PHOTIC REACTIONS OF TADPOLES 273 



peatedly stimulated at one-minute intervals, revealed no sub- 

 stantial change in the reaction-time. The difference in behavior 

 becomes clear when we recall that in Ciona the process of recovery 

 requires from two to three hours, while in the tadpoles it is only 

 fifty minutes. From this it is not difficult to see that the more 

 rapidly an animal is able to recover decomposed material in 

 the sense organs the less rapidly will it become fatigued. 



IX. DISCUSSION 



1. The nature of the process 



At various points in this paper it has been stated that the 

 process active in the sense organs of Rana clamitans tadpoles 

 during illumination is of a photochemical kind. This conclu- 

 sion was based on the regularity with which responses took 

 place under measured amount of radiant energy and also upon 

 the definite relation between reaction-time and the amount of 

 active mass diu'ing the photosensory recovery and fatigue. 

 That the process in the photic sense organs of animals is of a 

 chemical character rather than of the nature of diffusion or some 

 other physical process finds strong support in a comparative 

 study of temperature coefficients of photochemical reactions 

 as compared with those of physical phenomena. No attempt 

 was made to determine the temperature coefficient of the tad- 

 poles in their responses to light, for preliminary study revealed 

 that the animals placed in water below 10°C. become very active 

 and show numerous spontaneous movements at short intervals, 

 while tadpoles placed in water above 22°C. become sluggish and 

 unresponsive. It is a well-known fact from general chemistry, 

 however, that the velocity of a chemical reaction increases with 

 a rise of temperature. The temperature coefficient of a true 

 chemical process is between 2 and 3 for a change of temperature 

 of 10°C. In chemical changes due to light the temperature 

 coefficient has much lower value than in chemical process proper, 

 and it is found to he between 1.1 and 1.2. This is evident from 

 the fact that the rate of photochemical change where light is 

 the stimulating agent is dependent upon the amount of light 



