NUCLEAR DIVISION 63 



clear that the precise appearances are sure to vary with the 

 state of nutrition of the cell. 



The nucleus contains the same elements as in Amoeba 

 (see p. 7). It is bounded externally by a delicate mem- 

 brane (iiud. memb.} within w r hich is a granular substance 

 traversed by a fine network, the nuclear protoplasm or 

 achromatin. The chromatin or deeply-staining element 

 presents various appearances in different cells : sometimes 

 it takes the form of a network, sometimes of isolated granules 

 or nucleoli : but in some instances, at any rate, it consists 

 of a long tangled thread (ckrom.} which is said by some 

 observers to be in reality a tube filled with the deeply stain- 

 ing substance to which the name chromatin is properly 

 applied. It should be noticed that a coil of this kind some- 

 what loosely woven might easily be mistaken for a network, 

 and that if it were alternately constricted and dilated instead 

 of being regularly cylindrical it would present the appear- 

 ance of isolated granules or nucleoli. 



The cells in the young growing parts of many plants have 

 much the same structure as this (Fig. n, A) except that the 

 delicate cell-membrane is replaced by a true cell-wall of 

 cellulose. In the older portions of the plant the protoplasm 

 is usually vacuolated (Fig. 8, F). 



The precise changes which take place during the fission 

 of an animal 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. 



First of all, when the cell is about to divide, the achro- 

 matic fibres of the nuclear protoplasm become arranged in 

 the form of a spindle (Fig. 10, A). At the same time the 

 chromatin filament unwinds itself, as it were, forming a loose 

 coil (A) : it then becomes broken up (B), and forms a series 



