CELLS IN DIVISION 



such pre-metaphase movements are of the same nature as those in 

 anaphase, but clearly this cannot be answered unless we can form some 

 idea of the nature of the forces by which chromosomes are then moved. 

 Broadly we may distinguish between two kinds of hypotheses of chro- 

 mosomal movement, namely field theories which postulate attractions 

 and repulsions of some kind between chromosomes and spindle poles, 

 usually electrostatic in nature, and those theories which hold that the 

 chromosomes are directly moved by the spindle. It is still true of all 

 such field theories that they rest almost wholly on inference from 

 chromosome movements ; some authors speak of forces within the cell 

 without specifying of what nature they may be, but this practice has no 



Figure 43 Pollen mother cell of Tradescantia after thermal 



shock. One bivalent is not attached to the spindle, and 



does not show the normal 'repulsion' of the centromeres. 



From SwANSON^^i {By courtesy, American Naturalist). 



more than linguistic convenience to recommend it. There is abundant 

 evidence that the spindle is directly involved in anaphase movement, 

 and that unattached chromosomes (Swanson^^^) (Figure 43) or frag- 

 ments of chromosomes (White^^^) do not behave normally during 

 anaphase. 



The chromosomes may be moved by the spindle in two ways, either 

 by shortening of the half spindles between chromosomes and poles, or 

 by interzonal expansion between the daughter chromatids, as in the 

 'Stemmkorper' theory of Belar.^^^ Either contraction of spindle 

 fibres, or growth of a negative tactoid (Bernal^^*) may bring about the 

 former movement. It is probable that polar contraction and interzonal 



115 



