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THE AMERICAN NATURALIST [Vol. XLIX 



The cross-over classes between vermilion and furrowed are 

 the bar and vermilion furrowed classes of which there are nine, 

 which is 3.75 per cent, of the entire number. Vermilion is at 

 33, hence the gone for furrowed lies at 36.75. 



The cross-over classes between furrowed and bar are the fur- 

 rowed bar and th 1 vermilion classes of which there are 43 which 

 is 18 per cent. Then furrowed lies 18 points to the left of bar 

 or at 39. 



The discrepancy in these results is due to the low viability of 

 the furrowed flies, yet the results agree fairly well, varying from 

 36.75 to 39, giving an average of 38.1, which is considered as the 

 relative position of the gene for furrowed in the sex chromosome. 



> \ / Dr_ 



The accompanying diagram will aid in understanding the 

 cross-over classes. The heavy straight lines represent the paired 

 sex chromosomes which a heterozygous female has received from 

 her parents. The upper one, which carries vermilion bar, was 

 received from the female parent and the lower, carrying fur- 

 rowed, was received from the male parent. Each of the sons 

 of this heterozygous female receives one of these chromosomes 

 which determines what it shall be with reference to these special 

 characters. In about 75 per cent, of the cases the sons receive 

 these chromosomes without any interchange of substance be- 

 tween the two as is shown by the two straight lines which rep- 

 resent the non-cross-over classes. When there is an interchange 



