Cytological and histological studies on ovaries from cattle fetuses of various stages indicate that the factor leading oöcytes in first meiotic prophase to enter dictyotene is contact with follicular cells. Early in embryonic development, each ovigerous cord in the upper layer of the ovarian cortex appears to contain a disproportionately greater number of germ cells than potential follicular cells. Many of these oöcytes enter meiosis and proceed beyond diplotene to diakinesis, only to degenerate. As the fetal ovary develops, the correct ratio of germ cells to potential follicular cells is gradually established, from deep in the cortex upward. An individual oöcyte in first meiotic prophase is now surrounded by a single dense layer of follicular cells; only these oöcytes are capable of entering dictyotene and forming primordial follicles. Precocious oöcyte maturation appears to be forestalled by the intimate contact of the follicular cells, which seems to force the oöcyte nucleus to maintain the non-homologous association of its biva