92 THE STORY OF THE LIVING MACHINE. 



rather closer than in the outer cell substance, but 

 their general character appears to be the same. 

 But a more close study of the nucleus discloses 

 vast differences. In the first place, the nucleus is 

 usually separated from the cell substance by a 

 membrane (Fig. 23, c). This membrane is almost 

 always present, but it may disappear, and usually 

 does disappear, when the nucleus begins to divide. 

 Within the nucleus we find commonly one or two 

 smaller bodies, the nucleoli (Fig. 23, /). They 

 appear to be distinct vital parts of the nucleus, 

 and thus different from certain other solid bodies 

 which are simply excreted material, and hence 

 lifeless. Further, we find that the reticulum 

 within the nucleus is made up of two very differ- 

 ent parts. One portion is apparently identical with 

 the reticulum of the cell substance (Fig. 23, d). 

 This forms an extremely delicate network, whose 

 fibres have chemical relations similar to those of 

 the cell substance. Indeed, sometimes, the fibres 

 of the nucleus may be seen to pass directly into 

 those of the network of the cell substance, and 

 hence they are in all probability identical. This 

 material is called linin, by which name we shall 

 hereafter refer to it. There is, however, in the nu- 

 cleus another material which forms either threads, 

 or a network, or a mass of granules, which is very 

 different from the linin, and has entirely different 

 properties. This network has the power of ab- 

 sorbing certain kinds of stains very actively, 

 and is consequently deeply stained when treated 

 as the microscopist commonly prepares his speci- 

 mens. For this reason it has been named chro- 

 matin (Fig, 23, ^), although in more recent times 

 other names have been given to it. Of all parts 

 of the cell this chromatin is the most remarkable. 



