No. 592] THE MECHANISM OF CROSSING-OVER 199 



lower part of L ; in this way a recombination of parts is 

 accomplished. Morgan and other workers on Drosophila 

 base their acceptance of this essential point in Janssens's 

 chiasmatype theory upon the evidence (from cytology) 

 that homologous chromosomes do twist about each other 

 during synapsis, taken together with the evidence (from 

 genetics) that these chromosomes emerge as new combina- 

 tions. Janssens, on the other hand, maintains that certain 

 details in the appearance of the chromosomes during that 

 stage in synapsis called ' ' strepsinema " give ocular evi- 

 dence that crossing-over occurs at this particular period 

 and in a particular manner. As it would seem possible, 

 however, to put another interpretation upon his figures, 

 this question may be deferred until later. 



Janssens had intended the chiasmatype theory to explain 

 the supposed fact that there might be more pairs of factors 



capable of recombination than there were pairs of chro- 

 mosomes. (It might be mentioned in passing, however, 

 that at that time this fact had not yet been demonstrated ; 

 there are even now probably no published facts except 

 those recently discovered in Drosophila which prove this 

 point). As shown above, Morgan went further than this 

 with the chiasmatype theory by applying it to explain, spe- 

 cifically, the recombination of linked factors (8). More- 

 over, he pointed out at the same time an important corol- 



