6O CHARLES PACKARD. 



period occurs. The number of eggs dividing with the full com- 

 plement of chromosomes falls gradually up to the 3O-minute 

 period. Then it drops very suddenly so that after a treatment 

 of 35 to 40 minutes very few eggs divide with the diploid number. 



The proportion of eggs dividing wholly under the influence of 

 the sperm rises rapidly at the critical period and remains high 

 until prolonged radiation prevents cleavage altogether. Of the 

 eggs which remain uncleft after the others have divided with 

 either the diploid or the haploid number of chromosomes, most 

 are in the condition shown in Fig. 9. These may divide at a 

 much later time but I have no data on that point. It is evident, 

 therefore, that during the critical period the egg nucleus has been 

 injured to such an extent that it can take no part in cleavage, and 

 in some way prevents or greatly retards the cleavage of the sperm 

 nucleus. After that time it acts like a foreign body, exerting 

 very little influence, if any at all. 



The form of the curve shown in the dotted line of the text 

 figure bears a striking resemblance to the curves figured by 

 Hertwig ('i i) in his article on the effect of radiations upon the un- 

 fertilized frog egg. In his experiments Hertwig exposed dif- 

 ferent lots of eggs to preparations of radium of different strengths, 

 and then fertilized them with normal sperm. The criterion in 

 estimating the effect of the treatment was the length of life of the 

 embryos. In the three sets of experiments it appeared that with 

 brief exposures the length of embryonic life varies inversely with 

 the duration of the exposure. Thus, the eggs treated for 5 

 minutes before insemination with a preparation having the 

 strength of 7.4 mg. of RaBr 2 lived for 5 days, at which age they 

 were abnormal in many respects. An exposure of 15 minutes 

 resulted in much greater injuries, and the embryos lived but 3 

 days. But with longer radiation, development was normal 

 through the gastrula stage and the embryos lived 8 or 9 days. 

 They were weak and small but fairly well formed. With weaker 

 preparations of radium the results were less pronounced but of 

 the same order. 



Hertwig's explanation is that brief radiations injure the egg 

 nucleus but do not prevent it from fusing with the sperm nucleus. 

 However, the injury is sufficient to bring about the production of 



