RESIDUAL ATTRACTION 497 



The associations found at meiosis therefore require no assumption 

 of forces of attraction that is not already justified by the study of 

 mitosis. 



We are not concerned here with the precocity theory of meiosis 

 which is derived from this principle. We must consider how the 

 specific attraction inferred is related to chromosome movements 

 under special conditions and in exceptional organisms. When the 

 chromosomes are brought close together at metaphase and anaphase 

 of mitotic and meiotic divisions they show a secondary attraction 

 (Ch. VI). This attraction is residual with respect to the primary 

 attraction, and like it shows itself between similar parts of chromo- 

 somes. It is, as a rule, very slight, so that it only slightly affects 

 the equilibrium position of the chromosomes on the metaphase 

 plate. A number of variable conditions affect its expression. It 

 shows itself more readily at meiosis than at mitosis because the 

 chromosomes are then shorter and can move with less resistance. 

 It shows itself more readily between smaller chromosomes, whether 

 in organisms like most Dicotyledons, in which they are all small, or 

 in the smaller members of a set. Thus fragment chromosomes 

 reduplicated in large numbers usually lie in groups. On the other 

 hand, it is genotypically and environmentally controlled. Its 

 expression must depend upon the substrate. Thus pears show this 

 association less than apples, although the relationships must be the 

 same (Moffett, 1934). The chromosomes of the Diptera, on the 

 other hand, have such a high residual attraction that it shows even 

 in the prophase at mitosis where, according to the illustrations (Metz, 

 1916 ; Moffett, 1936) it is enough to permit the development of 

 relational coiling. Probably the rare crossing-over in the male 

 Drosophila occurs at somatic mitoses and is conditioned by this 

 relational coiling (cf. Sect. 4). 



At meiosis in the male this highly-developed residual attraction 

 expresses itself in an entirely new type of segregation mechanism 

 (Ch. IX) and at the permanent prophase of the salivary glands in a 

 new type of chromosome relationship (Ch. VI). In meiosis the 

 equilibrium position, with the four chromatids of the bivalent 

 almost equally spaced, shows that the primary and residual attrac- 

 tions are almost equal and the distinction between them has there- 



