1 6 PROBLEMS OF FERTILIZATION 



spermatozoon, but he observed the sperm nucleus and 

 its aster so soon after that he had no doubt of the 

 correct interpretation; he also observed the approach 

 of the sperm nucleus and the egg nucleus to the center 

 of the egg and their apparent fusion. 



Fertilization has been previously interpreted as a fusion of 

 two cells, but we have now seen that the most important process 

 involved is the fusion of the two nuclei. The union of the egg 

 nucleus with the sperm nucleus is necessary to produce a nucleus 

 endowed with living forces adequate effectively to stimulate the 

 later developmental processes in the yolk, and to control them in 

 many respects. 



Fol's observations, made partly independently of 

 Hertwig's and partly after the publication of Hertwig's 

 first paper, supplemented Hertwig's in several important 

 respects: (i) He observed the details of penetration of 

 the spermatozoon with a clearness that has never been 

 surpassed for these forms. (2) He gave the first correct 

 account of the maturation divisions and origin of the egg 

 nucleus (Hertwig regarded the latter as being the per- 

 sistent nucleolus of the germinal vesicle). (3) He paid 

 special attention to the origin of the fertilization mem- 

 brane and founded the classic theory that it was an 

 adaptation to prevent polyspermy. (4) He was the 

 first one adequately to present the harmful effects of 

 polyspermy. 



The period initiated by these two men was charac- 

 terized mainly by the repeated demonstration of pene- 

 tration of the spermatozoon, the formation of a nucleus 

 from the sperm head, and the fusion of this nucleus with 

 the egg nucleus. It was also gradually demonstrated 

 that the egg nucleus is genetically derived from the 



