174 SOME RECENT DEVELOPMENTS 



nucleoplasm, and apparently independent of the 

 main nuclear reticulum. 



As regards the chemical constitution of the 

 various parts of the nucleus, very little is as yet 

 known. The network and the nucleoli take up 

 stains more readily than the rest of the nucleus : 

 the substance of which they consist is hence con- 

 trasted as chromatin with the non-staining nucleo- 

 plasm. In some cases the threads of the reticulum 

 appear to consist of rows of minute chromatin 

 granules arranged in an achromatin basis. 



We are now in a position to consider the changes 

 which occur in the nucleus during the process of 

 indirect nuclear division or mitosis. The first 

 change is that the secondary threads of the 

 reticulum disappear, apparently through being 

 absorbed into the primary threads. The primary 

 threads thus become much more conspicuous, and 

 their arrangement in loops is clearly seen. The 

 threads forming the loops next become somewhat 

 shorter and thicker ; they may also by transverse 

 division become more numerous. There is some 

 reason for thinking that in cells of the same kind 

 from the same animal, the number of threads 

 is constant, but this has not yet been proved to be 

 a general rule. In the epidermal cells of the 

 Salamander twenty-four loops are said to be 

 constantly present. The next stage is an extremely 

 important one, for the discovery of which we are 

 indebted to Flemming. Each of the looped threads 

 splits longitudinally along its whole length into 

 two parallel threads. The pair of threads formed 



