20 GENETICS 



anaphase (Fig. 10) these half chromosomes commence 

 to separate from each other and to move toward 

 the poles, while the mantle fibers shorten. During 

 the anaphase (Fig. 11) the cell body lengthens and 

 begins to divide, while the migration of the half 

 chromosomes toward the poles is completed. In 

 the beginning of the telophase (Fig. 12) 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 re-form around the 

 chromosomes. Finally, at the end of the telophase 

 (Fig. 13) 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. 4). 



5. AMITOSIS 



Amitosis, or the formation of two cells from one 

 without the machinery of mitosis, is comparatively 

 rare. It occurs in certain rather isolated instances 

 among animals and plants, particularly in old cells 

 late in their life-cycle or in cells that are on the road 

 to degeneration. When amitosis takes the place 

 of the more elaborate process of mitosis it is fre- 

 quently, though not always, a signal of the death- 

 warrant for that particular cell. 



6. SEXUAL REPRODUCTION 



The mechanism by means of which two cells unite 

 to make one in sexual reproduction is quite as com- 



