222 GENETICS 



envelopes of protective or nutritive function, and it is 

 usually distended with stored up yolk, in consequence 

 of which it is comparatively large and stationary. 

 The result is that whatever locomotion is necessary to 

 bring the two cells together for union devolves upon 

 the sperm-cell. Consequently the sperm-cells are prac- 

 tically modified into nuclei with locomotor tails of cyto- 

 plasm, and frequently, in addition, with some structural 

 modification for boring a way into the egg-cell. They 

 are, moreover, much more numerous than the egg-cells, 

 so that although many go astray, never fulfilling their 

 mission, the chances are nevertheless good that some 

 one of them will reach the egg and effect fertilization. 



Ordinarily only one sperm enters the egg, but when 

 several succeed in penetrating into the egg-cytoplasm 

 only one proceeds to combine with the egg nucleus, that 

 is, only one sperm nucleus is normally concerned in the 

 essential process of fertilization. 



It was formerly thought by the school of "ovists" 

 that in fertilization the essential process is a stimula- 

 tion of the all important egg by the sperm. The 

 opposing school of "spermists," on the other hand, re- 

 garded the egg simply as a nutritive cell the function 

 of which is to harbor the all important sperm. It is 

 now known that both the egg- and the sperm-cell are 

 equally concerned in fertilization, which consists in the 

 union of their respective nuclei within the cytoplasm of 

 the egg. 



