THE CARRIERS OF THE HERITAGE 21 



plicated as tliat of mitosis by which one cell is trans- 

 formed into two. 



In sexual reproduction there are two kinds of germ- 

 cells, the egg and the spermatozoan respectively, 

 which take part in producing a new organism. These 

 cells are structurally unlike each other in nearly 

 every particular, but each is a true cell, which von 

 Kolliker made clear as early as 1841, and each has 

 typically the same number of chromosomes in its 

 nucleus, a fact more recently determined by van 

 Beneden in 1883. 



The egg-cell is often supplied with one or more 

 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 practically nuclei with locomotor tails of cyto- 

 plasm, and frequently, in addition, with some struc- 

 tural 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 cyto- 

 plasm only one proceeds to combine w^ith the egg 

 nucleus, that is, only one sperm nucleus is normally 

 concerned in the essential process of fertilization. 



