32 CELL DIVISION [CH. 



the nuclear membrane disappears the chromosomes show 

 clear evidence of a split, but commonly as the chromosomes 

 shorten and thicken it becomes obscured, and is not recog- 

 nisable in the early equatorial plate stage. Now, however, 

 the split reappears, and each chromosome becomes accu- 

 rately divided into two longitudinal halves. The halves 

 then separate, and, possibly by the contraction of the 

 spindle-fibres attached to them, travel towards the centro- 

 somes or poles of the spindle, one half of each chromosome 

 going towards each pole (PL II, E, F). Each daughter group 

 of chromosomes thus contains the same number as was 

 present in the equatorial plate. As the chromosomes travel 

 to the poles, the interzonal spindle-fibres are seen still run- 

 ning continuously from pole to pole, and as the chromo- 

 somes approach the poles these interzonal fibres often 

 become somewhat thickened in the equatorial region of the 

 spindle. When the chromosomes get near the poles of the 

 spindle they become crowded together, and often change 

 their form; in some cases they become swollen and vacuo- 

 lated, each looking like a minute nucleus, in others they 

 appear to send out branches which begin to interlace with 

 others of neighbouring chromosomes. Gradually from this 

 crowded mass, by further swelling and coalescence of the 

 chromosomes, a nucleus is formed, and at the same time the 

 interzonal spindle fibres and the rays of the asters become 

 indistinct, and the cytoplasm of the cell begins to divide, 

 the line of division running exactly across the plane of the 

 equator of the spindle (PL II, F, G; Text-fig. 4, p. 38). 



Put quite shortly, then, the main stages of mitotic nu- 

 clear division are as follows: First, the chromatin of the 

 nucleus becomes aggregated either in isolated masses or in 

 a spireme, in preparation for the formation of chromosomes, 

 and at the same time the centrosome appears, divides into 



