5o8 



CELL MECHANICS 



arises from those having chiasmata on both sides. Metaphase 

 configurations of chromosomes with unequal arms agree with this 

 assumption. 



This conclusion is favoured by the behaviour found in many 

 organisms, especially those with small chromosomes. The paired 



L.Dk. 



L.Dip. 



Fig. 142. — Percentage frequency polygons of numbers of chiasmata 

 (abscissae) in bivalents of Rosa " Orleans " at late diplotene, 

 early diakinesis, late diakinesis, and metaphase of the first 

 division in pollen mother-cells. (From Erlanson, 1931 c.) 



chromosomes repel one another at diakinesis to such an extent that 

 all the chromosomes, paired and unpaired, appear to be distributed 

 evenly in the nucleus. Where interstitial chiasmata are still present 

 (as in Prunus) it is then found that the repulsion is effectively 

 between the centromeres and the parts of the chromosomes between 

 these and the first chiasma are therefore drawn out into a fine 

 thread while the other parts show no tension. 



