The factors length of life, daily egg production and especially yield of adults from counted eggs were studied for the wild type and for the mutants vestigial, hairy, singed, singed3, reduced, scraggly, rudimentary7, and rudimentary14. It is concluded that fertility is governed by constitution of zygotes of parents and offspring and not by genie contents of gametes. In all outcrosses, substantial increases in fertility occurred, and the hybrids themselves thus showed increased fertility, explained as due to numerous genie differences between all Drosophila strains analogous to the heterosis effects in corn crosses.