THEORIES OF FERTILIZATION 



267 



These parthenogenetic merogons develop to the blastula stage only. Gyno- 

 merogony is the parthenogenetic development of an egg fragment containing 

 the egg pronucleus. 



J. Theories of Fertilization and Egg Activation 



Boveri, T., 1887, 1895. In somatic mitoses, the division center or centro- 

 some is handed down from cell to cell. In the development of the female 

 gamete, the division center degenerates or becomes physiologically incapable 

 of continuing the division of the egg either before or after the maturation 

 divisions. The mature egg thus contains all the essentials for development 

 other than a potent division center. The sperm, on the other hand, lacks the 



REFRACTIVE SUBSTANCE CHROMOSOMES 



WALL OF NUCLEUS 



SUPERNUMERARY SPERM 





•■^v,-.- •;^;;:-; R 



G. 



Fig. 135. Fertilization phenomena in the pigeon. (After Harper, '04.) (A) Germinal 

 vesicle of late ovarian egg. The chromatin material is shown in the center of the vesicle; 

 the nuclear wall is beginning to break down. (B) Spindle of first maturation division. 

 Egg just ovulated and entering the oviduct. Sperm enters the egg at this time. (C) 

 Second polar spindle and first polar body. (D) First and second polar bodies; egg 

 pronucleus reorganizing. (E) Two pronuclei approaching, preparatory to fusion. Sperm 

 nucleus to the left. (F) Two pronuclei fusing. (G) Accessory sperm nuclei to the 

 left of this figure; fusion nucleus to the right. 



