10 CYTOLOGY chap. 



daughter chromosomes, by which the division of the nucleus as a whole 

 is effected. 



The remaining processes of mitosis are concerned merely with the 

 separation of the daughter chromosomes and the reconstruction of new 

 nuclei out of them. 



In many species the division of the chromosomes is not apparent till 

 a later stage (metaphase), and indeed the moment at which division 

 occurs seems to vary greatly in the different cells of a single organism. 

 It must, however, be remembered that we are dealing with very minute 

 bodies, in which a narrow cleft may easily be obscured by the certain 

 amount of distortion (swelling, shrinkage, etc.) inevitable during fixing 

 and staining. The question of the division of the chromosomes will be 

 returned to again (p. 13). . 



Another striking and important characteristic of the prophase chromo- 

 somes in many animals and plants is their alternate expansion and 

 constriction, giving them the appearance of a string of beads. This is 

 also a very variable phenomenon, generally most conspicuous in early 

 or middle prophase. The beads are known as chromomeres, and are 

 discussed in Chapters V. and VI. 



In the later prophase shown in Figs. 2, C, 3, D, the spireme consists of 

 obviously separate chromosomes. The nucleus, which has been steadily 

 increasing in size since the inception of the prophase, has now attained 

 its maximum volume, and the chromosomes are usually evenly spaced 

 out through it. This stage of the prophase is conspicuous enough to 

 have earned the special name of diakinesis (Hacker, 1897, b). 



Except in the rare cases where the whole mitosis takes place within 

 the nuclear membrane, diakinesis ends with the disappearance of this 

 membrane, so that the chromosomes lie naked in the cytoplasm. 



During prophase the centrosome (in animal cells) divides, if it has not 

 already done so, and the two resulting daughter centrosomes move apart 

 to take up positions at opposite poles of the nucleus. As they separate, 

 connection is maintained between them by fine fines or fibres, the spindle 

 fibres, and at the same time similar fibres radiate out from the centro- 

 somes into the cytoplasm, the asters. The whole system of fibres (together 

 with the centrosomes in animal and certain plant cells) is often known as 

 the achromatic figure. After the dissolution of the nuclear membrane, 

 some of the spindle fibres grow in and attach themselves to the chromo- 

 somes (Fig. 2, D). 



Soon after the disappearance of the nuclear membrane the chromo- 

 somes typically become arranged in one plane, at right angles to the 

 spindle fibres, in the manner shown in Figs. 2, 3, 4. The plate of chromo- 

 somes so formed is known as the equatorial plate. 



As we have already seen, each chromosome is already, or now becomes, 



