Differentiation and ultrastructure of the oncospheral envelopes in the hymenolepidid cestode Staphylocystoides stefanskii (Zarnowski, 1954)

Ultrastructure of the oncospheral envelopes in developing and fully formed eggs of the hymenolepidid cestode, Staphylocystoides stefanskii (Zarnowski, 1954), is described. The uterus in this species is saccular, with deep inf oldings of the uterine wall which form pocket-like structures. The uterine wall is composed by a flat syncytial uterine epithelium containing elongate d nuclei with prominent nucleoli. The differentiating and mature oncosphere s are surrounded by three envelopes: (1) an outer envelope; (2) an inner en velope consisting of three layers - an extraembryophoral cytoplasmic layer, a dense and relatively thick embryophore, and an intraembryophoral cytopla smic layer; (3) a thin oncospheral membrane, surrounding the oncosphere. Th e outer envelope usually contains 2 nuclei in the preoncospheral stage, how ever, no nuclei were observed in this layer in the fully formed eggs. The i nner envelope shows in sectioned material 1-2 nuclei in its intraembryophor al layer. The extraembryophoral layer of the inner envelope increases in th ickness during the egg maturation. The embryophore was initially discontinu ous, formed by the blocks of the electron-dense substance, and situated dir ectly under the outer limiting membrane of the inner envelope. Later the ne ighbouring blocks fuse together and finally produce a continuous dense laye r of embryophore. The embryophore remains slightly vacuolised for some time and finally forms a thick homogeneously electron-dense layer. The oncosphe ral membrane appears striated on the high-power micrographs. The ultrastruc ture of oncospheral envelopes in S. stefanskii is compared with those in ot her mammalian hymenolepidids.