A complete reconstruction of the constituents of the mature egg of N. dispar has been attempted at the light microscope level based on serial semithin sections, and results have been correlated with partial reconstruction from ultrathin sections. The outer coat of the oncosphere consists of an anucleate cytoplasmic layer of tegument, a basal lamina, and two layers of peripheral, somatic musculature. The oncospheral hooks and their associated muscle system, situated in the anterior pole of the larvae, together with penetration gland secretion appear to play an important role in host tissue penetration. The bases of each lateral hook pair are joined by a common zone of “connective” material whereas the medial hook bases are embedded in individual cups of this material. Five major types of oncospheral cells have been distinguished. These consisted of: (1) a bi-nucleate medullary centre (= subtegumental cell); (2) a bi-nucleate, U-shaped penetration gland; (3) two nerve cells of neurosecretory type; (4) about 34 somatic cells (= cell bodies of somatic and hook muscles); and (5) about 12 germinative cells, arranged in two groups of six cells, situated in the posterior pole of the hexacanth. The position of oncospheral structures remains fixed in relation to one another but at the same time is somewhat arbitrary due to the high plasticity of the hexacanth during movements.
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