SUSCEPTIBILITY OF AMPHIBIAN OVA TO X-RAYS 485 



per cent (F-5d); in a third lot (A-6d); after two and one-half 

 hours exposure all died within fifteen days after exposure. In a 

 fourth lot (D-4^d) an exposure of fifty-five minutes had little 

 noticeable effect. 



After the tapdole becomes well developed and swims about 

 freely it is still susceptible to prolonged although not to short 

 exposures. Contrast D-8d and A-20d with A-9d and A-12d, 

 table 8. Out of sixteen newly hatched frog tadpoles exposed 

 for two and one-half hours to the rays fourteen died within three 

 days after the exposure, one died on the fifth day, and the last 

 specimen on the sixth day. Five frog tadpoles exposed ten days 

 after fertilization for three and one-half hours all died within two 

 weeks after the exposure. 



Larvae exposed after hatching and while becoming differen- 

 tiated into tadpoles show the effects internally chiefly in the cen- 

 tral nervous system and eyes and to a less extent in the heart and 

 blood vessels. The walls of the spinal cord may be so thick that 

 the lumen is obliterated. The constituent cells in these speci- 

 mens are quite abnormal. The pronephic tubules may be abnor- 

 mally dilated. The alimentary canal and its appendages are 

 usually relatively normal. The body cavity may be abnormally 

 dilated. 



When older specimens are exposed abnormal accumulations of 

 fluid between the epidermis and the underlying pigmented con- 

 nective tissue are not infrequent. 



7 Period of metamorphosis 



My experiments on the exposure of toads to the X rays during 

 metamorphosis have been quite limited in number. Out of four 

 specimens with small immovable hind legs exposed for one hour 

 to the rays one died sixteen days later, two twenty-two days later, 

 while the fourth lived for a month but did not develop visible 

 fore-legs nor hind legs sufficiently differentiated to move. Out 

 of five specimens with small hind leg buds exposed for one hour to 

 the rays all died within three weeks without undergoing further 

 metamorphosis. Out of three specimens with small immovable 



