164 S. O. MAST 



obtained vary from a trifle over one hundredth to nearly six 

 hundredths of a minute. Thus it is evident that the reaction 

 in question continues, at times, for only a very short period; 

 indeed, so short that it resembles the reaction of skeletal muscles 

 to a single induction shock. Under certain conditions, it is, how- 

 ever, in all probability the result of a series of impulses, for if 

 the illumination is immediately increased again after it has been 

 decreased, the tail always immediately bends in the opposite 

 direction. Thus the reaction can be reduced to a very small 

 fraction of a minute. It is consequently clear that the time 

 the tail remains bent depends upon the condition of illumina- 

 tion. If the illumination is reduced and held there the tail 

 remains bent longer than it does if the illumination is imme- 

 diately increased again. Increase in illumination, therefore, 

 inhibits the bending effect of decrease in illumination. 



The degree of bending of the tail in the response to shadows is 

 (like the extent of the stroke of the tail) within wide ranges 

 largely if not entirely independent of the extent of the reduction 

 in illumination. This was clearly established in numerous obser- 

 vations made in the same way as the observations on the extent 

 of the stroke of the tail previously described. 



The amount of reduction in light energy required to induce the 

 bending reaction of the tail was not accurately ascertained, but 

 it is certainly relatively small. Judging from the results ob- 

 tained in numerous observations, the reduction necessary is 

 about the same as that required to induce the activating response 

 previously described; and it clearly depends upon the time-rate 

 of reduction, just as in the case of the activating response, for it 

 does not follow if the reduction is sufficiently slow, no matter 

 how extensive it may be. 



REACTIONS TO INCREASE IN ILLUMINATION 



If the illumination is increased, resting Amaroucium tadpoles, 

 as previously stated, remain quiet. There is no response even 

 if intense direct sunlight is flashed upon them. But if they 

 are active the tail at once bends toward the abocular side. Is 

 this bending due merely to the inhibition of the effect of pre- 

 vious reduction in illumination or is it a response to a stimulus? 



