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The cellular organisation of the infective eggs of I. madagascariensis has been reconstructed at light (LM) and electron microscopy (EM) level from serial semithin (LM) and ultrathin (EM) sections. The oncospheres are surrounded by parenchymatic capsules. The total number of oncospheral cells is about 44 (50 nuclei). Among them, five major cell types have been distinguished. These consisted of: (1) a six-nucleate, U-shaped penetration gland; (2) a bi-nucleate medullary centre (= sunken perikaryon of oncospheral tegument); (3) two nerve cells of neurosecretory type; (4) about 30 somatic cells (=mycytons of hook and somatic musculature); and (5) about 12 germinative cells (two groups of 6 cells), localised in the posterior pole of the hexacanth. The functional ultrastructure of hook-muscle system and penetration gland and their role in host tissue penetration are discussed.