REACTIONS OF AMPHIBIAN LARVAE TO LIGHT 199 



minutes in which to react, but the reactions were usually quick 

 and definite, the reaction time varying between ten and one 

 hundred and twenty seconds, the average being about fifty sec- 

 onds. Sometimes a response did not take place for a longer 

 time, but in the majority of such cases it was difficult to say 

 whether the movement was made with reference to the light. 

 Such reactions were therefore called indifferent, that is locomo- 

 tion straight ahead, or nearly straight ahead, without apparent 

 reference to the light. The typical positive or negative reaction 

 consists, in the first place, of a turning of the anterior end of the 

 body toward, or away from, the light. The turning is usually 

 assisted by the fore-legs, though sometimes they are relaxed and 

 are dragged passively, in which case the turning must be brought 

 about solely through the contraction of the body muscles. I'his 

 turning of the anterior end of the body is usually followed by 

 a swimming, or creeping on all-fours, in the given direction. Some- 

 times, however, a larva remains bent in a curve for several sec- 

 onds, with its anterior end in the given direction and with its 

 posterior end still at right angles to the rays of light. In a few 

 cases this constituted the entire response, the larva remaining in 

 this position for five minutes. In a few other cases, always posi- 

 tive, the whole response is carried out so rapidly that no distinc- 

 tion between turning and swimming toward, or away from, the 

 light could be made. There were, in addition, cases where a 

 larva after having been oriented remained absolutely still for 

 five minutes. In this event it was replaced by the next in 

 order. 



In the reactions of the same and different individuals there 

 were the usual inconstancies which one observes in the study of 

 the light reactions of almost any animal, and which have been 

 referred to, for want of a definite explanation, as being due to 

 the changing physiological conditions of the organism. In some 

 cases an individual in a set of ten, on one day would be per- 

 sistently negative, indifferent, or show no reaction, while two or 

 three days later it would be as persistently positive. Again an 

 individual might show no reaction within five minutes for one 

 trial, and thirty minutes later it might be decidedly positive. 



