562 CELL-DIVISION CHAP. 



becoming attached to the chromosomes. D, perfected nuclear spindle with 

 the two centrospheres at the poles (csph) and the eight chromosomes (chrs) in the 

 equator of the spindle, all now longitudinally split. E, daughter chromosomes 

 diverging from one another, but still united by filaments ; the centrosomes (cs) 

 are already doubled for the next division. F, daughter chromosomes, quite 

 separated from one another, are already beginning to give off processes, only 

 the four of one aspect of the spindle being indicated ; the cell-body is beginning 

 to be constricted. G, end of the process of division ; two daughter-cells (tz) 

 with similar nuclear network (tk) and centrospheres (csph) as in A. (From 

 Weismann's Evolution Theory, adapted from E. B. Wilson.) 



any given species of animal or plant, although it may 

 vary in different species from two to a hundred and 

 eighty-six or more. 



The chromosomes become arranged in the equatorial 

 plane of the spindle and each of them splits along its 

 whole length, so as to form two parallel rods or loops 

 in close contact with one another (D), and arranged in 

 a radiating manner so as to present a star-like figure or 

 aster when the cell is viewed in the direction of the long 

 axis of the spindle : in this way the number of chromo- 

 somes is doubled, each one being represented by a pair. 

 Everything is now ready for division, to which all the 

 foregoing processes are preparatory. 



The two chromosomes of each pair now gradually 

 pass to opposite poles of the spindle (E), two distinct 

 groups being thus produced and each chromosome of 

 each group being the twin of one in the other group. 

 The mechanism by means of which this is effected is not 

 definitely known : possibly the fibres of the spindle are 

 the active agents in the process, the chromosomes being 

 dragged in opposite directions by their contraction : on 

 the other hand, it is possible that the centrosomes may 

 possess an attractive force, or that the movement is due 

 to the contractility of the chromosomes themselves. 



After reaching the poles of the spindle (F) the chromo- 

 somes of each group unite with one another to form a 

 network around which a nuclear membrane finally makes 

 its appearance (G) . In this way two nuclei are produced 

 within a single cell, the chromosomes of the daughter- 



