490 



CELL MECHANICS 



where more than one chiasma is formed in a given pair of arms. It 

 is also determinate in its direction (D. 1936 d). At mitosis, how- 

 ever, as we saw, relational coiling is facultative. It is also, so far 

 as our present observations go, indeterminate in many chromosomes 

 although determinate in some (D. 1936 c). We cannot yet say what 

 the immediate cause of the mitotic coiling, still less of its variation, 

 may be. Like the chromosome coiling at meiosis, it must depend 

 ultimately on the specific attractions of the genes and on the mole- 

 cular coiling of the threads. If the effective agent of association is 

 the attraction between the particular pairs of genes nearest one 



Fig. 138. — Three possible interpretations of a first metaphase 

 chromosome in relation to its major and minor spirals. (D., 



1935 «^-) 



another in the two internal spirals, then an inequality in the ampli- 

 tude of the two spirals would give rise to a coiling of the chromatids 

 — but consistently in the opposite direction to the internal coiling. 



The reason why the relic spiral cannot be used in forming the new 

 internal spiral is, on the other hand, fairly clear : it has already 

 expanded to an amplitude far greater than an internal spiral ever 

 assumes, and an internal spiral always develops by increasing its 

 amplitude, not diminishing it. 



There is no direct evidence to show why the chromosomes coil 

 relationally at one stage and internally at others. The most obvious 

 possibilities are that the difference depends either on a difference in 

 the degree of attraction at work at different stages, or on the coiling 



