INHERITANCE IN HYDATINA 83 



20 per cent series, with one exception, yielded less than 21 per 

 cent (^1 to Li from H and Hi from H). Thus, in (C X D) X 

 (C X D), selection for viability of fertilized eggs was accompanied 

 by a differentiation with respect to the proportion of male-pro- 

 ducers such that the least viable eggs yielded the parthenogenetic 

 line with the most male-producers. Precisely the same result 

 appeared in L4, where selection for low viability produced a 

 parthenogenetic line with 37.5 per cent of male-producers, while 

 the high viability eggs gave rise to a line with only 13.6 per cent 

 of male producers. 



One can not safely generalize, however, and say that a high 

 percentage of male-producers is associated with low viability of 

 fertilized eggs; for in Hz sl similar selection for viability of eggs 

 was made, without, as figure 2 shows, any appreciable effect on 

 the percentage of male-producers in the descendent partheno- 

 genetic lines. 



INTERPRETATION OF RESULTS 



Viability of fertilized eggs appears to be dependent on segre- 

 gating factors or genes. Otherwise, two eggs from the same 

 source could not give rise to parthenogenetic lines whose ferti- 

 lized eggs differ markedly in viability. Selection for viability 

 was effective in a hybrid (heterozygous) lot of eggs because of 

 this segregation. 



Since a single selection produced the maximum effect on via- 

 bility, since this effect was not augmented by subsequent selec- 

 tion, it appears probable that but few genes are concerned with 

 viability. The eggs of the first selected families must have been 

 practically homozygous, or subsequent selection would have aug- 

 mented the effect. Had the genes affecting viability been num- 

 erous, the chance of selecting a completely homozygous egg the 

 first time would have been quite small. I hesitate to suggest that 

 a single pair of genes is involved; but it is quite possible that 

 only one of the elements of viability discussed in a previous 

 section has been studied here, and that the irregularities noted 

 are but the effect of external conditions. 



