FERTILITY AND STERILITY IN DROSOPHILA 145 



Tables 1 to 7 show that in the beginning of the experiment only 

 a few sterile pairs were found (table 1). Moreover, the fertile 

 pairs were high producers (Part II, table 2) yet sterile individuals 

 appeared in their offspring and these were largely females. At 

 times fully half the females were affected, although there were 

 undoubted cases in which the males were also affected. 



It is to be noted in the eleventh generation, as shown in table 

 7, that out of 182 pairs only 6 sterile females appear. The steril- 

 ity as it affected the female had been practically eliminated, for 

 while it had been affecting 50 per cent of the females it affected 

 at the end of the experiment less than 4 per cent. It might seem 

 as though the character had been transferred to a certain extent to 

 the male as 22 males out of 182 are recorded in table 7 as sterile. 

 But the figures, as given here for the male, do not in all probability 

 represent the actual facts in the case. Many of these males were 

 very small, and after they had been paired for a few days prac- 

 tically all the males that proved sterile were crawling over the 

 food with their wings drooping at their sides. This may or may 

 not have prevented them from mating with the females. I was 

 not able to follow the case further, but I have found no evidence 

 in any of the other experiments that sterility, as it affects the fe- 

 males, can be shifted in heredity to the males. 



That sterility is not due to inbreeding, and that selection is 

 an effective agent in controlling it, is shown in the history of 

 another strain, to which I shall refer as the truncate stock. This 

 stock had been inbred for forty-two generations by Mr. Alten- 

 burg, a graduate student in the department. A great many 

 sterile pairs were appearing in the strain and the broods were so 

 small that it was somewhat difficult, as Altenburg told me, to 

 keep the stock from dying out. Table 8 gives the history of the 

 sterility as it appeared in this stock. When I took charge of the 

 stock fully half of the pairs were sterile and yet on continued in- 

 breeding a large percentage of the sterility disappeared. Table 

 11 shows that in F49 there were 3 sterile pairs in 21 pairs. That 

 there were but few sterile individuals in the truncate stock at this 

 time is also borne out by crosses that were made with fertile 

 races. Sterility had been largely eliminated, and moreover this 



