20S C. F. CURTIS RILEY 



camts (some of them immature), as described by Mast (I.e., 

 pp. 214-215, 219-220) appears to occur much in the same way 

 as observed by the writer in young toads. The intensities of 

 the lights employed by Mast in his experiments were 12.5 and 

 25 ca.m. It is probable, during the orienting response of young 

 toads, that the light acts in some such manner as described by 

 Loeb (1905, p. 32), and to which Mast (I.e., p. 223) practically 

 assents. Even so, though the light should act "at a constant 

 intensity," such a fact does not necessarily preclude the influ- 

 ence of the differences in intensity on the two sides of the ani- 

 mal's body, during orientation. 



In regard to the effect of light intensity upon the orientation 

 of young toads, mention should be made of Mast's (I.e., pp. 

 219-220) extremely interesting experiments with specimens of 

 Bufo americanus, some being immature. Two lights of different 

 intensities were employed, one being 12.5 ca.m. and the other 

 25 ca.m., and the source of illumination was two Nernst glow- 

 ers, the two beams of light crossing at right angles. When a 

 toad was placed, with one side turned toward the glower, in the 

 beam of light of lesser intensity, it oriented directly and accu- 

 rately, and then jumped toward the source of illumination. How- 

 ever, when the animal reached the intersecting beam from the 

 light of greater intensity, instead of orienting toward this light, 

 it continued to jump toward the weaker light. Altogether 42 

 trials were made, 36 of these being as described. In 6 cases 

 only did the toad turn toward the stronger light when it reached 

 the point of intersection of the two beams, and these six trials 

 were all with the same individual. Seven toads only were used 

 in the experiments. It would seem in a series of experiments 

 exhibiting results of this nature that the effect of light intensity 

 was modified as an orienting factor, or why did not the animals 

 orient toward the light of stronger intensity. The present 

 writer offers two suggestions which may prove to be partially, 

 if not fully explanatory of this. First, the eyes are strongly 

 stimulated by the light from in front, and the response to such 

 stimulation in itself may result in producing an inhibitory effect 

 upon the toad in so far as its response to the stimuli from the 

 intersecting light is concerned. While it is true that in toads 

 both skin and eyes are photoreceptors as Pearse (I.e.) has 

 proved, it is evident that the cross rays from the stronger light 



