THE CHROMOSOMES 167 



At present it seems better not to attempt to 

 commit the theory of crossing over to one rather than 

 to another of these stages; for, whether the process 

 occurs at the leptotene thread stage as suggested 

 above, or, as Janssens beheves, at a later stage 

 (strepsinema), the genetic result is the same. What 

 we wish to point out is that in the phases through 

 which the chromosomes pass at the maturation 

 stages there is given an opportunity for an inter- 

 change of parts. The genetic evidence shows very 

 clearly that interchanges do take place, as is best 

 illustrated in the case of the sex chromosomes, 

 whose history can be traced with some assurance 

 from one generation to the next. 



What we wish especially to insist upon and empha- 

 size is that the evidence from linkage in Drosophila 

 has shown beyond any doubt that crossing over is 

 not a process that involves only a particular factor 

 in relation to its allelomorph. Our work has shown 

 positively that there is a tendency for large sections 

 of the chromosomes to interchange whenever crossing 

 over occurs. 



Another idea that is likely to suggest itself in this 

 connection has also been disproven by the evidence 

 from Drosophila. It might be supposed that at a 

 resting stage the chromosomes go to pieces and the 

 fragments come together again before the next 

 division period. Linkage might then mean the 

 likelihood of fragments remaining intact, etc. But 

 if the chromosomes broke up completely into th(Mr 

 constituent elements at each resting period then 



