Structure and differentiation of the inner egg membrane of Trichocephaloides megalocephala (Cestoda, Dilepididae)

Electron microscope studies of the inner membrane of developing eggs of T. megalocephala were carried out. At early developmental stages the inner membrane is a syncytial cytoplasmatic layer lying on the basal plate of the embryo. At the preoncosphere stage the division of the membrane into two zones (external and internal ones) takes place. Initially the differentiation manifests itself in the cytoplasm polarisation; at the end of the middle preoncosphere stage the zones are divided by the "oncosphere membrane". The formation of the "oncosphere membrane" is accomplished by the external part of the internal zone. Embryophore is a derivative of the external zone, at the final stages of the formation the embryophore material is transformed from granular into thin-fibrillary. The origin of the external integument of oncospheres of cyclophillids, which, as it has been shown for T. megalocephala, is a derivative of the inner membrane rather than of specialized epithelial oncosphere cells, is considered.