42 



Oenetical and Cytological Studies on Pisum 



(4) The anaphase figures show that the points of contact between 

 the foui chromatids are chiasmata. The two chromatids associated at the 

 attachment constriction pass to the same pole. Where there has been 

 a point of contact at metaphase, at anaphase the chromatids are widely 

 separated distal to that point (distal with reference to the attachment 

 constriction). This shows that the two chromatids associated on one side 

 of the point of contact have not been associated at the other. There must 

 therefore have been an exchange of partners at each point where the four 

 chromatids were in contact at metaphase. There are different ways in 

 which the exchange of partners may occur. Taking a bivalent, which has 

 two chiasmata on one side of the attachment constriction, there are two 



Fig. 5. Single anaphase figures from two semi-sterile plants 696'° and 724'. x 4000. 



possible modes of behaviour. The second chiasma may restore the 

 relationship between the chromatids which existed between the first 

 chiasma and the attachment constriction, or an entirely new association 

 may be set up. The anaphase configurations show the various chromatid 

 relationships which exist between successive chiasmata. Figs. 5 b, and 

 10 b fifth from the left, show the anaphase configurations which result 

 from a metaphase bivalent in which the second chiasma restored the 

 relationship between the chromatids that existed before the first. 

 Fig. 4 b, the first from left, shows the asymmetrical figures obtained from 

 a bivalent in which the second chiasma does not restore the relationship 

 existing before the first chiasma. 



Fig. 5 e is probably derived from a metaphase bivalent with one 



