562 CELL MECHANICS 



these external movements is also to be inferred from the internal 

 rearrangements implied by the spiralisation cycle. 



While the iso-electric point of the surface molecules will determine 

 the surface charge at any one point, other things being equal, 

 the surface charges at different points will, of course, interact 

 and cause a secondary distribution not directly conditioned by 

 surface molecular structure. To this is probably to be ascribed 

 the " precocious condensation " or staining of the proximal parts of 

 chromosomes irrespective of their internal constitution. Nor must 

 we forget that this behaviour is correlated with precocious division 

 of these parts, division being itself controlled by electrical conditions, 

 and as we have seen, in certain cases by the possession of a centro- 

 mere. Still more important will be the effect of a localised charge on 

 the viscosity of the chromosome thread and its transmission of 

 torsions. The control of the distribution of crossing-over by the 

 centromere must depend partly on this property. 



Thus at every phase of their life one particle, the centromere, 

 governs the activities of the chromosomes. It can do so because 

 it is in them but not of them. The centromere agrees with the 

 chromomeres and the chromosome thread in its property of linear 

 arrangement and regulated reproduction. It disagrees with them 

 in its cycle of reproduction and repulsion, its surface charge and 

 staining capacity, its lack of longitudinal cohesion and spiralisation. 

 In all these respects it is related rather to the centrosomes, from 

 which it differs merely in its position on the gene-thread. On 

 these cardinal similarities and differences of structure and behaviour 

 depends that co-ordination of centric and genie reactions which 

 makes reproduction and heredity possible. 



Cell genetics led us to investigate cell mechanics. Cell mechanics 

 now compels us to infer the structures underlying it. In seeking 

 the mechanism of heredity and variation we are thus discovering 

 the molecular basis of growth and reproduction. The theory of 

 the cell revealed the unity of living processes ; the study of the 

 cell is beginning to reveal their physical foundations. 



