302 THE AMERICAN NATURALIST [Vol.L 



that the non-disjunction had occurred in the maturation 

 division in the mother fly, or else that both were derived 

 from the two halves of a single (cross-over) X-chromo- 

 some which underwent non-disjunction in an embryonic 

 cell division of the individual itself. But the fact that 

 the two chromosomes are not identical would rule out the 

 second possibility. The result, therefore, would mean 

 that in the same tetrad one strand may have crossed over 

 at a certain point and not another strand, i. e., that Jans- 

 sens 'a theory is correct and crossing-over takes place at a 

 stage when there are four threads, two of which may 

 cross over at a certain point while the others retain their 

 original composition. 



Up to the present, however, no exceptions of this type 

 have been found, although Bridges has obtained not a few 

 exceptions of the type that may as well be explained by 

 non-disjunction in an oogonial division (i. e., in which 

 one X had crossed over— but not the other), and also one 

 other exception, which had received two similar double 

 cross-over chromosomes. The latter peculiar circum- 

 stance must have resulted either from a non-disjunction, 

 at the maturation division in the mother, of two strands 

 of a tetrad, both of which had crossed over in the same 

 two places, or from a non-disjunction, in an embryonic 

 cell division of the individual itself, of the two halves of 

 the single (double cross-over) X-chromosome, which, on 

 this view, was originally present. But the latter explana- 

 tion is very improbable, for, unless the non-disjunction 

 occurred in the first cleavage, only a small part of the fly 

 would be composed of cells descended from the one into 

 which the 2 X's entered; most of the cells, therefore, 

 would contain only one X and these would necessarily be 

 male; thus the fly would be a gynandromorph. More- 

 over, all the cells derived from the one which, in the non- 

 disjunctional division, failed to receive either half of the 

 X-chromosome, would probably die. Hence the evidence 

 is fairly good that in this case the two double cross-over 

 X-chromosomes represent two strands of a tetrad. Since 

 these two strand's, although both double cross-overs, were 



