LIVING MATTER 



131 



tions which are hereditary. This nuclear material is called 

 chromatin (Figs. 70, 71). 



Reproduction of the Cell. There is only one way in which 

 new cells are formed, and that is by a process called cell 

 division. The usual method of division of a nucleus is by 

 a very complicated procedure called mitosis. By an intri- 

 cate series of stages the chromatin of the nucleus becomes 



, Centrosome 

 .Chromatin, 



Chromosomes' 



E " F ^ A " G 



Fig. 71. Diagrams showing a cell undergoing mitosis and division 



absolutely equally divided into two parts, and from each of 

 these a new nucleus is formed. The important steps in 

 this mitosis, or nuclear division, are outlined below. 



Division of the Nucleus. The scattered granules of chro- 

 matin within a nucleus that is about ready to start division, 

 join to form a continuous thread (Fig. 71, B). The thread 

 later breaks up into small masses known as chromosomes 

 (Fig. 71, C). While this has been going on, a small body, 

 the centrosome, in the cytoplasm just outside the nucleus 

 becomes active and forms a structure known as the spindle 

 (Fig. 71, C). While the spindle is being formed the wall 

 surrounding the nucleus breaks down and disappears. The 

 chromosomes, which are thereby set free in the cytoplasm, 



