PRE-REDUCTION AT THE CENTROMERE 261 



0/}//ga/-ory facultative 







Fig. 86. — Diagram showing different types of division of unequal 

 chromosomes owing, on the assumption of chiasmatypy, to the 

 different relationships of the inequality, the centromere and the 

 chiasma, which may or may not be formed between them. 

 (Where several chiasmata are formed the possibilities are more 

 numerous.) The arrows represent the direction of the change 

 undergone by the bivalent between diplotene and metaphase 

 with terminalisation (incomplete in B). The inequalities and the 

 centromeres are shown blank. A , first division regularly reduc- 

 tional owing to the centromeres lying next to the inequality. 

 B, first division regularly equational (second division reduc- 

 tional) owing to the centromeres lying at the opposite end from 

 inequality and one chiasma being formed between them. C, 

 first division reductional or equational according to whether a 

 chiasma is formed on one side of the centromere or the other. 

 Co shows a lateral chiasma. A, Trimerotropis, Circoieitix, Acri- 

 dium, Stenoboihrus, most sex chromosomes (and autosomes 

 fused with sex chromosomes). B, Phrynotettix , chromosome 

 " B " ; Melanoplus (Hearne and Huskins, 1935) ; C, Phry- 

 noUetix, chromosome " C " ; Mecostethus gracilis and Trimero- 

 tropis citrina (Carothers, 1931) ; Stauroderus (D., 1936 d) ; 

 Peziza (Matsuura and Gondo, 1935). 



and sometimes not. Such bivalents are found frequently in 

 Orthoptera, and we can now see why they behave in this way 



