FERTILITY AND STERILITY IN DROSOPHILA 203 



It will be recalled that over sixty generations of continous 

 selection has failed to produce pure truncate stock. May not 

 this result be explained on the grounds that only the heterozygous 

 forms with respect to this structure ever reach maturity; the 

 homozygous or pure truncate animals never coniQ into existence? 

 It is of interest in this connection to recall Cuenot's failure to 

 produce yellow mice that would breed true. 



The reduced fertility of the inbred stock in successive genera- 

 tions of inbreeding may also find its explanation on the assump- 

 tion that the gametes have reduced to a condition approaching 

 homozygosity for as I was able to show in the Fu generation a 

 larger percentage of zygotes developed as a result of outbreeding. 



FERTILITY AND HIGH PRODUCTIVITY OF THE HYBRIDS 



It has been pointed out that the hybrids that resulted from the 

 crosses between the low-producing truncates and the high-pro- 

 ducing wild stocks give rise to a large number of offspring. They 

 produce on an average many more young than their high-produc- 

 ing parents. The fertilizing power of their gametes when inbred 

 was only 56.6 per cent, while in the Woods Hole stock it was 75 

 per cent. In other words, the productivity of the hybrids is 

 higher than its high-producing parent, but the fertilizing power of 

 its gametes is lower. 



This apparent contradiction is accounted for by the fact that 

 the output of eggs of the hybrid female is greatly increased and de- 

 spite the fact that the fertilizing powers of the egg and sperm when 

 placed together is relatively low, nevertheless the production of 

 offspring is very high. This fact shows how easily one might be 

 misled by using the number of offspring as a measure of the fer- 

 tilizing power of the egg and sperm when not controlled by more 

 accurate means. A comparison of the number of eggs laid by 

 the hybrid, table 21, with the number laid by the Woods Hole 

 females, tables 14 a, 15 a and 15 d will show that the output of 

 eggs from the hybrid is practically twice that of its high-produc- 

 ing parent. 



The following controlled experiment, table 22, confirms the 

 foregoing conclusion and also shows that it makes no difference 



