No. 592] THE MECHANISM OF CROSSIXG-OYER 207 



Furthermore, admitting this conclusion that the linkage 

 diagrams really represent the chromosomes, the fact that 

 the factors are exchanged in sections proves that whole 

 pieces of the chromosomes change places at once, as oc- 

 curs in the process of "crossing-over" postulated by 

 Jannsens, instead of small parts or factors in the chromo- 

 somes being separately exchanged. The idea that the 

 interchange during synapsis may be a kind of exchange 

 of separate particles from one container to another seems 

 to have been held by a number of geneticists. On this 

 view, the chromosomes might be considered as a sort of 

 pod, containing the factors within them like so many 

 beans; when the chromosomes synapse, the pods open 

 towards each other, so that a factor in one might change 

 places with a factor in the other. Conceivably — if we 

 adopt this view— certain factors might be harder to dis- 

 lodge than others, and so different frequencies of separa- 

 tion would exist between different factors. But such a 

 mechanism of interchange would not result in a mode of 

 linkage that may, in the sense explained above, be called 

 linear, for separation of factor B from A would, on this 

 mechanism, have no influence at all on whether or not C 

 separated from A. This difficulty could be partially met 

 by supposing that interchange of one factor in some way 

 facilitates interchange of the neighboring factors, but the 

 type of linkage which is actually found goes much further 

 than this, and shows that the whole group of factors re- 

 mains intact except at one or two points, interchange being 

 in two or three entire sections. This can only mean, then, 

 that interchange is a process of crossing-over, if it occurs 

 by means of synapsis at all. 



It might be claimed, however, that this interchange of 

 whole sections of the chromosomes need not occur at syn- 

 apsis, and therefore need not be of the nature of crossing- 

 over at all. The only alternative to crossing-over, how- 

 ever, would be to suppose that, during the resting period 

 of cells, the chromosomes might break up into pieces, and 

 that then, in reuniting, a fragment of one chromosome 



