SUMMARYImmature sporangiospores of Rhixopus iaigricans and R. sexualis were relatively thin-walled, although thickened longitudinal ridges were already discernible. The position of the nuclei suggested that they had recently divided; the mitochondria were globose and similar to those of vegetative hyphae; the protoplast was surrounded by a thin plasmalemma and contained numerous food globules. The endoplasmic reticulum was sparse. Mature spores had a thick wall of reticulate structure and contained large contorted mitochondria. The first visible stage in germination was the formation of an inner cell wall of tangential elements resembling that of the vegetative hyphae and contrasting with the original spore wall. The mitochondria increased in number, probably by division of the large contorted ones of the dormant spore, and were again regularly globose. The original spore wall became considerably stretched and finally ruptured to allow the germ tube enveloped in the newly formed elastic inner wall to emerge.

SUMMARYUltrathin sections of vegetative hyphae of Rhixopus sexualis and R. homothallicus examined by electron microscopy revealed structure essentially similar to that reported for some other filamentous fungi and yeasts. The cell wall in section consists of elongated elements tangentially orientated. The nucleus is surrounded by a double membrane interrupted by pores and contains a denser body identified as the nucleolus. The cytoplasm contains well-defined mitochondria, vesicles (cisternae), unidentified spherical bodies, oil drops and vacuoles. There is a complex system of cytoplasmic membranes or membrane-like layers including the outer layer or plasmalemma, membranes (tonoplasts) surrounding the vacuoles and, in the zone of extension growth behind the hyphal tip, a more or less continuous, convoluted ' cortical membrane ' separating a central core of cytoplasm from a peripheral zone. The possible significance of such a membrane in relation to cytoplasmic streaming is discussed.

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SUMMARYThe fine structure of Pythium debaryanum Hesse differs from that of Rhizopus species and some other fungi in the abundant regularly distributed endoplasmic reticulum, the presence of typical Golgi-bodies and the irregularly tubular structure of the cristae mitochondriae. Some of these characters resemble those of some algae and the liverwort Anthoceros and the significance of this is discussed. Typical lomasomes are present. METHODS Young hyphae of Pythium debaryanum RESULTSThe cell wall shows no structure and is represented only by a narrow electron transparent zone (Pl. 8, fig. 5).