182 PROCEEDINGS OF THE AMERICAN ACADEMY. 



D. The Reactions of Eyeless Toads to Unilateral Stimulation 

 by Light from above. 



The last experiments described showed that a toad would turn toward 

 the illuminated side when only one eye was stimulated, even when such 

 a movement did not take it into a region of greater light intensity. 

 The next question which suggested itself was whether eyeless indi- 

 viduals would make similar movements when only one side was stimu- 

 lated. In solving this problem, the apparatus shown in Figure 4 was 

 used. It consisted of a wooden box (sixty centimeters high, forty-five 

 wide, and twenty-eight deep) which was lined throughout with two 

 layers of black cloth, except the floor, which was of slate. Light com- 

 ing from above (I) passed through oblong openings in two screens (a, s) 

 so that an area a little larger than a toad was illuminated on the floor 

 of the apparatus, where the light intensity was 413 candle-meters. 

 Each toad was so placed that the right and left sides were alternately 

 illuminated, and an accurate unilateral division of light and shadow was 

 secured by using a small movable screen (c) of blackened sheet iron. 



In preparing individuals for these and subsequent experiments, a 

 different method was used for excising the eyes from that followed 

 heretofore. Instead of removing the whole upper jaw, a horizontal cut 

 was made just above the nostrils, which met a vertical cut behind the 

 eyes. The roof of the mouth was thus left intact, and there was conse- 

 quently no interference with the respiratory movements. The plan 

 followed in experimenting was to orient the individual facing the ob- 

 server before each of the first ten reactions, while for the last ten it was 

 faced in the opposite direction. Before and after the tests with light 

 from above, each toad was tested ten times with light of the same in- 

 tensity (413 candle-meters) from the side. The results of the reactions 

 (Table IX.) with the light from above show a turning toward the side 

 illuminated in seventy per cent of the cases, and, while the positive 

 phototropism of the same individuals was slightly greater when they 

 were illuminated from one side, the difference does not amount to 

 enough to be significant. It may therefore be said that the positive 

 phototropism of eyeless toads is due to intensity differences on the two 

 sides of the body. 



Payne (:07) has performed experiments of the same kind with the 

 blind fish, Amblyopsis spelaeus, after the eyes had been excised, and 

 obtained similar results. Apparently the direction of the light rays, as 

 distinguished from intensity differences, has no influence on the reac- 

 tions of either of these species. 



