FERTILITY AND STERILITY IN DROSOPHILA 369 



isolated day by day and the corresponding number of eggs that 

 hatched. 



The truncates in this experiment gave a fertihty of 31.3 which is 

 from 5 to 10 per cent higher than in any of the previous experi- 

 ments. The Woods Hole stock shows a fertihty of 83.6 which is 

 much higher than that of previous experiments. The fact is that 

 all the stocks at this time showed a rise in fertility. Whatever 

 the meaning of this may be, the former relations hold as to the 

 effect on fertility when crossed, as is evidenced by the fact that 

 there is a marked rise in fertility when the truncate is crossed into 

 the Woods Hole stock; compare diagram J with diagram C, 

 Part II. 



Diagram I brings out the effect of crossing within the truncate 

 stock. The fertility of the truncates when tested together is 31.3 

 while that of their long-winged brothers and sisters when tested 

 together is 51.9. "VMien the longs are crossed to the truncates as 

 the diagram shows, the high fertility of the longs is able to bring 

 the fertility of the truncates up to its level and it would seem that 

 in the case of the cross between the truncate female and the long 

 male that there was a marked rise in fertility. The fact is that in 

 this experiment this cross is higher by 10 per cent than in the case 

 where the truncate female is tested with a wild male. I have made 

 many crosses between the truncate female and the Woods Hole male 

 with the result that the fertility of the combination stands almost 

 invariably at 55 per cent. It would seem from this experiment 

 and the evidence from former ones that the rise in fertility of this 

 particular combination is significant. 



A study of the diagrams will show a marked degree of incompat- 

 ibility to exist between the flies with the long wings for while 51.9 

 of their eggs hatch when mated together yet no less than 90.7 

 of the sperm and 73.8 of the eggs are capable of entering into a 

 combination that results in development, as is shown by crossing 

 into the Woods Hole stock (diagram K). 



The greatest degree of incompatibility however exists between 

 the flies with the truncate wings for in this experiment they pro- 

 duced no less than 54 per cent of good eggs and 88.5 per cent of 



