CASTLE. — INBREEDING, CROSS-BREEDING, AND SELECTION. 771 



On July 9 a male was introduced, and pupae were again found after 

 none had been found for over a week. These results show conclusively 

 that a single copulation or several within a period of a few days is not 

 necessarily sufficient to fertilize all the eggs in Drosophila. 



Abstract of Report op F. W. Carpenter on the Sixth 

 Inbred Generation op the A Series. 



To test the variability of the inbred flies, as compared with those not 

 inbred, the following variable character was selected. On the proximal 

 segment of the tarsus of the first pair of appendages there is present in the 

 male a secondary sexual organ, comb-like in appearance, containing a 

 variable number of chitinous teeth. This " sexual comb " has probably 

 the function of a clasping organ in copulation. It is the number of 

 teeth in this comb that has been taken as the variable character to be 

 dealt with, the counting being done under the compound microscope. 



The male Drosophila can often be distinguished with the unaided eye 

 from the female by its smaller size and the darker color of the posterior 

 segments of the abdomen. However, one can be quite certain of the sex 

 only by examining the fly under the microscope, where the presence or 

 absence of the sexual comb peculiar to the male can be noted. 



In counting the teeth in the sexual comb the procedure was as follows : 

 The sex of the preserved fly was determined under a low power of the 

 microscope. If the fly proved to be a male it was removed from the 

 stage of the microscope and the anterior pair of appendages was cut away 

 from the body with a sharp scalpel. In order to do this more easily the 

 head of the fly was first removed. The appendages were then examined 

 under the high power of the microscope and the number of teeth in the 

 sexual comb counted. 



The Inbreeding Experiments. 



The inbred material was obtained thus. A normal male and female 

 were placed in a jar with food. The pair lived here, and from their off- 

 spring several males and several females were selected. Of these, one 

 male and one female were placed in each of several jars with food. 

 Brothers and sisters were thus bred together. The most promising one 

 of these latter cultures was selected to perpetuate the line and the other 

 cultures were rejected. Certain of the offspring of this selected culture 

 were treated in the way just described for their parents, and so the suc- 

 cessive generations of inbred flies were obtained. 



When the sixth inbred generation was reached, three cultures, 



