CENTRIC REPULSION ' 507 



pure forms must throw light on the differences between chromo- 

 somes which constitute hybridity and would show the principles on 

 which unsupported metaphase observations of hybrids should be 

 interpreted. These expectations have now been fulfilled. 



Chiasmata must be supposed to arise interstitially, as exchanges 

 of partner amongst chromatids, on four grounds : (i) a strictly 

 terminal origin excludes the possibility that chiasmata arise always 

 as exchanges of partner, and demands therefore a dual explanation 

 of their origin ; (ii) a terminal origin is incompatible with the 

 chiasmatype theory, which supposes that chiasmata arise through 

 crossing-over, and therefore interstitially ; (iii) terminal chiasmata 

 are found at late diplotene more frequently in small chromosomes 

 (e.g., fragments in Fritillaria imperialis) than in large ones in the 

 same nucleus, and while it is intelligible that the conditions of 

 terminalisation are different in small chromosomes it is contrary 

 to observations of frequency to suppose that the conditions of 

 origin are different (Ch. V) ; (iv) the terminalisation coefficient 

 of bivalents having different numbers of chiasmata at diplotene in 

 Tulipa shows differences such as are accentuated later in the course 

 of development, and may therefore be attributed to terminalisation 

 (v. Fig. 142). Furthermore, the terminalisation coefficient is higher 

 at the earliest recordable stage in organisms with complete 

 terminalisation like Campanula than in those with slight 

 terminalisation like Tulipa. 



Where several chiasmata are concerned the process has been 

 most clearly shown in Campanula (D. and Gairdner, 1931) where 

 terminalisation is complete, in Tulipa (D. and Janaki-Ammal, 1932), 

 Stenobothrus (D. and Dark, 1932), Zea (D. ig:i4), Spironema (Richard- 

 son, 1934), Culex (Moffett, 1936) and Melanoplus (Heame and 

 Huskins, 1935) where it is incomplete. The observations show the 

 following rules of behaviour : — 



(a) The movement is a movement away from the centromere 

 because when terminalisation is complete a proportion of meta- 

 phase chromosomes are rod-shaped having a terminal chiasma 

 at one end, the rest ring-shaped having terminal chiasmata at 

 both ends. The first type arises from diplotene bivalents with 

 chiasmata all on one side of the centromere. The second type 



