SUSCEPTIBILITY OF AMPHIBIAN OVA TO X-RAYS 463 



merit. In these eggs a yolk plug projected from a large blasto- 

 pore. In the rest of the eggs the blastopore was completely or 

 nearly closed. In 10 per cent of the total number development 

 ceased with the closure of the blastopore; in 37 per cent it ceased 

 after the formation of abnormal larvae without definite head or 

 tail rudiments; in 14 per cent abnormal larvae with irregular 

 heads and tails were produced. The abnormal specimens in 

 the various lots resembled those previously described in the sec- 

 tions on the effects of exposure of the sperm and of the ova to 

 the rays. 



The relative immunity during this period of rest is still more 

 strikingly shown in Experiment C-1, in which eggs were exposed 

 for a period extending from one hour to an hour and a half after 

 fertilization. Of these eggs 4 . 7 per cent failed to undergo cleav- 

 age but this probably means little since 6 . 9 per cent of the control 

 specimens were unfertilized. The rest of the eggs passed through 

 the period of gastrulation and over one-half of these appeared 

 normal at the end of two weeks when the experiment was discon- 

 tinued. Of the total number of eggs 4.7 per cent appeared 

 abnormal early in larval differentiation, 19.8 per cent late in 

 larval differentiation, and 11.2 per cent early during the tadpole 

 period. 



In a series of experiments with another lot of eggs the period 

 of ' rest ' as measured by relative immunity to the rays came later 

 than in the two experiments above mentioned. The results of 

 the first two exposures in this series have been described above, 

 Experiments D-j and D-f, table 3. Nearly all the specimens 

 exposed were affected. The third exposure of eggs from this lot 

 was for one-half hour, beginning one and one-fourth hours after 

 the eggs were fertilized. Fifty per cent of these eggs appeared 

 to develop normally and several were followed to metamorphosis. 

 It is probable that in the batch of eggs used in this series of expos- 

 ures the periods of fusion of the nuclei and of rest were later than 

 in the other experiments, so that the period of relative immunity 

 was also later. 



