The developing ovarian oocyte of Bombus terrestris has been studied using electron-microscopical, histochemical and autoradiographical techniques. In this publication only the events taking place in the peripheral ooplasm and in the surrounding follicular epithelium are described.

During vitellogenesis three forms of yolk arise. Most of the lipid yolk, together with large numbers of cytoplasmic organelles, is derived from the accompanying trophocyte cells during the earlier phases of yolk synthesis. Later, the larger albuminous yolk spheres which form the major part of the yolk arise at the oocyte periphery as a result of micropinocytotic activity. Yolk precursor materials taken up in this way are probably derived from the haemolymph. Glycogen yolk particles appear in the ooplasm in large numbers during the later phases of yolk synthesis and arise from groups of small vesicles that have been derived largely from the trophocytes.

Two kinds of follicular epithelial cells occur. All the evidence suggests that the larger, more common form is extremely active during synthesis of albuminous yolk, although its precise role remains unclear. The narrower, less common form of follicular epithelial cell occurs only before albuminous yolk synthesis when its protein secretion is probably transferred to the oocyte.

After yolk synthesis the vitelline membrane is laid down in the intercellular space between the follicular epithelium and the oocyte, both of which contribute towards its formation. Next the chorion is laid down. Although only the earlier stages of its formation have been studied, it is apparent thatthe chorion is laid down in the intercellular space immediately adjacent to the vitelline membrane, and that it is formed by the follicular epithelium alone.

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