FERTILITY AND STERILITY IN DROSOPHILA 353 



fertility, and that crossing gives the proper constellation of factors 

 for a marked rise in fertility. 



It must be frankly admitted that this explanation at best is only 

 tentative and that the door must be left open for further investi- 

 gation. The possibility of transmissible lethal factors is not to 

 be overlooked. 



The third question under consideration relates to the effect on 

 fertility of crossing certain mutations, namely, pink-eye and white- 

 eye and the effect on fertility when these are crossed into the 

 inbred stock. The triangular portion of diagram A shows that 

 there is a rise in fertility in all cases. In four of the six cases the 

 rise in fertility is very high. This is analogous to what happens in 

 the case of the truncates when crossed into other stocks. 



I wish here to add the data from an experiment that has some 

 bearing on the foregoing considerations and also some bearing on 

 the question of inbreeding. The object of this experiment is to 

 serve as a check upon the controls used in the foregoing experi- 

 ments, namely. Woods Hole, white-eye, I2, I3 and the inbred 

 stocks. This experiment was carried out in the same manner as 

 the foregoing. The egg counts began the same day that the 

 former experiment closed, July 15, 1913. The number of eggs 

 isolated, together with the corresponding number that hatched, is 

 given in table 4. 



I have placed the percentage of fertility of these stocks in dia- 

 gram A and enclosed the numbers in circles. It will be noted that 

 the Woods Hole and white-eyed stocks remain practically the 

 same as in the previous experiment. There is a marked rise in 

 fertility, however, in case of the three stocks originally from the 

 same germ plasm. Later (Aug. 7 to 24, table 1) the Woods Hole 

 stock was tested. Its fertility had risen from 59.7 to 71.7. The 

 Woods Hole stock was again tested August 17 to September 4. 

 This stock now gave a fertility of 83 . 6. The fact is that all my 

 stocks at this time showed a marked rise in fertility, as is evi- 

 denced by the experiments to be dealt with in the next paper. 

 Even the truncate stock which had been tested many times through 

 the year and had varied from 20 to 26 per cent now gave a fertility 



