AbstractThe intrauterine, mature and fully embryonated eggs of the prosthogonimid trematode Mediogonimus jourdanei Mas-Coma et Rocamora, 1978 were examined by means of transmission electron microscopy (TEM), using high pressure freezing, freeze substitution and infiltration with resin techniques. Each embryonated egg is composed of a miracidium surrounded by three envelopes: (1) the egg shell, (2) the outer and (3) inner envelopes. Egg envelopes play an important role in the protection,

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... sm, storage of nutritive reserves and the general biology of the M. jourdanei egg. The inner envelope is characterised by large, flattened nuclei, and its syncytial cytoplasm contains a heavy accumulation of glycogen, lipid droplets, mitochondria and large vesicles. These traits indicate that this layer has the features of a metabolically-active syncytial layer with an energy storage capability. In the infective eggs observed before the hatching of the miracidium, areas of so-called "focal cytoplasmic degradation" were frequently observed, which may be involved in the autolytic process of all components of this envelopes.