172 SOME RECENT DEVELOPMENTS 



has been described by Ranvier in the leucocytes of 

 the Axolotl, in which case the process occupied an 

 hour and a half ; by Waldeyer in Infusoria ; and 

 since then by many other writers. 



Of recent years attention has been more 

 specially directed to a far more complicated series 

 of changes, which have been seen by many 

 observers to occur in the nucleus during division, 

 and which are spoken of as karyokinesisor mitosis, 

 or indirect nuclear division, in contrast with the 

 former or direct mode. To understand these 

 more complicated processes, we must first describe 

 the structure of the nucleus as determined by more 

 recent and more detailed investigations. It now 

 appears that in the cell-nucleus there may normally 

 be distinguished four elements : the network or 

 reticulum, the nucleoli, the nuclear membrane, and 

 the nucleoplasm. 



The network or reticulum consists of finer or 

 coarser threads, which differ in their arrangement 

 in different cells. In epithelial cells from a 

 Chironomus larva, Balbiani found the nuclear 

 reticulum to be one single complexly coiled thread. 

 In a large number of other cases it has been 

 found by Rabl and others that the typical arrange- 

 ment is as follows : Two kinds of threads may be 

 distinguished ; thicker primary threads and thinner 

 secondary ones. The primary threads may be 

 twenty or more in number : each is folded on 

 itself so as to form a loop, and the several looped 

 threads are placed in a symmetrical manner in the 

 nucleus ; the loops being arranged around one pole 



