562 CELL-DIVISION CHAP. 



for division, to which all the foregoing processes are pre- 

 paratory. 



The two chromosomes of each pair now gradually pass to 

 opposite poles of the spindle (E), two distinct groups 

 being thus produced, and each chromosome of each group 

 being the twin of one in the other group. The mechanism 

 by means of which this is effected is not definitely known : 

 possibly the fibres of the spindle are the active agents in the 

 process, the chromosomes being dragged in opposite 

 directions by their contraction : on the other hand it is 

 possible that the movement is due to the contractility of 

 the chromosomes themselves. 



After reaching the poles of the spindle (F), the chromo- 

 somes of each group unite with one another to form a 

 network around which a nuclear membrane finally makes 

 its appearance (G). In this way two nuclei are produced 

 within a single cell, the chromosomes of the daughter- 

 nuclei^ as well as their attendant centrosomes, being formed 

 by the binary fission of those of the mother-nucleus. 



But pari passu with the process of nuclear division, 

 fission of the cell-body is also going on. This takes place 

 by a simple process of constriction (F, G) in much the same 

 way as a lump of clay or dough would divide if a loop of 

 string were tied round its middle and then tightened. 



In comparatively few cases the dividing nucleus, instead of 

 going through the complicated processes just described, 

 divides by simple constriction ; but this seems to occur only 

 in the case of certain highly-differentiated cells or of worn-out 

 cells. We have therefore to distinguish between ///rar/and 

 indirect nuclear division. 1 



In this connection the reader will not fail to note the 



1 To the latter very elaborate method the name mitosis or karyo- 

 kinesis is applied : direct division is then distinguished as amitotic. 



