198 ROSCOE R. HYDE 



The percentage of eggs that hatched in this experiment is not 

 altogether rehable, due to unfavorable conditions which arose. 

 Previous to this time the flies had hatched normally and I still 

 believe that the foregoing experiments measure the absolute 

 fertility of the different stocks and combinations made, within 

 a range of 3 per cent. In this experiment, however, the food be- 

 came very sour and the flies did little more than craw^l out of the 

 pupae and die. The dead animals were searched for and counted. 

 The percentages as given in this table are in consequence too low 

 and are to be taken relatively to each other and should not be 

 compared with the percentages as given in the other tables. 

 Tables 19 and 20 give the details of a second and third repetition of 

 this experiment. The results express more accurately the facts in 

 regard to fertility in these combinations. The plan of the third 

 experiment, table 20, was modified somewhat for it seemed that 

 fertility could be determined fairly accurately by isolating the eggs 

 from mass cultures provided the experiment were controlled by a 

 stock the fertility of which was known. Accordingly hybrids 

 between the truncates whose fertility was low and the Woods 

 Hole stock whose fertility was high were produced and a number 

 of these hybrids were tested in mass culture. The hybrids were 

 also back crossed into the truncates as shown in table 20. In 

 each case a number (the actual number is indicated in the table) 

 of virgin females was placed in a bottle and twice as many males 

 from the stock, in w^hich the combination was desired, added. 

 The males and females were kept separate for four or five days 

 before they were placed together. 



At the time of placing the two sexes together ati epidemic of 

 mating took place. In each of the bottles about half the pairs were 

 to be seen mating at the same time. None of the eggs were 

 isolated until five days later in order to give all the females a chance 

 to mate and in order to avoid the first few eggs laid, which as I 

 have shown, are not likely to hatch, especially in the case of the 

 truncates. The eggs w^ere isolated day by day in the usual 

 manner. Table 20 gives the result of this experiment. It will 

 be noticed that it is in close agreement with the second experi- 

 ment of this series. 



