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Unactivated surf clam (Spisula solidissima) oocyte. A large tetraploid nucleus (germinal vesicle; GV) is present, within which lie a prominent nucleolus (arrow) and nucleolinus (arrowhead). The nuclear envelope begins to disintegrate within the first two minutes of activation and the GV is indistinguishable by 10–12 minutes post-fertilization. The nucleolus disappears within 5–6 minutes of fertilization, and the nucleolinus persists for 4–5 minutes by itself. Size bar = 15 µm.
