Morphology and Development of Oncospheres of the Cestode Oochoristica symmetrica Baylis, 1927

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Abstract
The hexacanth embryo (oncosphere) is the definitive embryonic stage of cestodes. Its development involves four periods. (1) MORULA: a solid ball of undifferentiated mesenchymal results from cleavage. (2) HOOK FORMATION, PARENCHYMA AND MUSCLE DEVELOPMENT: three different pairs of hooks develop from oncoblasts. Mesenchymal cells form muscle fibers. (3) MEMBRANE FORMATION AND EPIDERMAL GIAND DEVELOPMENT: a non-cellular membrane is secreted around the embryo of Oochoristica. An epidermal gland area covers part of the cortex. In other cestodes the glandular epidermis may detach to form an embryophore. (4) PLASTIN CELL FORMATION: old oncospheres become small and more compact and several plastin or germinative cells appear. The events of these periods vary with differ-ent tapeworms but are carefully considered for Oochoristica. Hook formation takes place in the oncoblast. Musculature of the hooks and parenchyma is described as large cells associated with contractile substance. One role of the epidermal glands is secretion of substances used for membrane formation. Suggestions for a theory of penetration are introduced. The embryonic epidermis is a possible true ectoderm that differentiates very early. The following cell types are recognized: epidermal cells, muscular-parenchyma cells, granular and compact germinative cells (plastin cells), hook-forming cells (oncoblasts), epidermal gland cells. Flame cells did not develop until the late metacestode. Principles important for tapeworm systematics, oncosphere morphology and development are summarized. Seven distinct types of embryo and their development patterns are identified: meso-cestoidid, taeniid, anoplocephalid, hymenolepid, linstowiid, proteocephalid, diphyllobothriid. Each has a characteristic embryo and hooks. Four basic types of information are needed for comparison: (1) Oocyte morphology; (2) Early cleavage pattern; (3) Morphology of oncosphere, hooks, fate of epidermal cells and nature of embryonic membranes; (4) Metacestode morphology. It is suggested that information concerning (1) and (3) be included in descriptions of new species.
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