248 RECENT CYTOLOGY 
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. Ig. 
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 mitosts, 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 
