MITOSIS IIS 



mitosis, and may be regarded as preparatory to the next and 

 most essential step, the metaphase. This consists in the divi- 

 sion of the chromosomata into equal halves. (Fig. 23, E.) If 

 they are rod-shaped or horse-shoe shaped the chromosomata 

 split into two lengthwise, if round or ovoid they simply divide 

 into two, and each half so-formed travels in opposite directions 

 along the spindle-threads towards the poles of the spindle. The 

 metaphase now passes into the final stages of the process 

 known as the anaphase. The divergent groups of chromo- 

 somata become closely crowded into a mass at each pole of 

 the spindle in the centre of the astral rays, being connected, 

 across the space previously occupied by the spindle, by a 

 bundle of fibres, known as the connecting or interzonal fibres ; 

 these are supposed to represent the central fibres of the 

 spindle. In plant cells, and in some animal cells {e.g. in 

 cartilage) the interzonal fibres are thickened in the equatorial 

 region of the spindle to form the so-called equatorial plate. 

 The last stages of mitosis are known as the telophase. The 

 groups of chromosomata at each pole of the spindle are 

 reconstituted into a new daughter-nucleus, usually going 

 through the processes of the prophase in a reversed direction. 

 Thus the chromosomata become united to form a spireme, 

 and the spireme breaks up into a chromatic reticulum, the 

 nuclear membrane re-appearing in the spireme stage. Whilst 

 this is going on, the cell-body is divided into two in a plane 

 passing through the equator of the spindle. In plant cells, and 

 some animal cells in which an equatorial plate is formed, the 

 division is effected by the formation of a septum across the 

 cell-body in the plane of the equatorial plate. But in most 

 animal cells division of the cell-body is effected by a simple 

 constriction which gradually deepens and divides the cell into 

 two. The asters generally disappear, or are reduced to a 

 spherical mass surrounding the centrosome, constituting a 

 centrosphere. The results of this truly remarkable process 

 are obvious. The nucleus of each daughter-cell received 

 exactly half the chromatin of the nucleus of the mother-cell. 

 Nor is this all ; the halves into which the chromatin of the 

 mother-nucleus is divided, are not only eqtial halves, but, 

 because of the longitudinal splitting of the chromosomata, are 

 like halves. When division of the cell is complete a resting 

 stage follows, during which the nucleus of each daughter-cell- 



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