PEARSE. — THE REACTIONS OF AMPHIBIANS TO LIGHT. 185 



the body," after the eyes have been excised. Parker (;05 b , p. 419) and 

 Reese (:06, p. 94) have, on the other hand, found the tail to be the most 

 sensitive region in Ammocoetes and Cryptobranchus respectively. These 

 few observations indicate that the comparative sensitiveness of the skin 

 to photic stimulation varies in 'different species of vertebrates. 



F. The Effect of Previous Conditions of Light Stimulation on 

 Photic Reactions. 



It had been noticed in a general way during the preceding experi- 

 ments that when a toad was placed near a strong light the first reac- 

 tion was more often away from the light than any of the subsequent 

 responses were, and that the first reaction was usually slower than 

 those which followed. G. Smith (;05) has shown that, when Gam- 

 marus is exposed to light, a pigment migration takes place toward 

 the proximal ends of the retinula cells, and that as this migration pro- 

 gresses the animal changes its reactions from indifferent to strongly 

 positive. As a pigment migration, as well as other changes, takes 

 place when the eyes of amphibians are exposed to light, it was thought 

 that there might be a similar influence on the reactions in this case, 

 and experiments were accordingly carried out to test this question. 



In these experiments toads were placed in the centre of a box which 

 was ninety centimeters long and thirty-eight wide. The floor and 

 sides were of slate, and both ends were closed by glass heat- screens 

 which contained a layer of water 3.75 centimeters thick. Light, which 

 had an intensity of 220 candle-meters at the spot where the toads were 

 exposed to it, was admitted from one end, and before each reaction the 

 individuals were placed with the right and left sides alternately toward 

 the source of light. Eleven toads were kept first in the dark for five 

 days and then in the light (three candle-meters) of a gas jet for an 

 equal period of time. The eyes were thus exposed continuously to 

 uniform light or dark, except when the animals were removed for the 

 experiments, which occupied about half an hour daily. By taking 

 twenty records from each individual each day, an attempt was made 

 to get a series of a hundred reactions from each individual under the 

 two conditions of previous exposure to light and to dark. In all but 

 three cases these attempts were successful. 



The results in Table XL show that the first reaction in a series of 

 twenty has the least tendency to be positively phototropic and that 

 subsequent reactions are increasingly positive. There is, however, no 

 great difference between the responses of individuals previously exposed 

 to light and those previously in the dark. In Table XII. the reactions 



