SHAPES OF BIVALENTS 



109 



their centromeres ; (iii) the number and position of the chiasmata, 

 and especially the distance of the nearest chiasma from the centro- 

 mere. Hence all metaphase bivalents fall into classes of which 

 types are illustrated in the diagram (Fig. 33). The three classes 

 as determined by the position of the chiasmata are as follows : 

 A, those with nearly random chiasmata ; B, those with chiasmata 



t IG. 34. — Diagram showing forms of bivalents at the first metaphase 

 of meiosis. M.P., polar or top view of the metaphase plate. M.E., 

 equatorial or side view of the same. A.E., equatorial view of anaphase 

 showing daughter bivalents (not shown in types R to W, which 

 correspond with earlier forms). Note that constrictions do not show 

 at metaphase except by their position at the centromere. Compare 

 with line diagrams of structure (Fig. 33) as follows : 



localised near the centromere ; C, those with chiasmata confined 

 to the ends or near the ends of the chromosomes, following ter- 

 minalisation. 



These classes are readily explained by reference to the observations 

 of the origin of chiasmata at diplotene and their later movement in 

 terminalisation. It is to be noticed especially that, at metaphase, 



