The structure of the ovarian follicle in the region of the germinal disc, which appears as a white plaque at the surface of the oocyte, was examined by electron microscopy and compared with the non-disc region which overlies the yellow yolk mass of the oocyte in the final growth phase. The main differences concerned the granulosa cell layer and the surface layer of the oocyte. In the disc the granulosa cells were less regularly arranged and the spaces between them varied in width. Their mitotic rate was higher than that in the non-disc region, where cell division was seldom observed at maturity. The perivitelline layer was comparatively poorly developed at the periphery of the germinal vesicle in 15 mm follicles, but eventually attained a uniform thickness throughout the follicle. In the intercellular and perivitelline spaces there were smaller amounts of granular material. Marked differences were observed in the ooxyte surface layer. In 15 mm follicles the surface of the germinal disc was thrown into numerous microvilli and some narrow indentations containing macrovilli from the granulosa cells. Coated vesicles, 120 nm diameter, appeared to be invaginating from the oolemma, whereas 70 nm coated vesicles were present in the deeper cytoplasm. In follicles of more than 25 nm diameter these structural conformations were evident only at the periphery of the disc; for the most part the 120 nm coated vesicles were absent, and over the germinal vesicle microvilli were of rare occurrence. On the other hand, the bulk of the oocyte surface was highly convoluted throughout this period of growth, numerous granulosa cell macrovilli extended into deep pouches associated with 300 nm coated vesicles, and the oolemma possessed a coating of fuzzy material. These observations suggest that there is a restricted passage of yolk precursors to the surface of the germinal disc, and that the inability to transport yellow yolk into the disc is related to differences in the oolemmal surface coat and the population of coated vesicles. The surface modifications, as well as the proliferation of the granulosa cells, are likely to be influenced by the presence of the germinal vesicle.