558 MARY B. STARK 



determinations of table 18 are all of the crossover value white 

 new lethal, and are expected to give a unimod^,! curve with 

 the mode at 15. 



SUMMARY 



Lethal factors were found in inbred stock only. Fresh wild 

 stock gave no unusual sex ratios. 



The first lethal (O was found in stock caught in 1911 and 

 has its locus at 23.7. One female of this stock gave an ex- 

 traordinarily high sex ratio; viz., 83 females to 3 males. That 

 this extraordinary sex ratio is due to the presence of two lethal 

 factors, one in each X chromosome, is shown by the fact that all 

 (not half) the daughters gave a 2 to 1 sex ratio. 



The now lethal (Isb) that appeared in the female with extra- 

 ordinary sex ratio crossed over, in one case examined, to the 

 X chromosome that carried the factor for white. Its locus is 

 at 16.7. Two other lethals were found in the 1910 stock. The 

 first (Isc) has its locus at 65.2. The other (Isd) dififers from all 

 other lethals in that 'the lethal bearing males emerge from the 

 pupa case, and die almost immediately on becoming adult flies. 



