throwing itself into a large gyred helix. The latter group (Sax, Wilson, 

 and Huskins), on the other hand, favors the idea of a differential length 

 change between the matrix and its enclosed chromonema. A stiff rubber 

 tube forced into a jar of about half its length will serve as a model in 

 this case, for the rubber tube will be forced into a coil. 



_^ 



Figure 7-1. Interpretative Drawing and Photomicrographs of Metaphase 

 Chromosome from Allium Root Tip Showing Nature and Direction of Coil- 

 ing in Sister Chromatids. (From Wilson, G. B. and Coleman, P. G., 1952. 

 "The Ontogeny of Chromosome and Chromonema Spirals. A Re-evaluation," 

 Cytologia, 17. Figs. 1 and 2, Plate IV.) 



The most complete hypothesis of the torsion school has been ad- 

 vanced by Darlington (1937). In brief, his idea is that a molecular coil 

 through torsion gives rise to an internal coil in the opposite direction, 

 which in meiosis produces the major coil. The same mechanism, by 

 assuming uniformity of direction of coiling in the half chromatids, is 

 presumed to explain relational coiling at prophase of mitosis through the 

 tendency of such pairs of coiled strands to reduce tension by entwining 

 in a compensating direction. In meiosis, homologous chromosomes with 

 uniformity in coiling direction would form a relational coil as the two 



THE CHROMOSOME COILING CYCLE / 165 



