristic of incomplete expansion at 2o°C\, 

 is independent of the intensity of diffuse daylight. This would 

 suggest that a high temperature is more efficient than light in 

 the regulation of pigment distribution, and that cold, that is, 

 the absence of heat, merel}- allowed light to act unrestrained. 

 Light and high temperature, then, are antagonistic in their effects. 



The results at 5°, 15*", and 25°C\ are, in a way, what might 

 have been expected. A temperature of 15° to 25°C\ probably 

 represents the greatest average warmth to which the animal is 

 subjected in nature; this range from 15° to 25°C., then, repre- 

 sents the limits of what may be called a warm environment for 

 the animal. In the same way from 0° to 10°C'. may be called 

 a cold environment, and from 10° to 15°C. a neutral environment, 

 neither particularly warm nor cold. Hence it is not surprising 

 that the results at 15°C. are more similar to those at 25°C. than 

 at 5°C. At 10°C. the distribution of pigment approximates 

 rather more closelv that at 5°C\ A curve, therefore, obtained 



