240 VASIL OBRESHKOVE 



most cases this condition was retained for much longer periods. 

 Light-colored tadpoles, however, which had not been subjected 

 to anesthetics, when removed to the dark room, remained in 

 the light condition. The work was confined, therefore, to 

 tadpoles as nearly as possible of the same color, and those some- 

 what pale in appearance were chosen. For this reason, when 

 blind animals were needed for some phases of the problem it was 

 found advisable to remove the eyes without previously placing 

 the individuals in chloretone. It is very possible that the slower 

 reaction-time of animals with expanded melanophores is due 

 to a nervous condition of the individual which may be associated 

 with the melanophores in an expanded state. There is another 

 and more plausible explanation of this condition, which will be 

 considered in connection with another phase of the problem. 



Preliminary experiments revealed the necessity of avoiding 

 another difficulty. Individuals differing in size showed variation 

 in the regularity with which they responded. The smaller 

 individuals were more active and more orderly in their responses. 

 Tadpoles between 4 and 5 cm. in length were found to be the 

 most favorable. 



Tadpoles which were chosen for the experimental work on the 

 basis of size and coloration were kept in the darkroom through- 

 out the period of experimentation. Previous to each experi- 

 ment, however, they were tested to determine whether they were 

 in normal state, free from any infection, such as saprolegnia, 

 and whether all of them were of equal photosensitivity. This 

 was done in the following manner. Five or six tadpoles were 

 placed in a white porcelain dish about 37 cm. in diameter and 

 with just sufficient water to cover them. Such individuals 

 were then repeatedly stimulated with a constant intensity of 

 light at intervals of one minute or longer. Only those tadpoles 

 were chosen which, with no exception, showed activity upon the 

 application of the light. Those which showed indifference to 

 the light stimulus were removed and placed in a special dish 

 and from time to time tested during the next few days. These 

 tadpoles usually died soon after being isolated from the chosen 

 stock. Previous to each experiment the selected anunals were 



