I93I The secondary association of chromosomes 355 



are completely terminalised, giving rise to the two types of terminally 

 associated chromosomes found at diakinesis with one and two terminal 

 chiasmata. 



The repulsion phase begins at earliest prophase and continues until 

 mid-diakinesis, by which stage the chromosomes are sharply defined 

 (Figs. la. and 2a). The spacing of the 18 pairs is fairly regular 

 and moreover the members of each pair are seen to be repelled to 

 approximately the same distance from one another. This distance is 

 considerable, so that the connections are extremely attenuated and 

 only visible in the best fixations. It is probably significant that 

 although a state of repulsion exists between the two chromosomes of 

 any given pair yet the attenuated connections of the bivalent type 

 with two terminal chiasmata do not always stretch directly across the 

 intervening space but are frequently curved in a manner similar to the 

 ' lines of force ' in a magnetic field. 



If all the chromosomes repelled each other equally then we should 

 expect a plant having a number of large chromosomes to show little or 

 no mutual repulsion of the members of each pair, and therefore no 

 attenuation of the terminal connections. This is seen in such illustra- 

 tions as those by BelaS (1928) in Stenobothrus and Newton and Dar- 

 lington in Tulipa (1929). 



From mid-diakinesis onwards the degree of repulsion gradually 

 diminishes. The chromosomes contract further, the members of each 

 pair come close together, and the previously attenuated connections 

 shorten and thicken. Simultaneously there is a general movement of 

 the pairs away from the periphery towards the centre of the nucleus 

 (Figs. lb,c and 2 6). 



P ro-metaphase 



Diakinesis is terminated by a sudden acceleration of this con- 

 verging movement, which results in the extremely close assemblage of 

 the chromosome pairs in the centre of the nucleus (cf . Figs. 2 c, d). 

 This stage is very short. It is characterised by a marked grouping of 

 hitherto unassociated bivalents. 



The chromosomes are now too closely packed to distinguish each 

 separate pair. At this point orientation of the bivalents begins, so that 

 they come to lie in the same direction as the spindle fibres, and by the 

 time the arrangement in one plane for the first division is accomplished 

 the pairs are again dispersed, as seen in a typical equatorial plate. 



Cytologia2, 1931 25 



