PEARSE. — THE REACTIONS OF AMPHIBIANS TO LIGHT. 203 



phibians in this respect. Cryptobranckus is most responsive when the 

 tail region is illuminated, but the skin of the toad is equally sensitive 

 on all parts of the body. 



The fact that both the skin and eyes act as photoreceptors in fishes 

 as well as amphibians has led to considerable speculation concerning 

 the origin of the retina in higher vertebrates. Various theories have 

 been put forward, but only two of them have direct relation to the 

 field included in the present paper. Willem ('91) advanced the view 

 that in its primitive condition light sensitiveness was distributed over 

 the whole skin and that it had become gradually localized in the eyes 

 of higher forms. Parker (:08) has pointed out an objection to this 

 view in the fact that photic sensitiveness is lacking in the skin of the 

 most primitive member of the vertebrate series (Amphioxus), though 

 it possesses direction eyes which are closely connected with the central 

 nervous organs. He believes that the development of photoreceptive 

 power in the skins of vertebrates has been a separate process from that 

 of the development of the retinas, which first arose in intimate connec- 

 tion with the central nervous system. This question cannot be re- 

 garded as definitely settled, and the results of the experiments 

 described in the present paper throw little light upon it. The fact 

 that photic sensitiveness is present in such a wide range of amphib- 

 ians seems to support Willem 's view, as the different forms have 

 developed along extremely diverse lines. 



Not only do the photoreceptive organs constitute important factors 

 in a consideration of the photic reactions of amphibians, but variations 

 in the light itself are important. Differences in intensity are signifi- 

 cant in the reactions of the toad, for the percentage of positively pho- 

 totropic responses decreases and the number of indifferent reactions 

 increases when the light intensity is decreased. The direction of the 

 incident rays of light which impinge on the photoreceptor is, however, 

 of no apparent consequence. A toad in which only one eye is illumi- 

 nated by light from in front turns toward the stimulated side instead 

 of going toward the light, and an eyeless toad subjected to unilateral 

 stimulation by light from above turns toward the illuminated side 

 without regard to the direction of the rays. In general, then, the 

 photic reactions of amphibians are brought about by intensity differ- 

 ences on the two sides of the body. Concerning the influence of the 

 quality of the light, it may be said that both the skin and eyes of am- 

 phibians are open to stimulation by light rays which include the whole 

 range of the visible spectrum. When the light is received through 

 both the eye and skin receptors, the rays toward the violet end of the 

 spectrum are most effective in producing tropic responses, but when 



