Caroline Pellew and Eva Richardson Sansome 37 



between certain lines of Pisum. These configurations are caused by the 

 cbromosomes touching each other at various points throughout their 

 length. At these points all four chromatids are brought into contact; 

 between the points the chromatids are in pairs. 



a 





Fig. 1. Somatic metaphases of Pisum sativum from root tips of (a) a cultivated variety, 

 (6) Th. 10, (c) a fertile F^ plant from Duke of Albany x Th. 7. The chromosomes are 

 4 to 5fi long, (a and c x 4875; 6 x 4800.) Drawn by Dr 0. D. Darlington. 



An examination of the drawings will show how closely these points 

 of contact resemble the chiasmata, which are to be clearly observed 

 at diplotene in those species in which this stage can be obtained. 

 "Chiasma" is used here to mean "the occurrence of a single exchange of 



