CROSSING-OVER 549 



two forces are in equilibrium, for pachytene may be indefinitely 

 prolonged without change. Further, the attraction must be posi- 

 tionally specific, not permitting the threads to slip round one 

 another. Such a specificity implies an asymmetrical molecular 

 pattern such as might (since the time of Pasteur) be expected in 

 the gene. The mechanical position may be imitated well enough 

 by releasing two woollen threads that have been placed alongside 

 one another in a state of torsion. Their internal molecular relations 



% 



Ur 



''\ fj 



n 



Fig. 158. — The coiling relationships of two chromosomes before 

 (left) and after (right) crossing-over has occurred. Right-hand 

 chromatid coiling (r) and left-hand chromosome coiling (L) and 

 chiasma coiling (L*). (D., 1936 df.) 



N.B. — The production of the equilibrium of pachytene is the 

 object of all spinning operations. 



then determine a longitudinal cohesion. The entanglement of 

 their fibres represents a positional specificity of attraction which 

 prevents slipping. The two threads take up a position such that 

 the torsion of their internal coiling is in equilibrium with the 

 opposite torsion of their relational coihng (Fig. 158). 



Such is the position when the chromosomes begin to divide. 

 The daughter chromatids will be relationally coiled in the opposite 

 direction to that in which the chromosomes are relationally coiled. 

 Division is bound to upset the equilibrium in two ways. First, it 

 abolishes the specific attraction which works only between pairs of 

 threads and therefore only between the new chromatids. The 



