EFFECT OF RADIUM ON DEVELOPMENT OF CJL-ETOPTERUS. 63 



diploid number of chromosomes. Her explanation for this 

 phenomenon is that the normal sperm chromosomes divided to 

 form the diploid number at some time before the first cleavage. 

 There is no evidence to prove this point. More probably the egg 

 was not radiated sufficiently and the nucleus was comparatively 

 uninjured so that it fused with the sperm nucleus and the sub- 

 sequent cleavage was normal. 



That sperms show varying resistance was shown by Gunther 

 and Paula Hertwig ('13). When methylene blue is allowed to act 



TEXT FIGURE i. 



for 1 8 hours on sperm from the same testis, some of the sperms 

 are killed while others retain their motility perfectly. The 

 same variation in susceptibility is probably found in the egg also. 

 Signs of injury in the chromatin do not appear until the chro- 

 matin increases in volume. The radiated sperm appears per- 

 fectly normal until it commences to grow inside of the eg^. In 

 the case of the sea-urchin it normally does not increase greatly in 

 size until after it has fused with the egg nucleus, and it is not until 

 that time that a radiated nucleus shows its abnormal condition. 

 When the eggs of Chcetopterus are radiated the chromosomes are in 

 the form of tetrads, the two successive divisions of the chromatin 

 having already taken place. The chromosomes divide regularly 

 and do not increase noticeably in size. Polar body formation is 



