INTERCHANGE 151 



diploids with rings or chains of chromosomes the whole course of 

 meiosis and the special genetic properties of the heterozygote 

 produced have now been determined. They may be summarised 

 as follows : — 



(i) Corresponding parts of the chromosomes can be seen to 

 associate in pairs at pachytene, where all of them can be distinguished 

 morphologically. There is an exchange of partner, where the 

 homology changes, like that at pachytene in a tetraploid but 

 obligatory (McClintock, 1931, Zea Mays ; Levan, 1935, Allium 

 ammophilum) . 



(ii) Chiasmata are formed at random in the paired segments at 

 diplotene (Gairdner and D., 1931, Campanula per sicif olio). 



(iii) These may remain nearly stationary (Pellew and Sansome, 

 193 1, Pisum sativum), or they may be terminalised (Campanula, 

 cf. (Enothera, Ch. IX), in which case a simple ring or chain of chromo- 

 somes joined end to end appears at diakinesis (as found in Datura , 

 (Enothera, etc) 



In Campanula, Rhceo and CEnothera unpaired chromosomes are 

 frequently found owing to failure of association of both their ends. 

 In Campanula even a ring of four may be replaced by a pair and 

 two univalents, while in Rhceo and CEnothera a ring of twelve may 

 break up in any of the conceivable ways — into 11 + i, 10 + 2, 

 9 + 3, etc., or 9 + 2 + I, 7 + 3 + 2, etc. 



(iv) The configuration arranges itself on the metaphase plate 

 just as a corresponding configuration would in a polyploid. This 

 means that the behaviour of the chromosomes at metaphase is 

 directly determined by mechanical conditions and is not influenced 

 by genetical conditions. Thus an association of four in Pisum is 

 arranged " non-disjunctionally " in about half the divisions, i.e., so 

 that chromosomes with associated segments will pass to the same 

 pole (Hakansson, 1931 a). This is just like a similar association of 

 four with interstitial chiasmata in a tetraploid Hyacinthus. 



A ring of four in Campanula, on the other hand, arranges itself 

 " disjunctionally " in about two-thirds of the divisions, i.e., so that 

 pairs of chromosomes with associated segments pass to opposite 

 poles (Fig. 52). The same holds good for a quadrivalent in tetra- 

 ploid Primula sinensis. The reason for this difference of behaviour 



