XI MITOSIS 545 



viewed in the direction of the long axis of the spindle. Every- 

 thing is now ready for division, to which all the foregoing 

 processes are preparatory. 



The two chromosomes of each pair now gradually pass to 

 opposite poles of the spindle (e, f), two distinct groups 

 being thus produced (g) and each chromosome of each 

 group being the twin of one in the other group. Perhaps 

 the fibres of the spindle are the active agents in this 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 the chromosomes 

 of each group unite with one another to form a network (h) 

 around which a nuclear membrane finally makes its appear- 

 ance (i). 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 (h) — 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. We have therefore to dis- 

 tinguish between direct ^nd, indirect nuclear division. To the 

 latter very elaborate method the name mitosis or karyokinesis 

 is applied : direct division is then distinguished as amitotic. 



In this connection the reader will not fail to note the 

 extreme complexity of structure revealed in cells and their 

 nuclei by the highest powers of the microscope. When the 



PUACT. ZOOL. N N 



