THE FIGURE-OF-EIGHT 263 



that only one division of the centromeres (and chromomeres) takes 

 place. Two successive divisions, not one special division, are 

 necessary therefore for reduction. It is no longer possible to 

 imagine that by tacking on a third, as in " brachymeiosis," two 

 reductions in the ordinary sense can be produced. 



(ii) Interchange Hybrids. Interchange may occur at any point 

 along a chromosome and the pachytene pairing of interchange 

 heterozygotes therefore shows the centromere lying in one of the 



Fig. 88. — The " figure-of-eight " in Pisum at first metaphase. 

 A chromatid diagram {cf. Plate VI) . The six chromosomes before 

 crossing-over had the constitution : AB-BC-CXD-DF-FE-EXA. 

 Crossing-over has given two new chromatids (or " daughter- 

 chromosomes ") : AXD and CXE [cf. Ch. IX, from E. R. 

 Sansome, 1932.) 



Note. — 'The chiasma formed in the X segment is represented 

 as having been arrested in terminalisation by the change of 

 homology. 



two arms of the cross formed by the four chromosomes. If chiasmata 

 are not formed between the centromere and the point of interchange 

 a simple ring is formed at metaphase, which segregates, as we have 

 seen. But if a chiasma is formed in one of these segments, then, 

 owing to the crossing-over which has occurred, simple disjunction 

 is rendered impossible (Fig. ^%). There must always be " chromatid 

 non-disjunction " (Sutton, 1935, cf. Sansome, 1933), whichever way 

 the four chromosomes arrange themselves. The occurrence and 

 length of such interstitial segments is therefore important for the 

 behaviour and especially for the fertility of simple interchange 



