18 



ZOOLOGY 



SECT. 



animals the chromatin is all concentrated in a relatively large 

 central mass with or without other smaller aggregations, or in several, 

 sometimes many, granules of about equal size. The plastin is 

 usually collected together in a rounded mass the nudeolus or 

 plasmosome which may contain some chromatin. 



When the nucleus divides during the process of division of the 

 cell, its contents, more particularly the chromatin, in most cases, 



chr 



ntun 



D 



' ' ' 





' 



FIG. 5. Diagrams illustrating karyokinesis. A, the resting cell, B, C, D, successive phases in 

 the formation and arrangement of the chromatin loops and of the nuclear spindle ; E,F,Q, 

 separation of the two sets of daughter-chromosomes and their passage towards the poles of 

 the spindle; H, I, division of the cell-body and formation of the two new nuclei; c. centro- 

 some ; chr. chromatin; c. pi. cell-plate; nu'. nucleoli; nu. m. nuclear membrane ; s. astro- 

 sphere ; sp. spindle. (From Parker's ttiology, after Flemming, Rabl, &c.) 



go through a remarkable series of changes, to which the term 

 Jcaryokinesis or mitosis is applied. At the time when this mitotic 

 division is about to be initiated, either one or two minute bodies 

 (Fig. 5, A, c) are to be distinguished situated close together in the 

 cytoplasm in the immediate neighbourhood of the nucleus. When 

 only one of these bodies is present at the outset it subsequently 

 becomes divided into two. These are the centrosomes minute 

 masses of a specially modified protoplasmic substance, capable of 



