I93I The secondary association of chromosomes 377 



once identical', have differentiated and now govern the same character 

 in different ways (cf . /, Y, A and B in Dahlia variabilis, Lawrence 

 1931). 



The theory of secondary association advanced in this paper has 

 been confirmed and amplified by further work on Cardamine pratensis 

 and Verbascum phoeniceum, an account of which is now in the press 

 (Lawrence). 



Summary 



1. A detailed account is given of the behaviour of the chromo- 

 somes from diplotene to second anaphase in Dahlia Merckii (2n=36), 

 D. coccinea (2n=32), D. coronata (2n=32) and D. variabilis (2n=64). 



2. Two kinds of chromosome association are found in these 

 species : 



a) Primary Association, arising from particulate pairing at zygo- 

 tene and determining segregation at metaphase. 



b) Secondary Association, arising at pro-metaphase and due to the 

 general affinity of homologous chromosomes. This does not affect 

 segregation. The chromosomes first associate at pro-metaphase 

 when they are in close proximity to each other. 



3. As first described by Darlington, the primary association of 

 chromosomes at metaphase is shown to be solely due to the mainten- 

 ance of chiasmata formed at pachytene. Only those chromosome pairs 

 or multivalents which are materially connected by chiasmata survive 

 the strong repulsion phase of diakinesis. This repulsion results in the 

 radial dispersion of the bivalents to the periphery of the nucleus, 

 coupled with an equal aversion between the pairs and the members of 

 each pair. The repulsion is fully maintained until mid-diaknesis, and 

 then gradually diminishes to pro-metaphase, when the close proximity 

 and general afl[inity of homologous chromosome results in groups 

 of secondarily associated chromosomes at metaphase. The frequency 

 and size of the group is characteristic for a given species. The impor- 

 tance of the diakinetic phase in differentiating primary and secondary 



association is emphasised. 



4. The theory of secondary association is discussed and evidence 

 from the literature presented. 



It is claimed that secondary association is evidence of more remote 

 affinities than can ever be expressed in primary association in a poly- 

 ploid. This is proved by the occurrence of true (primary) association 



