522 CELL MECHANICS 



the problem of the molecular structure underlying the spindle. That 

 it has a special molecular structure is shown not only by the distribu- 

 tion of its contained water and its reaction to fixatives but also by 

 special experiments. Centrifuged cells show a distortion of the 

 spindle after fixation of the kind that is to be expected of a body 

 that is both fluid and orientated (Schrader 1934 on Cyclops, etc.). 

 Moreover these observations leave no doubt on another point of 

 some importance. The fibres or channels that are visible in certain 

 organisms and indeed with special fixations in all organisms are 

 not uniformly distributed in the spindle but are of two localised 

 kinds at metaphase, viz. those " interzonal fibres " joining, or 

 rather lying between, the two poles, and those joining each pole 

 with the several centromeres of the chromosomes (F. Schrader, 1932 ; 

 Upcott, 1936 a). It seems likely that the differentiation is not 

 strictly confined to these types, but is rather locally exaggerated by 

 the poles and centromeres to give the differentiation we see. 



We may conclude therefore that the differentiation of the spindle 

 is a differentiation of its own water content. Such a differentiation 

 could be brought about by the orientation of molecules of suitable 

 shape such as chain molecules with which water is laterally asso- 

 ciated. Further, this differentiation takes place under the action 

 of centrosomes in the first place and is influenced also by the 

 centromeres. 



We next have to consider how the centrosomes and centromeres 

 can determine the orientation of the spindle. Hardy (1899) pointed 

 out that an internal heterogeneity such as that shown by the 

 structure of the spindle could arise from a stress such as stretching. 

 We shall see, however, from the repulsions inferred in the spindle 

 IM C that it lies in an electrical field. We know also that its heterogeneity 

 depends on a special distribution of molecules of a high dielectric 

 constant, molecules of water. Finally, we know that an orientation, 

 to give a structure of a liquid crystal such as that of the spindle, 

 is affected by an electrical field (Bragg, 1933). It seems more likely 

 therefore from our present knowledge that the differentiation of the 

 spindle depends on the orientation of particles having an anisotropic 

 dielectric constant in an electric field and that this orientation takes 

 place in such a way that the repulsions are most efficiently trans- 



