MITOSIS 115 



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, .) 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 equal 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 



