n] CHROMATIN, CENTROSOMES 21 



of the chromatin. In the "resting" nucleus the chromatin 

 stains very faintly as compared with the chromosomes in a 

 nucleus which is approaching division, probably because at 

 that stage the chromatin is combined with a relatively high 

 proportion of albumin, which separates from it as the 

 nucleus prepares to divide. And the conversion of the oxy- 

 phil substances in the nucleus into chromatin may similarly 

 take place by the separation from them of the nucleinic 

 acid which is one of their constituents. 



Since not only nucleinic acid and its compounds stain 

 with basic stains, but also other organic acids and their 

 compounds, it is not safe to conclude that any basiphil body 

 consists of chromatin. In suitably preserved material all 

 parts of the cell rich in nucleinic acid will take the stain, 

 but bodies containing other acids may do so also, so that 

 other tests may be necessary before concluding definitely 

 that any particular object consists of chromatin. 



Apart from the nucleus, there are certain other structures 

 which are so frequently present in cells of the most various 

 kinds that they are often regarded as typical cell organs. 

 Of these perhaps the most important is the centrosome 

 with its associated structures. These are bodies which play 

 a preponderant part in the mechanism of cell division, and 

 as the next two chapters will be largely devoted to them, 

 they need not be considered further here. Reference must 

 be made, however, to two other groups of bodies which are 

 now known to be of very general if not universal occurrence 

 in cells, and are of a more enigmatical nature. The first of 

 these consists of the structures grouped together under the 

 general name of mitochondria, known also as cbondriosomes, 

 or, >vhen rod-shaped, as chondrioconts. These were first 

 described in germ cells which are developing into spermato- 

 zoa, but recent investigation has shown that they are very 



