262 CHROMOSOMES IN HEREDITY: MECHANICAL 



(Fig. 86). They form chiasmata on either side of the centromere, 

 i.e., either between it and the difference, or not. 



TerminaHsation of a chiasma between two arms, one of which 

 lacks an end, gives a symmetrical lateral chiasma (Fig. 86). Such 

 chiasmata have been found in haploid CEnothera (Catcheside, 1932) 

 and in PcBonia (Dark, 1936) as well as in Orthoptera. 



In the light of these observations, we see how unfortunate was 



Fig. 87. — Pachytene pairing in a double interchange heterozygote 

 in Zea Mays, diagram and observed arrangement, showing the 

 differential segment where a chiasma will give a " figure-of- 

 eight," cf. Ch. IX (Brink and Cooper, 1932). 



the attempt made thirty and forty years ago to decide whether the 

 first or the second division was the " reduction division." The 

 behaviour in regard to reductional or equational division of a part 

 of a bivalent depends on the number and relationships of the 

 chiasmata that he between it and the centromere. The behaviour 

 of the whole of a univalent which forms no chiasmata depends 

 directly on the division or lack of division of the centromere. The 

 behaviour of the nucleus as a whole in having a reduction of chromo- 

 some number depends on the succession of two divisions so rapidly 



