162 S. O. MAST 



without any change in the environment and since the direction 

 of the stroke of the tail under the latter condition is deter- 

 mined by the structure and physiological states of the organism, 

 it may be determined likewise under the former conditions. If 

 it is thus determined, the response to shadows is purely an acti- 

 vating response, a response which is entirely independent of 

 the localization of the stimulus. However this may be, evi- 

 dence presented in the following pages shows that the tail in 

 active specimens bends toward the ocular side when the light 

 is reduced, indicating that reduction in illumination may be 

 more than merely an activating stimulus. 



Regarding the difference in the reaction to shadows in nega- 

 tive and in positive specimens, all that can be said with certainty 

 at present is that in positive specimens the structure and physio- 

 logical states and processes of the tadpoles are such that when 

 the light is reduced the muscles contract in such a way as to 

 cause the tail to bend toward the abocular side, while in negative 

 specimens they are such that the same change in illumination 

 causes them to contract in such a way as to make the tail strike 

 in the opposite direction. 



h. Active specimens 



In the preceding section it was demonstrated that when 

 the light is suddenly reduced resting tadpoles of Amaroucium 

 immediately become active and that the activity may consist of a 

 series of one or more strokes of the tail resulting in locomotion. 

 In this section we shall consider the reactions in active specimens. 



Numerous observations were made on the effect of suddenly 

 reducting the illumination on specimens in which the tail was 

 rapidly vibrating, and it was found that while there is no apparent 

 change in the rate of vibration, the tail bends toward one side 

 so as to vibrate in a new plane. In negative specimens it bends 

 toward the ocular side, in positive ones toward the abocular side. 

 This is particularly evident in specimens which are attached or 

 held under the cover-glass. 



If a number of tadpoles are put into a vaseline enclosure under 

 a cover-glass some usually become entangled in the vaseline at 



