Interpretation of freeze-fracture and thin-section results shows that fusion of the peripheral vesicle with the plasmalemma of a Phytophthora palmivora zoospore occurs at several discrete sites and results in the formation and expansion of a particle-free bilayer membrane diaphragm and in the appearance of a polymorphic network of membrane-bounded tunnels, the lumina of which are continuous with the cytoplasm. The outer half of the bilayer membrane diaphragm appears continuous with the outer half of the plasma membrane; the inner half of the bilayer membrane diaphragm with the inner half of the peripheral vesicle membrane; and the inner half of the plasmalemma with the outer half of the peripheral vesicle membrane.Interpretation of our results leads us to formulate a hypothesis for a sequence of several intermediate stages involved in membrane fusion. The initial fusion event is viewed as a local catastrophe (Thorn, R. 1972. Stabilit6 Structurelle et Morphogen~se. W. A. Benjamin Inc., Reading, Mass.) involving the sudden reorganization of apposed elements of the inner half of the plasmalemma and the outer half of the peripheral vesicle membrane. Fusion of apposed components at the rim of the perimeter of fusion results in the formation of a toroid hemi-micelle which provides continuity between the inner half of the plasmalemma and the outer half of the peripheral vesicle membrane. Simultaneously, apposed components at the site of fusion may reorganize into an inverted membrane micelle. A bilayer membrane diaphragm is then formed by apposition and flowing of components form the outer half of the plasmalemma and the inner (exoplasmic) half of the peripheral vesicle membrane. The existence of large areas of membrane contact before fusion may lead to several fusion events and the formation of a polymorphic network of membrane-bound tunnels.Membrane fusion is a frequent and important event in the life of most eukaryotic cells (17,23,29,30). Among other cellular processes, merebrane fusion mediates secretion as a necessary step to the transfer of macromolecules contained within cytoplasmic vesicles to the cell periphery or

I49Zoospores of Phytophthora palmivora were induced to encyst synchronously and changes in morphology, particularly in surface structure, were examined by freeze-fracture and freezeetching techniques.A uniform 'fuzzy coat', 20 to 25 nm thick, covers the plasma membrane. This hitherto unknown structure surrounds the entire zoospore body and flagella. No structural correspondence between cyst wall microfibrils and plasma membrane particles was found. The pattern of distribution of plasma membrane particles remains unchanged during encystment.