No. 593] THE MECHANISM OF CROSSING-OVER 293 



apart in opposite directions by mutual repulsion of the 

 strands or by pulling of spindle fibres, in breakage of 

 parts originally together. (So perhaps fusion might 

 occur during the amphitene and breakage in the strepsi- 

 nema stage; this would be a combination of schemes 1 

 and 2 which would account both for the exact apposition 

 of parts and for the phenomena observed by Jannsens.) 

 Be this as it may, at any rate, the negative argument may 

 be given that it is just as hard to account for recombina- 

 tion at a later stage in synapsis as at this stage, even 

 overlooking the objection of the thickness of the threads. 



There is a serious objection to the scheme just given, 

 however, in that, as the threads come together, they seem, 

 in many preparations, not to keep their original plane of 

 apposition, but to twist tightly about each other, like the 

 strands of a rope, throughout their entire length (see 

 Fig. 6). It is possible that the twisting of one thread 

 about the other is merely apparent, however, and that the 

 threads lie parallel but are simply coiling up in a spiral, 

 in the process of forming the shorter, thicker prophase 

 chromosomes ; for, unless the spiral were very delicately 

 preserved by the fixing agent, there would be apparent 

 knots in it as though there were a twisting of two strands 

 about each other. Moreover, there is evidence indicating 

 that this tight twisting occurs only in certain species of 

 animals. But let us assume for the moment that this very 

 tight twisting really takes place during the amphitene 

 stage in flies, and that crossing-over takes place at this 

 period (this we may call scheme of crossing-over number 

 three). Would there then be any way of explaining why 

 one crossing-over should interfere with another near by, 

 in view of the fact that the loops are of such small di- 

 mensions ? In seeking an answer to this question, it will 

 be helpful to bear in mind that crossing-over can be di- 

 vided into just three essential processes— a bending of the 

 chromosomes across each other, a breaking of the threads, 

 and then a fusion of adjoining pieces (or, perhaps, the 

 fusion of the homologous chromosomes comes first, and 

 then the breaking of the original chromosomes at that 



