FERTILITY AND STERILITY IN DROSOPHILA 157 



view paved the way for a study of the low production of the trun- 

 cates, which is dealt with in the second part of this paper. 



It is to be noted in 12 b (table 16 b) of this cross that 11 pairs 

 out of 26 are sterile. This proved to be due to the males. 

 Twenty-eight other males from this family were tested with the 

 hope of establishing a sterile male strain. Seventeen of the 28 

 were sterile. This gave promise of yielding a sterile male line; 

 but on inbreeding a few pairs the sterility vanished as suddenly 

 as it had appeared. This is the only case in all the experiments 

 in which sterility affected the males to any appreciable degree. 



Let us turn now to the evidence which this experiment gives on 

 the transmission of female sterility. A study of the tables where 

 the sterile female strain (inbred stock) was tested against the 

 truncate and Woods Hole stocks shows that the sterility reap- 

 peared again after skipping a generation and that it affected 

 primarily the females. The defect reappears in both the cross 

 and the reciprocal. A great deal of weight is to be attached to 

 this evidence since the sterility did not appear to any appreciable 

 degree when the truncate stock was crossed to the Woods Hole 

 stock. Moreover, the sterility as it was affecting the female was 

 now at its maximum. In table 14 a there are three inbred males 

 that did not transmit the defect. An apparent explanation is 

 found on the assumption, which is borne out by all the evidence 

 presented, that the defect behaves like a unit character and 

 accordingly some families are not affected. 



Crosses between inbred and truncate stock 



Before proceeding to a summary of these sterile cases I wish to 

 give the results of an experiment in which the inbred stock was 

 crossed into the truncate stock. Sterility was present in both 

 stocks at this time. Tables 17 a and 17 b give the result of the 

 sterility as it appeared in the Fi and F2 generations. 



Table 1, F3, gives the control for the inbred stock while table 

 8, F46, gives the control for the truncate stock. The inbred 

 strain up to the time this cross was made had been reUed upon as 

 a fertile strain. That sterility had appeared in this stock and 



