No. 594] THE MECHANISM OF CROSSING-OVER 357 



balloon with respect to speck is still unknown.) Various 

 " trial and error" matings were therefore made in the 

 hope of getting these unplaced factors in suitable combina- 

 tions, and crosses were also undertaken to secure such 

 data in regard to their position as would be useful for the 

 purpose in view. These attempts were often cut short, 

 owing to the information which was meanwhile being 

 accumulated by the other workers, but before the latter 

 information was obtained the positions of bifid, forked and 

 dachs had been determined, and several multiple stocks 

 that were later used had been made up. 



We may now consider specifically what combinations of 

 factors were actually employed in the experiment and 

 what special methods were used for securing and main- 

 taining these combinations. 



In the case of the first chromosome, it was desired, for 

 the final linkage determination, that the heterozygous flies, 

 whose gametic output was to be tested, should contain 

 most of the recessive factors (which are usually the mu- 

 tant ones) in one chromosome, and the dominant (usually 

 normal) factors in the other, for it was considered worth 

 while to test the possibility that a ehromosome containing 

 so many mutant factors might behave abnormally. Fur- 

 thermore, if the commonly accepted belief were correct, 

 that recessive factors are "absences," it was possible that 

 the chromosome with many recessive factors might be 

 shorter than the other, and that linkage disturbances 

 might arise for this reason. Two stocks were, therefore, 

 made up, of the factors in the first chromosome, to supply 

 these two kinds of chromosomes for the heterozygous 

 females to be tested. One stock contained the mutant 

 factors for yellow body color, white eyes, abnormal ab- 

 domen, bifid wings, vermilion eyes, miniature wings, sable 

 body color, rudimentary wings, and forked spines. All of 

 these factors are recessive except abnormal abdomen, 

 which is only partially and irregularly dominant. The 

 other stock contained only the mutant factors cherry eye, 

 elub wing, and bar eye. Cherry is an allelomorph to the 

 factor for white eyes carried by the other chromosome, 



