193' The secondary association of chromosomes 361 



The theory of secondary association of chromosomes 



The main features of the study of meiosis in Dahlia may be sum- 

 marised as follows. 



1. At zygotene the chromosomes pair laterally (parasynapsis). 



2. At diplotene two or more chiasmata per chromosome are found. 

 The chiasmata are completely terminalised before diakinesis. Coin- 

 cident with the onset of diplotene the paired chromosomes are distri- 

 buted to the periphery of the nucleus. 



3. Diakinesis is essentially a repulsion phase which is fully main- 

 tained until mid-diakinesis. 



4. The degree of repulsion gradually diminishes and diakinesis is 

 abruptly terminated by the sudden converging of the chromosomes on 

 the centre of the nucleus. This stage (pro-metaphase) is very brief 

 and is characterised by the secondary association of a number of the 

 bivalents. 



5. The chromosomes are orientated on the equatorial plate ready 

 for disjunction, this orientation being accompanied by a radial disper- 

 sion of the bivalents. 



6. A number of the bivalents are associated, but not materially 

 connected, in groups of 2 and 3. The bivalents which thus associate 

 secondarily are found to be of similar size and configuration. 



7. Anaphase proceeds normally and is followed by a short inter- 

 kinetic stage. 



8. Association is also seen in the second metaphase. Tetrad 

 formation is regular. 



Cytological observations by various workers (McClung 1917, Seiler 

 1926, Belling 1927, Darlington 1929, and Newton and Darlington 

 1929) have shown that the pairing of chromosomes at meiosis is particu- 

 late. At zygotene the chromosomes pair laterally, particle by particle, 

 insomuch as the parts are similar. During the pairing stage breaks 

 occur in the chromatids followed by reunion, of different strands. 

 At diplotene the chromatids separate in pairs and the points of inter- 

 change become conspicuous as ' chiasmata ' according to the chiasma- 

 type theory. The chiasmata apparently hold together the respective 

 chromosomes until disjunction at anaphase of the first division, when 

 the paired chromosomes separate and pass to opposite poles (Darling- 

 ton 1931). 



Now we have seen that, coincident with the onset of diplotene in 

 Dahlia, there is a general distribution of the chromosome pairs to the 



