For more than two centuries post-ovulatory internal self-fertilization has been ascribed to bryozoans. Since the last third of 19th century, some scientists have unsuccessfully tried to dispute this opinion. Only when the release of sperm through the terminal pores of the tentacles was directly observed by Silén in 1966, a paradigm was changed and cross-fertilization was admitted as a rule in Bryozoa. Additional experiments proved that opportunities for self-fertilization are very restricted in this phylum. Further studies showed that fertilization is internal in all Gymnolaemata: either intracoelomic or intraovarian. It provides high levels of fertilization success which is especially important for the species possessing small number of eggs developing to lecithotrophic larvae. In egg broadcasters sperm fuses with late stage ovarian oocytes at or near ovulation. In brooders sperm fuses with either late or early stage ovarian oocytes. These demonstrate the different stages of the transition towards early sperm-oocyte fusion culminating in a strong dependence of the oogenesis from fertilization in the brooding Cheilostomata.