The fine structural characteristics of oncospheral hook morphogenesis in the davaineid cestode Inermicapsifer madagascariensis are described. The primordia of the hooks appear in the oncoblasts of embryos in the advanced stage of development, in which there is a greatly reduced number of blastomeres that exhibit a bilaterally symmetrical pattern in their organization.

The hook primordium, adjacent to the invaginated part of the nuclear envelope, is surrounded by an abundance of free ribosomes, mitochondrial aggregations and extended Golgi regions. Simultaneously with its elongation and transformation into a blade, the hook primordium material becomes differentiated to form an electron dense cortex and a less dense, inner, crystal-like core. At the beginning of shank formation, the blade of the hook protrudes outside the oncoblast. The membrane-enclosed point of exit of the blade is surrounded by a cytoplasmic sheath which later forms a circular, septate desmosome.

With oncoblast degeneration, muscle fibres attach directly to the collar and the base of the hook.