The ovary of the chick embryo starts differentiation from the 7th day of development. The left ovary grows progressively, whereas the right one undergoes involution. With the purpose of determining the degree of regression of the right ovary in relation to the diverse types of granular cells that form it, an ultrastructural study was made of embryonic ovaries of 7, 11, 15 and 19 days old. Three types of granular cells are described: A cells, similar to granulocytes, with phagocytic functions, B cells with semi-dense granules and functionally active C cells with granules of low electronic density. During development, A cells remain in the right ovary until 19 days. Nevertheless, there is not an increase in their amount in the involuting ovary. B cells diminish in the right ovary from 7 to 19 days; however, no modifications are found in the left ovary. These cells are considered precursors of the C cells due to the transition forms found in both cell types. C cells are seen in similar proportions in both ovaries at all the embryonic stages studied. It is important to note that there is a difference in the mechanism of hormonal induction at the level of these cells in the ovary that progresses as compared to the one that undergoes atrophy. Precursor B cells would be destroyed in the right ovary before undergoing a complete functional differentiation. These observations would indicate that both B and C cells are involved in the hormonal control of differentiation and estrogen secretion in both ovaries. With respect to type A cells, further studies are needed to establish their function.