THE NUCLEUS 



67 



origin), by concentration of chromidia into a clump or mass which 

 acquires gradually the structure and organization of a true nucleus. 

 is a frequent and normal occurrence in the life-cycle, as will be 

 seen in subsequent chapters. Those who regard the chromidial 

 condition as the more primitive will see in the formation of secondary 

 nuclei from chromidia the ontogenetic recapitulation of the phylo- 

 genetic origin of the nucleus as a structural element of the cell-body. 

 From the foregoing it is seen that nuclei, in the Protozoa, do not 

 necessarily arise from pre-existing nuclei ; the generalization " Omnis 

 nucleus e nucleo," though it probably holds good universallv for 

 the cells of Metazoa, cannot be maintained for Protozoa if the term 

 " nucleus " be taken in its strict sense. On the other hand, there 



FIG. 32. Arcdla vulgaris, to show formation of secondary nuclei from the chro- 

 midia. A, Ordinary type of individual, with two nuclei and a ring of chromidia : 

 B, example in which secondary nuclei are being formed in the chromidial 

 ring. A 71 , Primary nucleus ; N 2 , secondary nucleus in process of formation ; 

 clir., chromidial ring ; o, aperture of the shell. After R. Hertwig (65). 



is no evidence that chromatin, within or without the nucleus, can 

 ever arise de novo or in any way except from pre-existing chromatin, 

 the particles of which grow and multiply as the result of processes of 

 assimilation such as constitute the most essential characteristic 

 of the living substance generally. 



There is no doubt, however, that chromatin may itself give rise- 

 to other substances of achromatinic nature, and probably of simpler 

 constitution, by a process of breaking down of its complex sub- 

 stance ; and also that there may be present in the cell various 

 substances very similar to chromatin in their properties and charac- 

 teristics, representing stages in the building-up of the complex 

 material of the chromatin-substance. In one or the other of these 

 two ways it is possible to account for bodies in the cell known by 

 names, such as " metachromatinic grains," " chromatoid 



