NUCLEAR DIVISION 231 



up of a material of comparatively weak staining 

 capacity ; embedded in this substance are numerous 

 granules of a very intensely staining material which 

 is known as chromatin. There are strong reasons for 

 believing that the chromatin of the nucleus is of 

 special importance from the point of view of the 

 mechanism of heredity. This reticular structure of 

 the nucleus is indicated in a diagrammatic fashion in 

 Fig. 19. 



Further light is thrown upon the detailed structure 

 of the nucleus by the changes which become visible 

 during the process of nuclear division. This process, 

 which is known as mitosis, we must now proceed to 

 describe. 



In the description of mitosis which follows, the 

 account of this process has been somewhat generalized 

 and simplified, and Figs. 19 to 26, which illustrate the 

 phenomena, are purely diagrammatic. It is hoped 

 that the most important features of this complicated 

 process may be in this way rendered comprehensible ; 

 and although in different organisms considerable 

 variations in the details of the process are to be met 

 with, yet in their general features all ordinary mitoses 

 in animals and plants are believed to conform to the 

 essential type of our description. 



The first change in the appearance of the nucleus 

 which indicates that a division is about to take place 

 consists in a rearrangement of the chromatin network, 

 which now takes on the appearance of a tangled 

 thread (Fig. 20). The outwardly-directed loops of 

 this skein often correspond to the separate portions 



