FAILURE OF CHI ASM AT A 



127 



proportionate to the length of the chromosome, or at least the 

 length paired at pachytene, but varying in different nuclei both in 

 number and position. The obvious conclusion is therefore that 

 failure of chromosome association and its asymmetry is due to 

 failure of chiasm^a formation owing to reduction in the length paired 

 at pachytene, and hence that any two chromosomes are associated 

 by virtue of a chiasma formed between them (D., 1929 h). 



In order to test this hypothesis a closer analysis of chiasma 

 formation in triploid Tulipa and Hyacinthus has been attempted 



^h 



^^%l^W\\^^i{ 



I fj 



Xhx: 9.2 8.2 4.. 3.1 3.2 1.0 I.I 2.1 2.\ l.oj.o 



Fig. 41. — Bivalents and multivalents in Hyacinthus, side view 

 (2w = 4,r — 2 = 30) ; four types of L, two of M and two of S. 

 Four chromosomes of each type except L^and M. x 2000 (after 

 D., 19296). 



(D. and Mather, 1932 ; Stone and Mather, 1932). It is found that 

 in Tulipa the proportion of univalents (with no chiasmata) as well 

 as of chromosomes with different numbers of chiasmata can be 

 approximately predicted on the assumptions : — (i) that the lengths 

 of the chromosomes which pair at pachytene vary, as might be 

 expected, from the randomness of association observed at this stage 

 amongst the three chromosomes (only two of which can be paired at 

 any one point), but the variation is very high, as though it were 

 due to random association of a small number of " pairing blocks " ; 

 (ii) that the numbers of chiasmata formed in these paired lengths 

 vary in the usual way ; and (iii) that the association of the 



