In eggs of many insects there exists a characteristic structure called an oosom that is required for the formation and differentiation of primordial germ cells (PGCs). Thermobia domestica (Packard) belongs to the group of insects in which there are no visible signs of such germ cell determinants and PGCs are formed at the later stages of development. Spontaneous and experimental polyembryony helps to resolve the problem of germ cells determinants existence. The stratification of cytoplasmic inclusions in T. domestica eggs causes the displacement of germ cells determinants. As a result, all twin embryos and some single embryos (experimental polyembryony) have sterile gonads.