46 



PROTOPLASM AND THE CELL 



arranged themselves around the nucleus in a broken ring (B), and the 

 centrosomes, still connected by the conspicuous rays of the asters, are 

 on opposite sides of the nucleus. The nuclear reticulum, arranged in 

 parallel lines, helps to connect them through the mass of the now fast 

 dividing nucleus. This whole structure, the two asters and the nucleus, 

 called the diaster; the connecting fibrils (possibly contractile), the 

 nuclear spindle. So far no actual division of any structure has taken 

 place, the process up to this point consisting only of the separation and 

 proper preliminary arrangement of structures, either dual or numerous. 

 These processes together, then, make up the "prophases" of mitosis 



FIG. 19 



Diagram of mitotic cell-division. (Flemming. 



The next step, the most difficult to explain (although not to describe) 

 in-" the whole marvellous mitotic process, consists in the longitudinal 

 halving of each of the chromosomes, each lateral half of each particle 

 of chromatin " thread" then beginning to separate from its former half (C) . 

 It gradually makes its way outward (D) through what- was formerly the 

 mother-nucleus, now called the nuclear spindle, to its centrosome and 

 aster, one of which still remains on each side. This separation (E) of 

 the chromosomes into lateral halves is called the "metaphase" and the 

 succeeding events the "anaphases" of mitosis. As the new chromatin- 

 masses approach the centrosomes on either side a constriction (F) is 



