MITOSIS 115 



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, and the space 

 previously occupied by the spindle is traversed by a bundle 

 of fibres, known as the connecting or interzonal fibres. 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 chro- 

 mosomata 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 chromoso- 

 mata become united to form a spireme, and the spireme 

 breaks up into a reticulum, the nuclear membrane re-appear- 

 ing 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 sur- 

 rounding the centrosome, constituting a centrosphere. The 

 results of this truly remarkable process are obvious. The 

 nucleus of each daughter-cell receives exactly half the chro- 

 matin of the nucleus of the mother-cell. Nor is this all ; one 

 half of each individual chromosome passes to one, the other 

 half to the other daughter-nucleus. When division of the 

 cell is complete a resting stage follows, during which the 

 nucleus of each daughter-cell grows to the size of the original 

 mother-nucleus, the chromatin sharing in the growth of the 

 whole nucleus ; and when the normal size is attained the pro- 

 phase is again entered into, and a fresh mitosis with a new 

 division of the cell takes place. The behaviour of the cen- 

 trosome demands some attention. This minute body seems 

 to lead the way in cell-division. The two halves into which it 

 divides form the centres of the asters and the poles of the 



