COILING AND CROSSING-OVER loi 



the first coiling, (iii) The chromosomes are coiled round one 

 another to a greater or less extent between chiasmata. 



This last system is evidently the direct successor of the pachytene 

 coiling of chromosomes, and its variation is particularly instructive. 

 Where the pairing at pachytene is intermittent (as with partial 

 localisation in some species of Fritillaria) the chromosomes are 

 relationally coiled in the unpaired intercalary parts. This coiling 

 must necessarily survive to diplotene. But in the paired proximal 

 regions there is little coiling ; the chromosomes are there asso- 

 ciated by chiasmata. Now relational coiling of the chromosomes is 

 a property shown by partner chromosomes before they divide into 

 chromatids and whether they form chiasmata or not. Each pair of 

 chromatids that is coiled round another pair at diplotene must be 

 derived from a parental chromosome. And since such coiling can 

 occur and does occur on both sides of chiasmata, chiasmata must 

 represent and result from breakage and crossing-over between 

 chromatids of the partner chromosomes. It seems probable that 

 this breakage and re-union of chromatids replaces the relational 

 coiling by permitting the chromosomes to uncoil after breaking 

 before they reunite (D., 1935 h). 



Between diplotene and diakinesis the chromosomes gradually 

 uncoil so far as the chiasmata allow them to. Thus since the 

 direction of coiling is always consistent in particular arms, so far as 

 we know, their uncoiling is bound to coil their chromatids round 

 one another, and this coiling cannot be undone in loops between 

 chiasmata, although in free ends it can. 



(v) Diakinesis. After chiasma-formation the chromosomes con- 

 tract still further in length and begin to assume a more rounded 

 outline until diakinesis, the stage of greatest linear contraction. 

 The chromosomes are then shorter and broader than at metaphase 

 of a somatic mitosis, except where, as in some Lepidoptera, the 

 mitotic contraction is already a maximum, and the chromosomes 

 are spherical at metaphase even at mitosis. 



This contraction is achieved, at least as far as organisms with the 

 largest chromosomes are concerned, by the assumption of a larger 

 spiral over and above the small spiral developed at mitosis. These 

 major and minor spirals were first discovered by Fujii in Tradescantia 



