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Ultrastructural aspects of fertilization were studied in three cestode species: one proteocephalid with biflagellate spermatozoa, Proteocephalus longicollis; and two cyclophyllideans with uniflagellate spermatozoa, Inermicapsifer madagascariensis (Anoplocephalidae) and Mesocestoides lineatus (Mesocestoididae). Fertilization in all three species occurs in the oviduct lumen or in the fertilization canal proximal to the ootype, where the formation of the embryonic capsule precludes sperm contact with the oocyte. Cortical granules are not present in the oocytes of any of the three species. Spermatozoa coil spirally around the oocytes and syngamy occurs by lateral fusion of oocyte and sperm plasma membranes. In the ootype one (Proteocephalus and Inermicapsifer) or two (Mesocestoides) vitellocytes associate with the fertilized oocyte, forming a membranous capsule which encloses both cell types. In this stage, spirally coiled sperm flagella adhere partly to the external oocyte surfaces, and partially penetrate into the perinuclear cytoplasm. Usually, several loops of the spermatozoon occur within the oocyte cytoplasm. The electron-dense sperm nucleus within the oocyte cytoplasm becomes progressively electron lucent after penetration. Simultaneously with chromatin decondensation, the elongate sperm nucleus changes shape, forming a spherical male pronucleus, which attains the size of the female pronucleus. Cleavage begins immediately after pronuclear fusion.