Changes in the fine structure of growing female germ cells of Paracentrotus lividus and the sequence of their stages were examined by the light and the electron microscope. The investigation showed that, in the previtellogenic stage, the cytoplasm contained numerous vacuoles and few yolk granules. In the vitellogenic stage, the number of yolk granules increased and, as the oocytes attained maturity, the cytoplasms became packed with yolk. The structure of the nutritive phagocytes is also described and their relationship to the developing oocytes is discussed.