Fine Structure of Silica Deposition and the Origin of Shell Components in a Testate Amoeba Netzelia tuberculata1

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Abstract
Netzelia tuberculata secretes a test composed of siliceous particles cemented together by organic plaques forming a single‐layered spheroidal shell. The siliceous particles are produced within cytoplasmic vacuoles by three mechanisms: 1) synthesis de novo by deposition of the silica on a matrix; 2) deposition of silica on particles remaining in digestive vacuoles, including starch grains and undigested walls of yeast cells; and 3) secretion of silica as a hollow sphere at the periphery of vacuoles enclosed by the silicasecreting membrane. The silicalemma (silica‐secreting membrane) originates as fibril‐containing vesicles (GFV) secreted by the Golgi body. Fusion of these vesicles with membranes surrounding digestive vacuoles or with membranes surrounding specialized vacuoles containing a silica‐binding matrix apparently converts the vacuole into a silica‐depositing organelle. Small spherules of silica occur on the vacuolar side of the membrane surrounding the developing test granules, marking the presence of silicalemma activity. These colloidal spherules become aggregated into larger spherules that condense to form the siliceous surface of the developing test particle. Other Golgi vesicles, designated Golgi plaque vesicles (GPV), produce the organic plaques that are deposited among the siliceous particles at the periphery of the cell during new test construction during cell division. The fine structure of the GFV and GPV and their role in test wall deposition are discussed in relation to other silica‐biomineralizing protozoa, including radiolaria.