specifically, with the number of gyres. Such a distribution would be ex- 

 tremely difficult to explain on any torsion theory. 



Whether two strands will twist about each other in a relational coil 

 appears to be a function of their distance from each other at the time of 

 formation of the helical coil. If two strands are widely separated and 

 have separate matrices, they will not entwine, and the helical coils 

 formed will be independent in direction. If two strands are clearly sepa- 

 rate but within the same matrix, they will not entwine, but their helical 

 coils will be in the same direction at the same loci; if two strands are 

 so close together as to be virtually single, they will be entwined with 



TABLE 7-2. Chromosome and Chromonema Lengths In Trillium erecium* 



* Data from Sparrow, A. H., Huskins, C. L., and Wilson, G. B., 1941. "Studies 

 on the Chromosome Spiralizaticn Cycle in Trillimn," Can. J. Research, C, 19, 

 Table I, p. 325. 



one twist for each turn of the helical coil. A bivalent may be considered 

 to consist of two pairs of chromatids, each chromatid a half chromo- 

 some, which coil in the same direction but do not entwine. Half chro- 

 matids which are very close together entwine to give the relational 

 coiling (Figure 7-3) seen at, for example, the first pollen-grain division 

 in Trillium (Figure 7-4). Sparrow, Huskins, and Wilson (1941) and 

 Sparrow (1942) showed a mathematical relationship between the pro- 

 phase relational coil and the association of half chromatids at the 

 previous anaphase. 



In general, the McGill group viewed the coiling cycle as being the 

 result of difi'erential length changes between the matrix and the enclosed 

 chromonemata, with relational coiling being a function of the degree of 

 association of two jointly coiling strands. If two strands are separate 

 and parallel before coiling, they must revolve about each other to form 



168 / CHAPTER 7 



