SEPARATION OF BIVALENTS 115 



and with terminalisation (Fig. 34, A-E). For regularity in the 

 separation of the paired chromosomes it is evident that uniformity 

 in chiasma distribution is the essential condition, and the ideal is 

 therefore complete terminalisation. 



Terminalisation and likewise localisation of chiasmata are 

 characteristic properties of species, and variation in the metaphase 

 distribution of chiasmata must be regarded as another instance of 

 the control exerted by the genotype over the behaviour of the 

 chromosome complement as a whole {v. Ch. III). Hence we 

 can see in the very general occurrence of terminalisation an 

 adaptation to the conditions of regular anaphase separation. The 

 opposite advantages of localisation will be considered in relation 

 to the genetical theory (Ch. VII). 



In organisms having complete terminalisation, as a rule, if an 

 occasional bivalent has an interstitial chiasma, it is sharply 

 distinguished at anaphase by its lagging. Such is the reason for 

 the lagging of chromosomes described in the following species. 

 (The list does not include cases where the cause of the appearance 

 of the interstitial chiasma is probably arrest of terminalisation by 

 change of homology in a structural hybrid, Ch. XII.) 



Table 13 

 Lagging of Bivalents with Interstitial Chiasmata 



Phragmatohia fuliginosa 

 Physaloptera sp. 



Stenobothriis spp. 

 Cycas revoluta . 

 Spinacia oleracea 

 Viola striata 

 Prunus hybrids 

 Yucca flaccida 

 Lachenalia glaucina 



Seller, 1914. 



Walton, 1924. (Large X chromosomes in 



homozygous sex.) 

 Belar, 1929 a ; D. and Dark, 1932. 

 Nakamura, 1929. 

 Maeda and Kato, 1929. 

 J. Clausen, 1929. 

 D., 1930 a. 

 O'Mara, 1932. 

 Moffett, 1936. 



The other side of the picture is shown by the precocious separation 

 of bivalents in certain organisms. This is particularly noted where 

 most of the bivalents have interstitial chiasmata. If then an 

 exceptional one has a terminal chiasma, and especially if this is 

 a short chromosome, it seems to divide precociously {e.g., Mus 

 musculus, Painter, 1927 ; Ranunculus acris, Larter, 1932). 



