362 W. J. C. Lawrence Cytoic^ia 2, 



periphery of the nucleus, accompanied by repulsion between the pairs 

 of chromosomes and members of the pairs. This repulsion force is 

 most evident at mid-diakinesis and in view of the considerable attenua- 

 tion of the chiasmata then found we may assume that maintenance of 

 pairing through diakinesis is conditioned solely by the maintenance of 

 chiasmata between the pairs, i.e. only those chromosome pairs which 

 are materially connected will survive the repulsion phase of diakinesis. 



Chiasmata formation is however dependent upon zygotene pairing 

 and therefore on the linear identity of the pairing chromosomes. If all 

 parts are similar, the pairing will be complete and the maximum of 

 chiasma formation will occur. If the chromosomes are similar only 

 in part or in scattered portions of their length, pairing will be incom- 

 plete and the chiasma frequency proportionately reduced. Darlington 

 (1929, 1930 b) has shown that chiasma frequency is proportional to 

 chromosome length, and that this frequency is further modified by 

 competition in pairing between more than two homologous chromo- 

 somes. Now from this we may postulate that chromosomes which are 

 homologous but whose number, linear differentiation and length do not 

 permit of chiasma formation may yet have a general affinity for one 

 another, and that under suitable conditions this general affinity would 

 result in their association at metaphase. In Dahlia, and probably in 

 most plants, this requisite condition is provided at pro-metaphase, 

 when the pairs of chromosomes are in close proximity to each other. 

 As we have seen, before pro-metaphase the bivalents are entirely un- 

 associated, but following this stage the typical groups of two or more 

 bivalents appear and often persist throughout the maturation divisions. 



On this hypothesis secondary association arises from the random 

 approach of homologous chromosomes during pro-metaphase, which 

 then remain in juxtaposition until they are again dispersed at the 

 next repulsion phase. In many plants this second repulsion phase is 

 interkinesis, but where this stage is short a certain proportion of 

 association survives until the second division. 



We have therefore two types of chromosome association during 

 meiosis and in particular at first metaphase, a) the pairing of homo- 

 logous chromosomes which begins at zygotene and persists to first 

 anaphase and b) association which arises from general aflSnity of 

 homologous chromosomes, resulting in their juxtaposition at meta- 

 phase of the first division. Both types of association may occur in 

 the same individual, the respective numbers of each being determined 

 from examination of diakinesis. 



