The structure of the nuclear and cytoplasmic membranes of Trichonympha, a complex flagellate, has been studied in the electron microscope. The nuclear membrane consists of two 70 A membranes, penetrated by numerous pores. Small (100 A) granules occur on the outer surface, around the rims of the pores. Granule-bearing membranes, only 30 to 40 A thick, form long, ribbon-shaped sacs, with 100 A granules on their outer surface. They apparently form close to the nucleus, from which they probably derive their granules. Smooth membranes occur in the parabasal bodies, which consist of stacks of 70 A membranes, joined at their edges in pairs to form flattened sacs. These can inflate and form cytoplasmic vesicles. A protein fibre is applied laterally to the pile of sacs. New sacs, replacing those lost by inflation, appear to form by a process involving the granular membranes, and there may be a transformation of one into the other. Starving eliminates granular membranes and results in a failure in the formation of new parabasal sacs. Refeeding reverses these effects. A parabasal body is a steady-state system, in which the rates of loss and gain of sacs are normally approximately equal. Parabasal bodies resemble the Golgi apparatus.

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