SUSCEPTIBILITY OF AMPHIBIAN OVA TO X-RAYS 475 



swimming tadpoles but all of the latter died early, none surviving 

 beyond nineteen days.^ 



In subsequent periods a greater and greater number of larvse 

 reached the tadpole stage and an increasing number of these 

 developed normally up to metamorphosis. In Experiment 15f , 

 exposure from fifteen and three-quarters to sixteen and one-half 

 hours after fertilization 19.2 per cent developed normally; in 

 Experiment A- 17^ exposure from seventeen and one-half to 

 eighteen and one-quarter hours after fertilization, 23.2 percent; 

 in Experiment A-25^, exposure from twenty-five and one-half 

 to twenty-six and one-third hours after fertilization (early in 

 the gastrulation period) 73 . 7 per cent ; in Experiment A-29 

 exposure from twenty-nine to twenty-nine and three quarter 

 hours after fertilization, (blastopore moderate or small) 84.6 

 percent; in Experiment A-30^, exposure from thirty and one- 

 fourth to thirty-one hours after fertilization, (small blastopore) 

 90.9 per cent; and in Experiment A- 1, exposure from thirty-one 

 and one-half to thirty-two and one-quarter hours after fertiliza- 

 tion, (blastopore small or closed) approximately 100 per cent. 

 Exposure to the X rays for two successive periods of forty-five 

 minutes each during this period caused all the eggs to develop 

 abnormally and none reached the tadpole stage (Experiment A-20). 



In Experiment B on another batch of fertilized eggs, successive 

 groups of which were likewise exposed for forty-five minutes, the 

 susceptibility proved at first to be much greater than in the batch 

 just described. The causes for this greater susceptibility are 

 uncertain. In the various groups exposed before the twentieth 

 hour after fertilization in only one instance did a larva reach the 

 free swimming tadpole stage, but this went on to metamorphosis. 

 (Experiment B-18|.) In Experiment B-15|, exposure from fifteen 

 and three-quarters to sixteen and one-half hours after fertiliza- 

 tion, one egg failed to complete the process of gastrulation, 3 . 3 

 per cent stopped developing as soon as the blastopore was closed, 



^Hertwig, 1910, has found that exposure to radium for from one-half to one 

 hour causes the blastulae of the oxolotl and the frog to die before gastrulation 

 is complete. 



