546 CELL MECHANICS 



organisms having large chromosomes, are, however, capable of 

 exact comparison and accessible to experiment. They allow of a 

 general account of the sequence of events in mitosis, which shows 

 how the several agents concerned combine to produce a regulated 

 or balanced result (D., 1935 g). 



1. Two charged centres of repulsion or centrosomes, lying in the 

 cytoplasm, orientate the substrate in their neighbourhood in such 

 a way that it transmits their repulsions most efficiently. This 

 change is produced by the orientation of long chain molecules with 

 water associated laterally to give indirectly an orientation of water 

 which could not be produced directly. An effective orientation 

 therefore depends on the relative concentration of water and chain 

 molecules. The orientated region of the cytoplasm invades the 

 nucleus, when its surface breaks down, and constitutes the spindle 

 of which the centrosomes are the poles. 



2. The result of the orientation is to give a medium of the type 

 of a liquid crystal, with a directionally differential dielectric con- 

 stant. Hence, secondarily, the polarised centromeres of mitotic 

 chromosomes and the paired centromeres of mitotic bivalents 

 are held on the spindle and are orientated along the spindle-arcs 

 where their repulsion is most efficient. 



3. The centromeres bearing a similar charge to the centrosomes 

 are forced to lie midway between them in an equatorial plate, which 

 is an equilibrium position for two poles and one centromere at 

 mitosis and for two poles and two centromeres at meiosis. The 

 repulsion of the poles does not push the centromeres off the spindle, 

 since this, in reducing the repulsion between paired or within the 

 polarised centromeres, would increase the potential energy of the 

 system. 



4. Being themselves charged, the centromeres secondarily react 

 with the centrosomes to modify the shape of the spindle, so that 

 from having been a centrosome-spindle it becomes a joint centro- 

 some-centromere spindle. The different distributions on the plate 

 (solid or hollow) are equilibrium positions determined by the body- 

 repulsions of different numbers of chromosomes and modified by 

 different pole-repulsions. 



5. This repulsion equilibrium is upset by the decline of the polar 



