20 



THE GROWTH OP ANIMALS 



myriads of cells. Consequently, the one cell with which 

 they began has been increased in number, many times. 

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

 crease in number is important and interesting. 



Perhaps the simplest manner in which cells may increase 

 in number is shown by the amceba. We have seen that a 



new amoeba is formed 

 simply by division, or 

 fission of the body. 

 In this process of the 

 formation of new 

 cells, no remarkable 

 changes occur. But 

 in the case of higher 

 animals, when a cell 

 divides, the nucleus 

 usually passes through 

 a remarkable and 

 complicated series of 

 changes to which the 

 term karyokinesis, or 

 mitosis, is applied 

 (Fig. 7). A spindle- 

 shaped body with a 

 starlike organ at each 

 end forms in the cell. 

 The substance of 

 the nucleus, which 

 has meanwhile been 

 transformed to rodlike bodies called chromosomes is then 

 drawn to the spindle. Finally, each of the chromosomes 

 splits in two equal parts, and one half of the resultant 



Fig. 7. — Stages of cell division by mitosis. 

 Diagrammatic. 



