No. 593] THE MECHANISM OF CROSSING-OVER 



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when the threads that crossed would normally have sep- 

 arated from those that did not, the egg would come to 

 contain two X-chromosomes, one of which was a cross- 

 over but not the other. In the usual type of non-disjunc- 

 tion, the X's never cross over— presumably because they 

 paired with the Y (which was present in these cases as an 

 extra chromosome), so this type of non-disjunction could 

 not afford a test of the theory. But it is to be expected 

 that non-disjunction should sometimes occur by mere ac- 

 cident without the interference of a Y, and since in these 

 cases the X's could have crossed over, such cases of non- 

 disjunction might furnish a test of Jansscns's theory, hi 

 1913, in an experiment designed for this purpose, I ob- 

 tained a fly which had received two maternal X-ehromo- 

 somes by reason of non-disjunction in its mother, and in 

 which one of these X-chromosomes proved to be a cross- 

 over but not the other ! The fly resulted from a cross of a 

 female which contained in one X-chromosome bifid and 

 vermilion, and in the other chromosome eosin and bar, by 

 a normal male. It itself contained in one of its chromo- 

 somes bifid and vermilion, and in the bifid, vermilion and 

 bar. Since then Bridges has obtained other exceptions 

 of the same general sort. But on further consideration 

 it appears that this result really proves nothing, for the 

 non-disjunction may just as well have taken place in an 

 oogonial division, "in this way an oocyte would result 

 that contained three X-chromosomes. At synapsis two 

 of these could cross over with one another, and the egg 

 could then receive a cross-over chromosome and also an 

 X that had not crossed over. To prove that the non-dis- 

 junction was not of this type, but really occurred in a 

 maturation division, i. e., that the two threads originated 

 from one tetrad, it would be necessary to obtain individ- 

 uals in which both of the X-chromosomes received by non- 

 disjunction had crossed over, but each at a different point 

 (or one of them at two points). 



In a case of the latter sort the fact that both chromo- 

 somes had crossed over at some point would prove either 

 that both of them had been in the synaptic tetrad, and so 



