PHOTIC REACTIONS OF TADPOLES 239 



were not favorable material for immediate experimental work. 

 These individuals were kept in the laboratory for at least one 

 week before they were subjected to experiment. In that time 

 the mud and other matter which filled the stomach and intestine 

 when the animal was captured were discharged and the animals 

 became accustomed to the laboratory condition, were easily 

 stimulated, and reacted with regularity to light. 



1. Selection of animals for experimentation 



Although the melanophores in the tadpoles of Rana clamitans 

 are not subject to as much contraction and expansion as is ob- 

 served in other tadpoles and in certain fishes, yet during the 

 course of the experimentation, as well as when tadpoles were 

 just brought from the pond or from the general aquarium, they 

 showed considerable variation in the state of these bodies. A 

 knowledge of the condition which induces contraction and ex- 

 pansion of the melanophores and the general state of the animals 

 when in either condition proved of importance early in the work 

 since tadpoles darker in color from expanded pigment cells 

 showed a slower reaction time than those that were lighter in 

 color. It was decided, therefore, that that condition of the 

 melanophores is an important factor to be kept in mind for a 

 study like the one undertaken, where the reaction-time of the 

 individuals to light constituted the only measurement for the 

 examination of the validity of a law. 



Tadpoles caught in the shallow water of a pond where the 

 background and bottom were chiefly white clay, and tadpoles 

 which had been left in white porcelain dishes in a light room for 

 several days were found to be light in color. On the other hand, 

 tadpoles w^hich were captured from deep portions of the pond, 

 and had been concealed under grass or debris, were dark brown 

 in appearance. Light colored tadpoles, however, which had 

 been placed in 0.2 per cent chloretone solution for anesthetizing, 

 preparatory to the removal of the eyes, expanded the melano- 

 phores in the course of a few minutes. Such animals, when 

 placed in the dark room, remained dark for several days, and in 



