486 CHARLES RUSSELL BARDEEN 



hind legs exposed for two hours to the rays one died within three 

 weeks after exposure while the others lived nearly a month with- 

 out undergoing metamorphosis. Out of five specimens with 

 small hind leg buds exposed for two hours to the rays all died 

 within three weeks without undergoing further externally visi- 

 ble metamorphosis. Out of eight specimens used for control, 

 five with distinct but immovable hind legs and three with small 

 leg buds at the time of exposure of the other specimens, one was 

 accidentally destroyed, two failed to develop vigorously and died 

 before completing metamorphosis, while the others developed 

 apparently normally. One underwent metamorphosis within three 

 weeks but in the others metamorphosis was somewhat later. In 

 another experiment out of six toad tadpoles with small hind leg 

 buds exposed for two and one-half hours to the rays all died within 

 sixteen days after the irradiation. Thus far I have been unable to 

 expose tadpoles sufficiently to inhibit development of leg buds, 

 without at the same time so injuring the organism as a whole as 

 to cause death. 



From the experiments made it seems probable that during 

 metamorphosis the toad is more susceptible to the rays than dur- 

 ing the earlier tadpole stages. More extensive experiments are, 

 however, necessary to warrant decisive conclusions. 



4. SUMMARY OF EXPERIMENTS 



The effects of exposures to X rays of the sex cells and of devel- 

 oping ova and larvse are illustrated in the following diagrams 

 and tables. 



Diagram A illustrates representative experiments to test the 

 effects of irradiation of sperm, unfertilized ova, ova during fer- 

 tilization, ova in the early cleavage stages, ova early in gastru- 

 lation, larvae before hatching, young tadpoles and tadpoles with 

 small visible hind leg buds. Each batch of eggs is represented 

 by a column divided into the stages of fertilization, cleavage, 

 gastrulation, larval differentiation before hatching, tadpole 

 differentiation after hatching, growth of the tadpole and meta- 

 morphosis. While these stages differ very greatly in the time 

 required for their completion, for the sake of convenience they 



