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 nearly 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 

 real 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 1 



