Caroline Pellew and Eva Eichardson Sansome 53 



(3) The frequency and distribution of these points agrees with that of 

 demonstrable chiasmata in other organisms (e.g. Tulipa and Stenoboihrus). 



(4) The method of separation of the pairs at anaphase, the chro- 

 matids associated at the attachment constriction always passing to the 

 same pole, leads to the separation of chromatids in the regions distal to 

 the first point of contact. The regions distal to the second point of 

 contact are either not separated or are asymmetrical, according to the 

 relationship of the exchanges at the two points. 



The nature of the associations of four, whether in a ring or a chain, is 

 shown to depend upon the formation of chiasmata between the homo- 

 logous segments. Its appearance is determined by the number and 

 distribution of chiasmata present and by its orientation. 



In about half the cases examined non-disjunction occiirs in the ring, 

 two adjacent chromosomes passing to each pole. It is suggested that 

 this gives rise to non- viable gametes through genetic unbalance, brought 

 about by chromosome segment deficiency and reduplication. 



The writer is indebted to Dr C. D. Darhngton for much help through- 

 out the course of this study. 



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