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Transmission of Pseudodiplorchis americanus is restricted to the brief period when its host, a desert toad, enters water to spa,vn. The parasite accumulates its entire annual reproductive output within an elongated uterus during the 10-11-month period of host hibernation, Embryos of P. americanus, at all stages of development, are retained within the uterus which eventually becomes packed with around 150 encapsulated infective larvae. Recently formed eggs, which comprise a fertilized ovum and 2-3 vitelline cells, are closely surrounded by a primary eggshell which stains positively for acidic proteins and keratin. Initially, during passage along the proximal uterus, the egg capsule is only 60 mu m in diameter, but as it passes to the distal uterus it expands to 800 mu m in diameter to accommodate the growing larva. Due to chemical alterations or complete replacement of the shell, the final (secondary) egg capsule is a large sac-like structure composed of elastin, The flexible nature of this shell maximizes the numbers of infective larvae which can be stored in utero. TEM studies have revealed this capsule to be composed of multi-laminate membranes with a specialized cytoplasmic lining involved in a unique mechanism for embryo nutrition, This is the first report of an elastin-type eggshell within the Monogenea. (C) 1997 Australian Society for Parasitology. Published by Elsevier Science Ltd.