CONDITIONS OF MOVEMENT 509 



{b) All the chiasmata, both near the centromere and further 

 away, move at the same time towards the ends. The nearer ones 

 do not catch up and meet the further ones. Thus the first change 

 observed in terminalisation is that the distal chiasmata become 

 terminal — without any reduction in the total number through a y^ 

 fusion taking place (e.g., in Fritillaria, Tidipa, Stenobothrus) . 

 Moreover, if the proximal chiasmata caught up the distal ones they 

 would in a proportion of cases cancel one another out, for the 

 changes of partner at adjacent chiasmata can often be seen at 

 anaphase to be reciprocal. The movement is not, therefore, merely 

 an opening out of the proximal or centric loop at the expense 

 of the distal loops, but a simultaneous movement of all chiasmata 

 towards the ends and leading to fusion only at the ends. 



(c) In Fritillaria and Stenobothrus there are long chromosomes 

 with several chiasmata having incomplete terminalisation and very 

 short ones with single chiasmata having complete terminalisation. 

 In neither is there any reduction in the total number of chiasmata. 

 There is merely a change in the distribution, the interstitial chias- 

 mata nearest to the ends becoming terminal. In Campanula, where 

 several chiasmata are completely terminalised, they all move 

 towards the ends before they fuse. They must therefore fuse at the 

 ends. Where there are two interstitial chiasmata fusing with one 

 terminal one they presumably do so simultaneously, since in the 

 intermediate stages the diminishing distal loops are always of the 

 same size in any one arm. 



We now have information which shows in a general way the 

 changes that may take place in the course of terminalisation. 

 Numerous conditions have been found to influence these changes 

 in each paired arm of a bivalent, of which the following are the 

 chief : — 



[a) Length of the arm. 



(6) Original frequency and distribution of chiasmata. 



(c) Agreement in the homologous linear sequence of the pair. 



(d) Rate of movement of the chiasmata. 



(a) Length of the Arm. In Nicandra (Janaki-Ammal, 1932) 

 terminaHsation is complete in the shortest chromosome pair at late 

 diplotene, and in the other short pairs at diakinesis, while in the 



