CELLS IN DIVISION 



through the asters. He concludes that the precise pattern of these 

 changes can only be accounted for by supposing the chromosomes to be 

 responsible, most probably by the liberation of some active substance. 

 If the drop in birefringence is really caused by the release of an active 

 substance from the chromosomes, and if it involves the contraction of 

 the spindle fibres, it follows that the chromosomes bring about of their 

 own movement. A number of cases are now known which clearly 

 demand a degree of chromosome autonomy (Schrader^^*^), but as 

 CoRNMAN^fis has pointed out, no existing scheme, except perhaps that 

 of Belar^*^ provides any mechanism for it. In theory at least, Swann's 

 suggestion seems to allow for this possibility. 



The objection is often raised against a traction mechanism, that there 

 is nothing for the chromosomal fibres to pull against. Some authors 

 have tried to overcome this difficulty by supposing the centrioles to be 

 anchored to the cell walls, or to the bulk of the cytoplasm, and in those 

 cells that contain asters this seems not unreasonable ; but it is question- 

 able whether any such mechanism is necessary. The spindle is known 

 to be an appreciably rigid body, held together at either pole by cen- 

 trioles, or possibly by the type of forces operative in tactoids. In either 

 case, it contains, besides the 'chromosome fibre' material, considerable 

 amounts of 'continuous fibre' material, and there is no reason why 

 this interstitial component should not act as the necessary brace against 

 the contraction. The rates of chromosome movement are so slow that 

 the forces involved must be extremely small. Such an idea receives 

 some support from the work on spindle elongation by Hughes and 

 Swann.^^^ a few of their curves show a perceptible shortening of the 

 spindle at or just before the beginning of anaphase. This shortening is 

 just what might be expected if the contraction of 'spindle fibres' were 

 causing a slight compression in the rest of the spindle. On the other hand 

 the majority of their curves do not show a shortening; nor do the curves 

 of Ris^fis 267 Qp SwANN.255 It would bc interesting to know if perceptible 

 shortening occurs more often in those spindles where the chromosomes 

 are large, and the proportion of 'continuous fibre' material is small. 



The extent of the contraction of chromosomal fibres is not always 

 easily determined. In cases' where the centriole is small and clearly 

 defined throughout mitosis, measurement presents no great problem; 

 but where the centrosomes are large and ill defined, as in the sea- 

 urchin, it is more difficult. In Table IV that follows some figures are 

 given for the extent of this contraction, measured from the centre of the 

 centriole or centrosome. In no case is the contraction particularly large 

 compared with that of smooth muscle, which may contract down to 

 about one-tenth of its extended length. 



Curves of the anaphase movement of chromosomes have been 

 obtained by a number of workers. Hughes and SwANN^^a found in the 



127 



