FERTILITY AND STERILITY IN DROSOPHILA 211 



SEX RATIO 



It is not the purpose here to enter into a discussion of sex-de- 

 termination. I merely wish to bring together the data from the 

 foregoing experiments that bear on the question of sex-ratios. 

 Since the viabihty has been so great in some of my strains it seems 

 remarkable that disturbances in the sex-ratio have not been 

 encountered. The tables show the ratios to be remarkably con- 

 stant, with a slight excess of females in almost all cases. The 

 females emerge first and if a count is made at this time there is 

 iu almost all cases a large excess of females. At times the ratio 

 may be as high as two or three females to one male. Toward the 

 middle of the count, however, the ratio approaches equality and 

 toward the end of the count the males are usually in excess and this 

 tends to equalize the sex ratio. It is not that the first eggs laid 

 produce females but that the egg which is to develop into a female 

 carries its development through on an average from twelve to 

 twenty-four hours more rapidly than the male. This is evident 

 from the hundreds of bottles from which my counts have been 

 made. 



Different strains of these flies vary somew^hat in their rate of 

 development. The truncates always emerged from two to four 

 days later than the wild stocks used in control. The hybrid fly 

 carries its development through to the hatching stage more rapidly 

 than either of the parent strains. 



I have said that the sex-ratio is remarkably constant, with the 

 females slightly in excess. This statement applies to the inbred 

 stock, the truncate stock and the Woods Hole stock when brothers 

 and sisters are paired. Diagram 1 shows the ratio of males to 

 females to be 100 to 103; 100 to 103 and 100 to 107 respectively. 

 I think that the slight excess of females is due to the factors to 

 which reference has already been made. When the different 

 races are crossed into each other a rather large excess of 

 females appears. This statement also holds for the children 

 and grandchildren when tested together in pairs. I do not believe 

 that the early emergence of the female from the pupae will ac- 



