ARREST OF MOVEMENT 511 



centromeres at a form a chiasma at c, terminalisation will be impos- 

 sible without the particles xyz coming to pair with the particles def 

 and leaving their identical partners. If we suppose that identical 

 partners exert a certain attraction on one another and that dis- 

 similar potential partners exert none [i.e., that the specificity of 

 chromosome attraction observed at zygotene still continues at 

 diakinesis and metaphase), then it is possible that whatever forces 

 determine terminalisation are unable to overcome the resistance to 

 change from identical to non-identical partners, or rather to no 

 partner at all, and in consequence movement of the chiasma is 

 arrested. This conclusion has been borne out by the observation of 

 exceptional interstitial chiasmata in (Enoihera (D., 1931 c) between 

 parts of chromosomes whose distal segments were dissimilar and 

 were therefore, on this hypothesis, bound to prevent terminalisa- 

 tion (Ch. VII). 



Table 72 

 Examples of Arrest of Terminalisation by Change of Homology 



Established cases : 



CEnothera biennis . . . D., 1931 ^. 



Pisum sativum . . . E. R. Sansome, 1932. 



CEnothera blandina , . Catcheside, 1932 ; (haploid) 



1935, Fig. 4b (X-rayed). 



Rosa blanda . . . Erlanson, 1931 c. 



Possible cases : 



Tradescantia virginiana . . D., 1929 c ; Roller, 1932 c. 



Aucuba japonica . . . Meurman, 1929 a. 



Forficula sp. ... Payne, 1914. 



Datura hyhxiA . . . Bergner and Blakeslee, 1932. 



CEnothera (triploids) . . Hakansson, 1930. 



Campanula persicifolia . . Gairdner and D., 1931. 



Dahlia hybrid (6x) . . Lawrence, 1931 a. 



Cardamine pratensis . . Lawrence, 1931 d. 



Zea Ma;jys, haploid gametophyte Beadle, 1931 (cf. Plate X). 



Anthoxanthum odoratum. . Katterman, 1931. 



Zea-Euchlmia derivative . Beadle, 1932. 



