THE MITOTIC CYCLE 



(Plate XI (14) ). It is difficult to see how elastic chromosomal fibres 

 could be attached simultaneously to widely scattered chromosomes 

 with the appropriate degree of tension to bring each finally into the 

 metaphase position. 



Observations such as this suggest that the metaphase configuration 

 is determined by forces more complex than elastic tensions in the 

 chromosomal fibres, though it is difficult to suggest alternatives. A 

 significant point about the movements of chromosomes is their extreme 

 slowness. It is usual to envisage these movements in terms of their 

 appearance in speeded-up films, but in fact the rates are never more 

 than a few microns, and more often only fractions of a micron per 

 minute. Rates of this order of magnitude are quite incompatible with 

 the existence of tensions as ordinarily understood. They are, however, 

 of the right order for the building up and breaking down of orientated 

 structures. It seems more likely therefore that the metaphase configura- 

 tion is brought about by the forces of aggregation at work between the 

 various hydrated fibrillar systems combined in the spindle. In trivalents, 

 the existence of two orientated systems on one side of the chromosome 

 and only one on the other, would presumably upset the normal balance 

 and might account for Ostergren's findings. The observation of 

 Hughes and Swann^^^ that metaphase chromosomes in chick cells 

 move very slowly up and down the spindle on either side of an equili- 

 brium position may also be relevant to such an idea. Elastic fibres 

 should not give rise to such a slow movement, whereas shifts in the 

 precise patterns of orientation in the spindle might conceivably do so. 



Although in one respect there seems to be a case for regarding the 

 metaphase spindle as a complex dynamic system, in another respect it 

 is certainly static. Having grown to a certain size in prophase, it 

 remains so throughout metaphase, whether this lasts for only a few 

 minutes, or for a much longer period. Chick cells in tissue culture for 

 instance, may stick in metaphase for hours ; some marine eggs are shed 

 in the metaphase of the first maturation division, in which condition 

 they remain until they are fertilized. In no case, however, does the 

 metaphase spindle show any tendency to increase in size, and as regards 

 the intake and orientation of new material, it is clearly in a state of 

 equilibrium. 



With the start of anaphase, this equilibrium is upset, and the bire- 

 fringence of the spindle declines as the chromosomes move apart. This 

 drop in birefringence was first noticed by Schmidt,^^! ^52 ^nd was 

 interpreted by him as being due to a contraction of the extended protein 

 elements of the spindle. An essentially similar view has been put for- 

 ward by SwANN,^^^ who has made a quantitative study of the pheno- 

 menon. He finds that the drop in birefringence starts from the equator 

 of the spindle, and moves towards each pole; it then spreads outwards 



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