PHOTIC REACTIONS OF TADPOLES 243 



meters. In the first set of experiments the photosensitivity 

 of tadpoles with eye and skin was tested. 



The use of such low degrees of stimulation seems to be in 

 marked contrast with the methods used in the study of the 

 reactions of amphibians to hght by other workers. A re- 

 view of the extensive literature on the subject reveals that 

 the intensities used in various investigations range approxi- 

 mately from 200 candle-meters to 1000 candle-meters, and in 

 some cases much higher degrees of stunulation were used. The 

 instances where low intensities were used are few, but in all 

 cases the object of the studies was to test the orientation of the 

 animals in relation to a source of light and no attempt was made 

 to determine extensively any quantitative relation which might 

 exist between the degree of stimulation and response. Cole and 

 Dean ('17), working with the tadpoles of Rana clamitans, ob- 

 served photokinesis when three intensities were employed, 

 500 c.p., 100 c.p., and 48 c.p. (distance from source of stimulus 

 was not given). The present study revealed that, in addition 

 to this, these tadpoles, when kept in the dark and then suddenly 

 and repeatedly illuminated by relatively low intensities within 

 a certain range, show a period elapsing between the moment of 

 stimulation and response which can be accurately measured by 

 a stop-watch. This can be called the reaction-time and includes 

 a sensitization period, during which light energy is taken up by 

 the receptors and thus initiates a change which acts as stimulus 

 to the nerve endings, and also the time for certain secondary 

 processes, such as diffusion in the receptors to the nerve endings, 

 transmission of the unpulse to the adjustor and effector and the 

 time occupied by the muscles in their contraction. This secon- 

 dary period involving the processes just enumerated is dis- 

 cussed by certain authors under the term of 'latent period.' 

 Others use this term to designate the total time from the appli- 

 cation of the stimulus to the observable response. In this 

 discussion we shall use reaction-time to designate the time 

 occupied by sensitization, that is, the minimum period during 

 which the light must act in order to produce a change in the 

 receptors of sufficient strength to initiate a reflex, plus the time 

 occupied by any secondary processes. 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 34, NO. 2 



