KARYOKINESIS. 



187 



The chromosomes now pass along the fibres of the achromatic spindle with their apices 

 directed towards the poles of the spindle. The convergent direction which they thus 

 assume as they approach the pole, gives them a star-like appearance when viewed in 



Fig. 216. STAGES. OF METAKINESIS. (Rabl.) 



A, commencing separation of the split chromosomes. 



B, the separation further advanced. 



C, the separated chromosomes passing along the fibres of the achromatic spindle. 



the axis of the spindle, and since there are now two such converging groups in the 

 original nucleus the stage has been termed the dyaster (fig. 211, /, m). 



In the next phase the chromosomes, which are thus passing to form the daughter- 

 nuclei, tend to become convoluted and to arrange themselves in the manner typical 

 of a resting nucleus, with their loops directed towards one pole (that of the half 

 spindle along which they have passed), and their free ends towards the other pole 

 (fig. 217) ; if much convoluted they have the appearance of a skein (fig. 211, ), but 

 this is not so complicated as that of the mother-nucleus. Finally from the convo- 



Fig. 217. FORMATION OF CHROMATIN NET- 

 WORK IN DAUGHTER-NUCLEI. (Rabl.) 



In the upper daughter-nucleus the 

 chromosomes are still separate and com- 

 paratively simple loops ; in the lower one, 

 secondary filaments are growing out from 

 them to form a network. The cell-pro- 

 toplasm is now completely divided. 



arects 

 wound, nucleus 



luted primary loops of the daugh- 

 ter-nuclei secondary filaments are 

 given off (lower half of fig.' 217), 

 which anastomose with one an- 

 other, and convert the chromo- 

 plasm into the network of a rest- 

 ing nucleus (fig. 211, p, q). 



The cell-protoplasm divides during or immediately after the stage of metakinesis. 

 Sometimes a constriction is seen around the equator of the cell, and this becomes 

 gradually more pronounced until the division of the cell is complete. In other 

 cases and this is especially frequent in plant cells, but is also seen in some animal 

 cells (fig. 218, d} points of enlargement make their appearance on the achromatic 

 uniting filaments which connect the daughter-nuclei in the stage of metakinesis, 

 and these points gradually increase in size until they mark a plane of separation 

 between the two halves of the cell, which then divides along this plane. 



The above is the typical mode of division by karyokinesis, but there are many 

 varieties. Sometimes, as in most plant cells and in many animal ova, the achromatic, 

 spindle is very distinct (fig. 218), and the chromatin is in very small amount, the-, 

 chromosomes being in the form of short rods instead of relatively long filaments^ 



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