KAKYOKINESIS 



called oxychromatin. The karyoplasm occupies the meshes of the reticulum, and 

 seems to be of the same nature as the fluid part of the cytoplasm. There are 

 often, but not always, one or more rounded bodies staining somewhat differently 

 from basichromatin, the true nucleoli or plasmosomes. 



CELL-DIVISION. While in a few cases it is believed that cells divide 

 directly, by constriction of the nucleus and then of the cell-body (amitosis), the 

 almost universal rule is that they divide by a complicated mechanism called indirect 

 division, mitosis, or karyokinesis. The process is an elaborate device for the exact 

 partition of the chromatin between the daughter-cells. 



Karyokinesis (figs. 3 to 11). The process of 

 not present the same picture in every detail 



doe& 



in 



10 



indirect cell -division 

 all classes of cells, varia- 

 tions occurring accord- 

 ing to the relative size 

 of nucleus and cell- 

 body ; but, apart from 

 i minor variations, there 

 is one type of mitosis 

 which, in certain par- 

 ticulars, differs essenti- 

 ally from that seen in 

 ordinary somatic cells. 

 This, which is known 

 as the heterotypical 

 'I mitosis, is characteristic 

 of the sex-cells, and will 

 be dealt with later ; but, 

 in order that its cha- 

 racter and significance 

 may be more readily 

 understood, a brief de- 

 scription of ordinary 

 mitosis, as it occurs in 

 somatic cells, will be 

 here given. 



The process is ini- 

 tiated by the division of 

 the centrosome (fig. 4). 

 As the two centro- 

 somes draw apart in a 

 direction tangential to 

 the nucleus, protoplas- 

 mic radiations become 



centred on them, and a spindle system of fibres is drawn out between them (fig. 5). 

 The network of the nucleus meanwhile takes the form of an apparently continuous 

 skein (fig. 4), which then arranges itself into loops directed towards the 

 developing spindle system (fig. 5). The loops then break apart at the opposite 

 pole of the nucleus, to form a series of V-shaped filaments or chromosomes (fig. 6). 

 The nuclear membrane meanwhile disappears, the spindle system gradually takes 

 up a position of equilibrium in the centre of the cell, and the chromosomes arrange 

 themselves round the equator of the spindle with their apices applied to it (fig. 7). 

 The chromosomes have in the meantime split longitudinally along their whole 

 length, and now fche two halves become separated from one another, the apices 

 of the daughter-V's being drawn towards opposite spindle-poles (fig. 8). The 



11 



FIGS. 9 TO 11. KARYOKINESIS IN BED BLOOD-CORPUSCLES 

 OF LARVAL LspiDOSiHEN (continued). (T. H. Bryce.) 



