FERTILITY AND STERILITY IN DROSOPHILA 347 



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We have then six different strains, two from different localities 

 and some that had the same germ plasm. I now propose to exam- 

 ine the fertility of the different stocks and their behavior on cross- 

 ing. The combinations were made up as shown in diagram A. 



METHODS 



In order to get an exact measure of fertility the eggs were iso- 

 lated as described in Part II. From the stock bottle in each case 

 15 virgin females and 30 young males were selected. The flies 

 that hatched from June 19 to June 26 were separated every 

 twelve hours from the stock bottles. There can be no question as 

 to the virginity of the females. The males and females were kept 

 in separate bottles and the different combinations made up on June 

 26. An epidemic of mating took place in all the bottles a short 

 time after the flies came out from under the influence of the ether.^ 

 I commenced to isolate the eggs two days later. This process was 

 carried out in the same way as in previous experiments with one 

 exception. The weather was very warm and the larvae emerged 

 from the egg in less than twenty-four hours. In order to exclude 

 this source of error I added another bit of food after isolating the 

 eggs. This served as food for the parents. After six or eight hours 

 this food was removed and a new bit added from which the eggs 

 were later isolated. Tables 3a, 3b, and 3c give the number of eggs 

 isolated each day and the corresponding number that hatched. 



DISCUSSION 



A study of diagram A brings out the essential relations that 

 concern the questions propounded at the beginning of this paper. 

 1. What is the effect on fertility when germ plasm originally from 

 the same source separately inbred for several generations is recom- 

 bined by crossing? It will be recalled that I, I2 and I3 represent in 

 this case the stocks under study. It is to be noted that the three 

 stocks although originally brothers and sisters and descended from 



' Mr. A. H. Sturtevant has shown that the mating habit in this species is largely 

 associated with the sense of smell and this fact probably accounts for the phe- 

 nomenon here observed. 



