THE NUCLEUS 247 
In entering into a detailed description of the changes 
which take place in the finer structure of the nucleus, 
it must be clearly understood that the more minute 
features alluded to are only to be seen with any degree 
of definiteness in dead cells which have been killed 
practically instantaneously by the action of some 
powerful chemical poison. Under suitable conditions 
it is believed that treatment of this kind fixes the con- 
stituent parts of the nucleus in very nearlv the same 
relative positions as they occupied in life at the 
moment immediately preceding the death of the cell. 
The tissues containing the cells to be examined are then 
usually cut into very thin sections, and other chemicals 
are applied to them, the result of this treatment being 
to stain different parts of the nucleus of different 
colours and with different degrees of intensity. “It is to 
the behaviour of the structures thus made visible that 
our description applies, since it is impossible to follow 
these changes in actually living cells except to a very 
imperfect extent. It may be pointed out, however, 
that we have every reason for believing that the 
differential effect produced by the processes of fixing 
and staining only serves to render more clearly visible 
rea] differences which actually existed during the life 
of the cell, and some indications of many of these 
differences have even been actually seen in living cells 
under exceptionally favourable conditions. 
The nucleus, when treated in the manner described, 
is seen to be built up of a network of branching fibrils, 
the meshes of which enclose a comparatively clear and 
hyaline substance. The fibrils of the network are made 
