Caroline Pellew and Eva Richardson Sansome 47 



shows a chain in which one pair of segments has three chiasmata. An in- 

 terstitial, and a terminal chiasma together give the appearance described 

 by Hakansson as a "Querarm." It is possible that this "Querarm" is a 

 relatively constant feature of two particular chromosomes, as Hakansson 



^0# 



5, v3 



2 3 



3 4 



^^^i^^ 



Fig. 8. Three metaphase divisions from semi-sterile plants. The association of four is 

 orientated so that adjacent chromosomes are directed towards opposite poles, x 4000. 



beheves to be the case in the ring he describes. Sometimes the other two 

 chromosomes in the ring have two chiasmata and form a second 

 " Querarm." Fig. 6 b shows such a case, in which both chiasmata are 

 interstitial. It is, however, more usual to find one of them terminal. 



Hakansson (1931) considers that the presence of a chiasma' in a 

 constant position at metaphase indicates that crossing-over is limited to 



