MOVEMENTS WITHIN BIVALENTS 105 



from diplotene to diakinesis is unaccompanied by any marked 

 change in the structure of the paired chromosomes. The chiasmata 

 are ahnost stationary and are therefore to be found distributed at 

 diakinesis as they arose — either locaUsed or at random. 



In most other organisms a change takes place. This is seen in its 

 extreme form in Primula (D., 1931 a) and Campanula {Gairdner 

 and D., 1931)- It has three obvious characteristics : — 



(i) The total number of chiasmata is reduced in each bivalent. 



(ii) The chiasmata come to be concentrated nearer the ends. 



(iii) These changes taking place pari passu eventually leave all 

 the bivalents associated terminally and the number of chiasmata 

 reduced to the number of ends associated. It still remains true 

 that the connection is not between two whole chromosomes, but 

 between their component chromatids. Each chromatid, which has 

 been connected side-by-side with one partner, is connected 

 end-to-end with another, and hence the terminal association means 

 a change of partner after the lateral association. It therefore has 

 the essential mechanical properties of a chiasma (Ch. XII). 



A true terminal chiasma can be readily distinguished from a 

 sub-terminal one both at diakinesis and at metaphase, when the 

 limbs of each of the chromosomes connected by it are repelling one 

 another ; this happens when they include the centromere. The 

 connections between the chromatids are then reduced to the 

 minimum and are drawn into fine threads by the tension they 

 undergo. In the absence of this tension there is no sharp demarca- 

 tion between the terminal chiasma and the interstitial chiasma that 

 is nearly terminal. 



There can be no doubt that this change of arrangement in the 

 chromatids is essentially the same as that described in certain 

 bivalents of Phrynotettix by Wenrich (1916). It was then said that 

 the chiasma arose from the meeting of splits separating the four 

 chromatids along two planes and that the movement was due to one 

 split gaining at the expense of the other. It may equally be 

 described as two types of association of chromatids replacing two 

 others (a description that will be found most convenient for 

 theoretical consideration) or as the movement of a chiasma along 

 the chromosome towards the end or away from the centromere. 



