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Ultrastructural aspects of the oncospheral hook morphogenesis in the dilepidid cestode, Dilepis undula (Schrank, 1788), are described. Oncospheral hook primordia appear within specialised cells, so-called oncoblasts, at the preoncospheral stage of the embryogenesis. Extended Golgi regions, numerous free ribosomes and mitochondria are involved in the hook development. During the hook growth, the blade and base gradually protrude outside the oncoblast plasma membrane. The nucleated oncoblasts persist around the handles of the fully formed hooks even in the mature oncospheres. The mature hooks in the cross-section consist of two to four layers of different electron density, depending on the level of the section; two of them, namely the highly electron-dense cortex and a moderately electron-dense core, are observed in all parts of the hook. A circular, septate desmosome and two electron-dense rings on its both sides are formed around the exits of the hook blades from the oncospheral tegument. Each hook blade has a protective moderately electron-dense "cap" on its tip. A hook region membrane, similar to that described in other cyclophyllideans, forms on the surface of the oncosphere a cavity covering the hook blades. The hook muscle attachment zones at the hook guard and base are represented by a relatively thick layer of fibrous material. The peculiarities of the oncospheral hook formation in Dilepis undula are compared with the results of our previous studies and literature data on other cestodes so far studied in this respect.