20 



THE GROWTH OF ANIMALS 



ni)aiads of cells. Consequently, the one cell with which 



they began has been increasetl m number many times. 

 The manner in which the cells in an animal's body in- 

 crease in nunil^er is important and interesting. 



Perhaps the simplest manner in which cells may increase 



in number is shoA\'n l^y the ama' 



)a. 



We have seen that a 

 new anitt'ba is formed 

 simply by tlivision, or 

 fission of the body. 

 In this process of the 

 formation of new 

 cells, no remarkable 

 changes occur. But 

 m the case of higher 

 animals, when a cell 

 ( livides, the nucleus 

 usuall}' passes through 

 a remarkable and 

 complicated series of 

 changes to which the 

 term l^aryokinesis, or 

 mitosis, is applied 

 (i'ig. 7). A spindle- 

 siiaped body with a 

 starlike organ at eacli 

 end forms in the cell. 

 The substance of 

 the nucleus, which 

 has meanwhile been 

 transformed to rodlikc In. dies called ('hroniDsoiiics, is then 

 drawn to the spindle. Finally, each of tlie ciiromosomes 

 splits in two equal pa.rts, and one half of the resultant 



Fic. 7. — Stages of coll diviriion by mitosis. 

 Diugramixiatic. 



