294 CHROMOSOMES IN HEREDITY: MECHANICAL 



see this beginning in Hyacinthus orientalis, where with a size range 

 of I : 4-2 there is a chiasma range of i : 3-1 (Stone and Mather, 1932), 

 and also in Spironema fragrans (Richardson, 1934) and S cilia 

 peruviana (Sato, 1934). Where the size disparity is greater, the lack 



/ / 

 / / 



(3fe/to6ffMrus) 



10 20 30 40 



50 100 150 200 



Mean /Vumher c^ cM/as/rtaf-a)/f^r bt'ya/ert^. 

 Units of crossing- ot/er ) 



Fig, 97. — Graph showing three types of relationship possible between 

 chiasmata or crossing-over and the length of the chromosomes : 

 direct proportion in Fritillaria imperialis (two clones) ; fixed 

 value with localisation in certain bivalents of Mecostethus and 

 perhaps in some Tettigidcs ; the compromise in Stenoboihrus 

 and perhaps in Drosophila melanogaster " D.m." (From D. and 

 Dark. 1932 ; cf. White, 1933, 1936 ; Hearne and Huskins, 1935.) 



of proportionality is more serious. In Stenoboihrus parallelus (D. 

 and Dark, 1932) the chromosomes with median centromeres are 

 seven times the length of the shortest, yet they have only three 

 times as many chiasmata. The same is true of Syrbula acuticornis 

 (Robertson, 1916), Hyacinthus amethystinus (D., 1932 g), Melanoplus 



