466 CHARLES RUSSELL BARDEEN 



The increasing susceptibility of the ova as the period of cleav- 

 age is approached may likewise be seen by comparing Experiment 

 A-lf, exposure from one and three-quarters to two and one-half 

 hours after fertilization, with Experiment A-f, exposure from 

 three-quarter to one and one-half hours after fertilization. In 

 Experiment A-f, table 4, 61 per cent of the ova completed gas- 

 trulation and 14 per cent did not show marked deformities until 

 after the rudiments of the head and tail were fairly well developed. 

 In Experiment A-lf gastrulation was not completed in a single 

 individual. In 7 per cent there was no cleavage. Since in the 

 control practically all eggs were fertilized there is a possibility 

 that in these eggs showing no cleavage the process was inhibited 

 by the exposure to the rays. In 89 per cent of the eggs develop- 

 ment stopped very early in gastrulation. In 4 per cent there was 

 marked abnormality of gastrulation with a yolk mass projecting 

 through a large blastopore. It is the period of greatest sus- 

 ceptibility. 



Following the first cleavage there seems to be a second period 

 of lessened susceptibility. In Experiment All a lot of eggs 

 which had been recently laid in the lake and which were fertil- 

 ized naturally showed practically all of the eggs to be in the two 

 cell stage. Exposure of this lot of eggs to the X rays for one-half 

 hour caused 73.3 per cent of them to cease developing early in 

 the blastula stage, 11.7 per cent to become spina bifida speci- 

 mens in which organ differentiation was slight, 6.7 per cent to 

 stop developing as soon as the blastopore was closed, and 8.3 

 per cent to develop into very abnormal larvae without well 

 marked heads or tails. 



In Experiment A-2§ eggs exposed from two and one-half to 

 three and one-quarter hours after fertilization likewise showed 

 less marked effects than Experiment A-lf. ,While in the latter 

 experiment no eggs developed beyond the early gastrulation 

 stage in the former 14.6 per cent developed into spina bifida 

 specimens and 4 . 9 per cent into abnormal larvse without definite 

 heads and tails. 



On the other hand, immediately preceding the appearance of 

 the fissure of the second cleavage plane the susceptibility once 



