560 CELL-DIVISION CHAP, xn 



fine network of threads extends : this is called the linin I 

 network, and in it are embedded granules of chromatin 

 (ch. g, chr), which are distinguished by their 'strong 

 affinity for aniline and other dyes (compare e.g. Part I, 

 Chaps. VII and VIII). Frequently, as we have seen, 

 one or more minute globular structures, the nucleoli 

 (nucl, kk), occur in the nucleus and are usually looked 

 upon as transient structures and not as actual living 

 parts of the cell : they also have a strong affinity for dyes 

 although often differing considerably from the chromatin 

 in their micro-chemical reactions. 



Close to the nucleus a small globular body, called the 

 centrosphere (csph), enclosing a minute particle or centro- 

 some, is present in the cell ; it is inconspicuous and often 

 difficult to recognise, but probably occurs in all animal- 

 cells which are capable of division. 



Cell-division. The precise changes which take place 

 during the fission of a cell are, like the structure of the 

 cell itself, subject to considerable variation. We will 

 consider what may probably be taken as a typical case 

 (Fig. 147). 



First of all, the chromatin granules (chr) come together 

 so as to form a loose coil or skein, the spireme (B) . The 

 centrosome and then the centrosphere (csph) divide, 

 and from the latter fine cytoplasmic filaments are seen 

 to radiate ; the products of its division gradually separate 

 from one another (C), ultimately passing to opposite 

 poles of the nucleus (D), delicate protoplasmic threads 

 extending from one to the other in the form of a spindle 

 (ksp) . At the same time, the nuclear membrane and the 

 nucleoli usually disappear, and the chromatin skein 

 divides into a number of separate pieces of equal length 

 called chromosomes (C, chrs), the form of which varies in 

 different cases : they may be spherical, rod-shaped, or 

 V-shaped, and, in general, their number* is constant in 



