1 8 CELL ORGANS [CH. 



the same as the similarly stained granules to which the term 

 linin is sometimes given. 



The nuclear constituent to which the greatest attention 

 has been directed is the chromatin, the amount and appear- 

 ance of which varies greatly according to the condition of 

 the nucleus. In the "resting" condition, that is, when the 

 nucleus has not yet begun to show any signs of approaching 

 division, the chromatin is finely divided, and in fixed pre- 

 parations scattered in the meshes of the network. Its 

 apparent quantity in this condition is often very much less 

 than when nuclear division is approaching, and there is 

 reason for believing that the granules of chromatin, and 

 the oxyphil granules which make up the greater part of the 

 "linin" threads, are not very different in chemical constitu- 

 tion, and that the latter may be converted into chromatin 

 as division approaches, thus increasing its amount. The 

 behaviour of the nucleus before and during division will be 

 considered more fully in the next chapter. 



In addition to the network, with its granules, two other 

 constituents are commonly, though not always, present in 

 the resting nucleus. These are sometimes spoken of collec- 

 tively as nucleoli, but are of two distinct kinds, known as 

 chromatin nucleoli or karyosomes and true nucleoli or plas- 

 12leson}^2S. Most frequently there is only one of each kind, 

 but two or more may be present, and either one or both 

 may be absent. The karyosome or chromatin nucleolus is a 

 mass of chromatin of varying size, usually round or oval 

 but sometimes less regular in shape. It appears to serve as 

 a reservoir of chromatin from which the chromosomes (see 

 below) may draw part at least of their supply when nuclear 

 division is approaching. In other cases the chromatin nu- 

 cleolus represents a single chromosome which remains com- 

 pact during the "resting" phase of the nucleus while the 



