Ultrastructural characteristics of developing eggs in the late preoncospheral and oncospheral stage and that of the uterine capsules in the hymenolepidid cestode, Pseudhymenolepis redonica Joyeux et Baer, 1935, are described. The uterus in this species breaks down very early into uniovular capsules. The uterine wall consisted of a syncytial flat uterine epithelium separated from the medullary parenchyma by a thin extracellular basal matrix. The uterine epithelium contained elongated nuclei with prominent nucleoli in the juxtalumenal cytoplasm. Its apical plasma membrane was folded into long microlamellae. The differentiating and mature oncosphere were surrounded by three envelopes: (1) an outer envelope, still containing the nuclei in the preoncospheral stage; (2) an inner envelope consisting of three layers - an extraembryophoral cytoplasmic layer, a thin and discontinuous embryophore, and intraembryophoral cytoplasmic layer; (3) a thin oncospheral membrane, closely surrounding the oncosphere. The relative thickness of the extraembryophoral and intraembryophoral layers of the inner envelope was changing during egg maturation. The numerous small mitochondria which were initially present only in the intraembryophoral layer, were concentrated later in the extraembryophoral layer and in many cases were observed in the embryophoral pores. The above data may suggest that these cytoplasmic organelles are pushed through the embryophoral pores as a result of the pressure of the developing oncosphere. The oncosphere surface was covered by the cytoplasmic oncospheral tegument, basal lamina and a layer of subtegumental somatic muscles. Several cell types were distinguished in the differentiating and mature oncospheres, namely: the germinative cells; somatic cells (= myocytons of somatic and hook muscles); the bi-lobed penetration gland with its secretory granules; the “neurosecretory” cells with their characteristic dense-cored membrane-bound vesicles. Each oncosphere had three pairs of embryonic hooks: one median, one dorso-lateral and one ventro-lateral pair. The degenerating hook-forming cells or oncoblasts remained visible around the hook handles. The details of the ultrastructure of the uterine capsules, oncospheral envelopes and different cell types of differentiating and mature oncospheres of P. redonica are discussed in comparison with literature data on other hymenolepidids, parasites of mammals and birds.
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