BODY -REPULSIONS 



515 



larger than others, but again little or no fusion of chiasmata 

 occurs, and this repulsion even is scarcely effective to move chias- 

 mata when the centromere lies in an open arm. In organisms 

 with still more terminalisation such as Rosa and Matthiola the 

 proximal loops expand considerably at the expense of the others, 

 and most of the chiasmata are forced to the ends, where the 

 supernumerary ones fuse. Finally, in organisms with complete 

 terminalisation such as Primula and Campanula these changes are 

 already well advanced at the stages of diplotene observed, and all 

 the chiasmata, with rare exceptions, have reached the ends and fused 



J^^^ 



■it 

 abed e 



Fig. 145. — Diagram showing how differential repulsions will determine 

 the movement of chiasmata. G, g, the generalised repulsions 

 between all parts of all chromosomes. L, I, the localised repul- 

 sions between centromeres (marked by arrows). Capitals 

 represent the forces within a closed loop, small letters between 

 open arms. In all types G overcomes g. In Tulipa g + I does 

 not overcome g^ but in Campanula it overcomes even G. In 

 Fritillaria G may be greater than g -\- I. 



before diakinesis. It can therefore be supposed that the generalised 

 repulsion is a common property of all chromosomes after they have 

 divided into chromatids and that the differences in degree of 

 terminalisation observed in different organisms are due to differences 

 in the degree of repulsion between centromeres. 



This difference seems to depend to a less extent on differences in 

 surface charge on the centromere than on differences in the size of 

 chromosomes. Where the chromosomes are small the centromeres 

 are held closer together ; the repulsion between them and the result- 

 ing movement is therefore greater in relation to the size of the 

 chromosomes. In other words, the repulsions between centromeres 

 are related less to the sizes of the chromosomes than to their own 

 functions in cell-division, which, as we shall see later, depend on the 

 size of the cell. 



17—2 



