SUMMARY

Ultrathin sections of vegetative hyphae of Rhixopus sexualis and R. homothallicus examined by electron microscopy revealed structure essen-tially similar to that reported for some other filamentous fungi and yeasts. The cell wall in section consists of elongated elements tangentially orien-tated. The nucleus is surrounded by a double membrane interrupted by pores and contains a denser body identified as the nucleolus. The cyto-plasm contains well-defined mitochondria, vesicles (cisternae), uniden-tified 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