TERMINAL AFFIXITY 517 



(hi) The Theory of the Terminal Chiasma. The conditions 

 under which interstitial chiasmata are converted into terminal 

 chiasmata are of three kinds, of which four examples may be 

 given (illustrated diagrammatically in Fig. 146) : {a) The chiasma 

 is single, and there are two associations of chromatids. 

 (b) There are two chiasmata, and therefore three associations of 

 chromatids, but the third is a continuation of the first. The 

 chiasmata are then comparate. They result from reciprocal or 

 complementary crossing over, (c) There are two disparate chias- 

 mata : the second exchange does not restore the relationship found 

 before. One chromatid which has crossed over at the first also 

 crosses over at the second chiasma. The chiasmata are also bound 

 to be disparate where (as in (^) ) a third chromosome associates 

 at the second chiasma with one of the other two. 



The maintenance of terminal association following the movement 

 to the ends by chiasmata having all these different relationships leads 

 to the following conclusions : (i) The proximal or penultimate 

 association replaces the distal or ultimate one in terminalisation 

 (Fig. 146 {a) ) in every particle save the terminal one. (ii) Compar- 

 ate chiasmata (Fig. 146 (b) ) do not cancel one another out, but 

 the penultimate association replaces the ultimate one. (iii) 

 Disparate chiasmata terminalise without replacement of the 

 ultimate association. The penultimate association simply dis- 

 appears (Fig. 146 (c)). 



These three imexpected properties can all be explained on a 

 single assumption : the terminal particles have, unlike the 

 intercalary particles, a double affinity, lateral and terminal ; the 

 latter is satisfied only on terminaHsation. That terminal particles 

 should inherently have a special affinity not found in intercalary 

 ones does not seem improbable on analogy with attraction based 

 on chemical, electro-magnetic or surface tension phenomena, 

 though we need not inquire for the moment which kind of attraction 

 is here concerned. 



The result of terminal affinity will be that when one thread 

 displaces another from association with a third, either the end of 

 the laterally displaced thread will retain its lateral association as a 

 terminal one (as in case {a) with a single chiasma, or in case [b) where 



