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Polyembryonic development in Copidosoma koehleri Blanchard is traced from the ovarian egg to the adult stage. Polyembryony apparently results from the change in cytoplasmic-nuclear balance when the formation of the polar region removes about half the cytoplasm of the egg. Furthermore, the extent of polyembryonic division is increased when the amount of nuclear material is increased by the addition of a sperm nucleus, more individuals being produced from fertilized than from unfertilized eggs. The mean number of adult parasites constituting an exclusively male brood was 21, while the mean number of individuals found in female broods was 31.4. This sexual differential in the amount of polyembryonic division is explained on the basis of a sperm-influence hypothesis. This theory is further substantiated by the androgenetic development of eggs inactivated by heavy dosages of x-rays. The polygerminal mass is invariably found to be associated with the fat body of the larval host in the region dorsal to the stomodaeum, and a remarkable ingrowth of host tracheae occurs to supply the respiratory demands of the developing parasite embryos. The extraordinary, precocious larval forms known as "asexual" are found to be present at a very early stage in the embryonic development of Copidosoma koehleri. The origin of broods of mixed sexes is explained upon the basis of superparasitism. The quantitative and experimental studies included in this paper are offered as a new approach to the investigation of insect polyembryony.