Abstract
Three different types of evidence are reported concerning the origin of yolk protein in the oocyte of the annelid Nereis virens. The fine structure of the oocyte and its different types of storage organelles are described; only few pinocytotic vesicles are found. Electron-microscopic autoradiography of oocytes incubated with 3H-leucine showed that the oocytes synthesize protein which, in part, becomes localized in the yolk bodies. On the other hand, electron-microscopic autoradiography of the oocytes incubated with a yolk-specific, radioactively labeled protein, Nereis vitellin, makes evident the uptake of this major yolk protein by the living oocyte. Incorporated Nereis vitellin is confined to small dense bodies and growing yolk bodies but never occurs in Golgi dictyosomes. Incorporation of labeled vitellin and intracellular transport into a specific storage compartment are regarded as strong evidence for heterosynthetic (somatic) origin of yolk protein in nereids, a view conflicting with the conventional interpretation of fine-structural and autoradiographic results from nereid oocytes.
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