PROOF BY COILING 



259 



The most significant type of interlocking is the double interlocking 

 discovered by Mather where two successive loops of a bivalent have 

 chromosomes interlocked with them. Each loop must then be 

 separating the original parental chromosomes, and the chiasma 

 between the two must be the result of crossing-over (Fig. 84). 



(iii) Relational Coiling. Another kind of evidence, showing that 



V/itk Crossing Onn- 



\(ifkouf Crossin.^ Oren- 



Fig. 85. — Diagram showing that crossing must have occurred before 

 a chiasma could be formed where relational coiling is found on 

 both sides of the chiasma. (D., 1935 d.) 



the partner chromosomes always separate at diplotene, is found in 

 the occurrence of relational coiling. This coiling develops, as we 

 have seen, during pachytene and before the separation of the 

 chromosomes and the appearance of chiasmata at diplotene. It 

 must therefore be a coiling of the partner chromosomes around one 

 another, yet it occurs on both sides of chiasmata (Fig. 85). If 

 crossing-over has not preceded and determined such chiasmata, we 

 must suppose (i) that the chromosomes were divided into chromatids 

 before coiling began, (ii) that these chromatids were re-assorted into 

 new pairs which acted as units in coiling, and (iii) that the chiasmata 

 were thus pre-formed at the beginning of pachytene and before any 



9—2 



