chore and subject to reversals within arms (Table 7-1). A number of 

 modifications have been proposed to explain these observations, but 

 though many of them agree superficially with data, they fail to make 

 mathematical agreement. 



TABLE 7-1. Frequency of Intrabrachial Reversals at First Anaphase.* 



'■'■' Data from Wilson, G. B. and Hutcheson, 1., 1941. "Further Studies on Changes 

 of Direction in the Major Coil of the Chromosomes of Trillium erectum L." Can. J. 

 Research, C, 19, Table III, p. 389. 



The most extensive studies of the matrical group were those made at 

 McGill University from 1935 to 1941. The lead was provided by Wilson 

 and Huskins' discovery (1939) that in Trillium erectum, when the major 

 coil is being developed, the over-all length of the chromosome changes 

 only very slightly in the direction of contraction, while the chromonema 

 length increases at least twofold (Table 7-2). All torsion mechanisms 

 demand either contraction of the chromonema or else more or less static 

 conditions. Wilson and Huskins, therefore, proposed the idea that the 

 major coil was the direct result of elongation of the chromonema within 

 a limiting membrane. The general theory is that all chromonema helical 

 coils are the result of differential length changes between the matrix and 

 the contained thread. Later work by Wilson and Hutcheson (1941), 

 Sparrow, et al. (1941), and Sparrow (1942) further supported this 

 general hypothesis and showed how it could explain the relational coiling 

 of half chromosomes at prophase. The most important evidence in favor 

 of the hypothesis proposed is the fact that reversals occur with random 

 frequency at chiasmata and the kinetochores and that when these are 

 allowed for, there remain a number correlated with length or, more 



THE CHROMOSOME COILING CYCLE / 167 



