PHOTIC REACTIONS OF TADPOLES 253 



The eyes of tadpoles of Rana clamitans were removed by 

 cutting the skin around them very carefully with a small sharp 

 dissecting knife and then placing curved scissors under the 

 eyeball in such a way that all muscles and nerves were cut with 

 one stroke and the eye removed. Such tadpoles were allowed 

 a few days in which to recover before being subject to experi- 

 mentation. 



When the work with blind tadpoles was begun, it was intended 

 to use the same number and range of intensities as were used 

 for testing the photosensitivity of tadpoles with eyes and skin. 

 Very early in the work, however, it became evident that the 

 reaction-time of blind tadpoles was the same as that of tadpoles 

 with eyes and skin illuminated. When blind animals were 

 repeatedly stimulated with light of 1.2-candle-meter intensity, 

 the individual readings corresponded very closely with those 

 obtained from the animals with eyes and skin stimulated. Simi- 

 larly, it was shown that both in tadpoles without eyes and 

 in tadpoles with eyes and skin, the same period elapses between 

 the stimulation and response under the influence of 15 candle- 

 meters. Table 3 gives the actual observations at these two 

 intensities, as well as the probable error, standard deviation, 

 and the intensity-time products. These products agree well 

 with each other, and likewise with those of corresponding inten- 

 sities when eye and skin were illuminated. 



These surprising results suggested that with light falling 

 vertically upon laterally placed eyes, such as those in tadpoles, 

 the eyes may not have received the full stimulus applied and 

 hence the similarity of results. For this reason the experiments 

 with other intensities were not carried further in the manner 

 described, but the subsequent tests and apparatus were modified 

 so that the stimulus was applied from the side. 



The individuals to be tested were placed in a rectangular glass 

 aquarium 38 cm. long and 10 cm. wide (fig. 3, m). By a tightly 

 fitting plate this width was subdivided longitudinally into two 

 compartments, one of which was just wide enough to allow 

 freedom of movement of animals forward and backward, but 

 always parallel to the side of the aquarium. At a distance in 



