226 JOSE F. NONIDEZ. 



Although, as stated above, the presence of sperm in the sper- 

 mathecse oi flies which have been laying eggs for several days 

 suggests that the first spermatozoa used in fertilization are those 

 stored in the ventral receptacle, which appears empty or contain- 

 ing few spermatozoa, still the same conditions may appear if the 

 amount of sperm received by the spermathecse surpasses by far 

 that stored in the ventral receptacle. In other words, the largest 

 amount of sperm will last longer, provided that spermatozoa are 

 simultaneously set free by all the seminal receptacles. 



Further experiments were carried out with the hope of clearing 

 up this point. These experiments consisted in allowing fertilized 

 females to lay eggs during a period of six days, and then cross 

 them with a mutant which would enable me to recognize the 

 hybrids in the F 1} without need of testing the offspring. The 

 mutant used was the bar-eyed male. Seven flies were used in the 

 experiment, which covered a period of eighteen days. The food 

 was changed daily and placed in separate vials to allow the eggs 

 to develop. 



These experiments showed the following facts : ( i ) When the 

 number of flies produced during the first six days is low, the off- 

 spring following fertilization by the bar male consists of hybrid 

 and normal females, the males being normal in all the cases. This 

 result can be accounted for by the presence of spermatozoa from 

 the first copulation in the ventral receptacle where they become 

 mixed up with those received during copulation with the bar 

 male. (2) When, on the contrary, the number of flies produced 

 during the first six days is high, only hybrid females are pro- 

 duced during the first days following fertilization by the bar 

 male. That is to say, the ventral receptacle was already empty 

 of sperm. It was then filled by the sperm ejaculated by the bar 

 male, and, therefore, only hybrid females were produced. In 

 such a case mixed offspring appear after a few days, when the 

 spermatozoa stored in the spermathecse begin to be used. 



The results obtained in the second case have been graphically 

 shown in the curve of Fig. 10, plotted after the data obtained in 

 a female which had produced a large number of flies during the 



