THE CARRIERS OF THE HERITAGE 



54) these half-chromosomes commence to separate from 

 each other and to move toward the poles, while the 

 mantle fibers shorten. During the anaphase (Fig. 55) 

 the cell body lengthens and begins to divide, while the 

 migration of the half-chromosomes toward the poles is 

 completed. In the begirmmg of the telophase (Fig. 

 56) the half-chromosomes grow until they attain full 

 size and the division of the cell body into two parts 

 becomes complete. The mantle fibers have disappeared 

 and the nuclear membrane begins to reform around 

 the chromosomes. Finally, at the end of the telophase 

 (Fig. 57) the nuclear membrane becomes complete, 

 the chromosomes break up into a chromatin network, 

 and two resting cells take the place of the single one 

 with which the process began (Fig. 48). 



5. SEXUAL REPRODUCTION 



The mechanism by means of which two cells unite 

 to make one in sexual reproduction is quite as com- 

 plicated as that 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 ^ermatozoan respectively, which 

 take part in producing a new organism. These cells 

 are structurally unlike each other in nearly every par- 

 ticular, 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 re- 

 cently determined by van Beneden in 1883. 



The egg-cell is often supplied with one or more 



