POSITIONS OF CHI ASM AT A 



107 



them to the first chiasma. If these segments are short they are 

 drawn out, as is the case with small chromosomes at diakinesis, into 

 a fine thread. 



The successive loops between chiasmata have, with the increasing 

 thickness and, presumably, rigidity of the chromosomes, come to lie 



Fig. 33. — Diagram to show the chromatid structure of representative 

 types of bivalents at metaphase. A, with unterminahsed, 

 random chiasmata. B, with unterminahsed, locaHsed chiasmata. 

 C, with completely terminalised chiasmata. x, xta : chiasma 

 and chiasmata. [a), with subterminal centromere (represented 

 by arrow) ; {b), with submedian centromere. (From D., 1931 c.) 



in planes at right angles to one another before diakinesis. This 

 condition is still more strongly developed at metaphase, and is 

 indeed the only direct criterion of the existence of chiasmata at this 

 stage in ordinary preparations. 



Metaphase bivalents are therefore found to fall into various 

 types according to (i^ size of the chromosomes ; (ii) position of 



