SUSCEPTIBILITY OF AMPHIBIAN OVA TO X-RAYS 459 



developing immediately before or immediately after the first 

 stages of gastrulation, while eight showed evidences of irregular 

 gastrulation and gave rise to forms with a large blastopore and 

 a protruding yolk mass. In three specimens the blastopore was 

 closed. In one of these development stopped very early in 

 larval development ; in the other two larvae with abnormal heads 

 and tails were produced. The effects mentioned indicate that 

 an exposure of three-fourths hour immediately after the beginning 

 of fertilization has a more profound effect on development than 

 exposure of either of the germ cells for an even longer period before 

 fertilization. (Compare tables 1, 2, and 3.) 



On the other hand, an exposure of half an hour immediately 

 after the beginning of fertilization in Experiment CO produced 

 effects apparently somewhat less severe than those in Experiment 

 1, table 1. In the latter experiment, in which the spermatozoa 

 were irradiated for fifteen minutes 11.9 per cent of the tadpoles 

 appeared fairly normal two weeks after fertilization. In Exper- 

 iment CO, 20 per cent appeared fairly normal at this period. 

 Since neither experiment was carried beyond two weeks after 

 fertilization the percentage of specimens capable of undergoing 

 ultimate normal development was undetermined in each case. 

 In Experiment 2, table 1, in which frog sperm was exposed thirty 

 minutes to the rays, and Experiment 3, table 1, in which toad 

 sperm was exposed thirty-seven minutes to the rays, the effects 

 were more severe than in Experiment CO, table 3. We have, 

 however, seen reason to believe that Experiment 2, table 1, 

 gave results quite atypical in nature and not to be attributed 

 wholly to the action of the X rays; while the longer exposure 

 (thirty-seven minutes) in Experiment 3 would perhaps suffice to 

 account for the greater severity of the results. In Experiment 4, 

 table 1, (exposure of frog sperm for twenty minutes), Experi- 

 ment 7, table 1, (exposure of frog sperm for twelve minutes, and 

 Experiment 8, table 1, (exposure of frog sperm for forty min- 

 utes) the results were similar to, although slightly more marked 

 than those found in Experiment CO, table 3. We may, there- 

 fore, conclude that exposure of eggs for one-half hour after the 

 beginning of fertilization gives results approximately equivalent 



