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 believes, 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 their 
constituent elements at each resting period then 
