1931 The secondary association of chromosomes 871 



Indeed if the number of the chromosomes is much greater than in the 

 diploid then the chances of false association, probably arising from 

 irregularities of fixation, are correspondingly increased. With good 

 fixation, secondary association is easily determined ; with a slightly 

 doubtful or variable fixation a final conclusion can only be made after 

 careful comparison of a number of plates. 



That secondary association is not essentially an artifact is shown 

 by the following :— 



1) It is found to be constant in the best of fixations. 



2) The average number of chromosomes per association and 

 average frequency of each kind of association (whether of 4, 6 or 8 

 chromosomes, etc.) is characteristic for a given species but varies from 

 species to species. (Cf . Dahlia Merckii, D. variabilis, Pyrus Mains etc.). 



3) At metaphase associated bivalents are seen to be similar in 

 size and configuration, i.e. they are structurally identical in respect of 

 the position and number of chiasmata and in the position of their 

 attachments. 



These are phenomena which cannot be attributed to a chance 

 "clumping" indticed by bad fixation. It is probable however that a 

 fixative which produced slight clumping would accentuate the associa- 

 tion of chromosomes already in juxtaposition. 



b) The Allopolyploid 



The homology, affinity and pairing of chromosomes in the polyploid 

 were examined by Darlington (1928). In the following pages the 

 relationship and behaviour of chromosomes in polyploids are discussed, 

 more particularly as they refer to secondary association. 



Secondary association is a phenomenon intimately connected with 

 allopolyploidy. The reason for this is clear on consideration. The allo- 

 polyploid arises from the hybridisation of two different species. As a 

 rule the hybrid will be sterile and under natural conditions will fail to 

 survive unless 1) it arises from the fusion of unreduced gametes, 2) 

 chromosome doubling follows hybridisation. In either case ea,ch chro- 

 mosome received from the parents is represented twice in the hybrid, 

 i.e. it h§is a mate, and conjugation and disjunction will, as a rule, pro- 

 ceed normally. As we shall see there is, on theoretical grounds, an 

 ideal condition of allopolyploidy with a high survival value, and for the 

 purpose of discussion the following remarks are based on this ideal 

 allopolyploid. In newly arisen allo-polyploids variation in the direc- 



Cytologia 2, 1931 26 



