I93I The secondary association of chromosomes 353 



(1929) and, in particular, Darlington (1930b) has since established 

 the nature and relation of prophase and raetaphase pairing, and it is 

 now possible to demonstrate the occurrence of two different types of 

 chromosome association in polyploids. 



We may define these two modes of association as follows : 

 Primary association 1) arises from prophase pairing and 



2) determines segregation. 



Secondary association 1) is a post-synaptic phenomenon 

 and 2) does not affect segregation. It is a differential approximation 

 of the bivalents in the equatorial plane. 



Secondary association is of widespread occurrence in polyploids, 

 and provides a valuable criterion of homology in hybrid polyploids. It 

 has been illustrated but not appreciated by numerous workers. The 

 purpose of this paper is threefold, 1) to present an account of the 

 observations substantiating the secondary association of chromosomes 

 and to distinguish it from primary association 2) to make a general 

 survey of the literature and drawings relating to this phenomenon, and 



3) to discuss its implications. 



Methods 



The observations have all been made on pollen mother-cells. Ma- 

 terial was fixed in Carnoy-Flemming and Carnoy- 2 B.E., sectioned in 

 paraffin wax at 16 ;u, and stained by the gentian-violet method. All the 

 P.M.C. counts have been verified by observations on root-tip prepara- 

 tions. Drawings were made at bench level with a Spencer Abbe 

 camera lucida, a Leitz 2 mm. objective (N.A. 1.4) and a Zeiss x 30 com- 

 pensating eyepiece to give a magnification of 4100. All drawings are 

 reduced to a magnification of 3400. The finer structural details have 

 been omitted in most of the drawings which show the chromosomes in 

 silhouette. I am indebted to Mr. L. La Cour for the preparation of the 

 material. 



Observations 



Dahlia Merckii (2n=36). 



In these studies the essential facts of secondary association have 

 been deduced from observation on several species of Dahlia. To avoid 

 needless repetition, the cytology of D. Merckii will be dealt with in 

 detail, that of the other plants being described only as it differs from 



