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Changes occur in the infant ovary which develop the structurally simple organ at birth to a highly complex one in the maturing animal. The ovary in the immature mouse is not a dormant organ but one that is characterized by a continuous growth and development and a concurrent continuous degeneration of oocytes and follicles. The stroma cells which are present in the ovary at birth are apparently a pool of cells with multiple potentialities, which contribute to the development of the earliest granulosa and theca cells. The interfollicular stroma of the juvenile ovary, however, originates from collapsing and contracting follicles. The quantitative changes in the oocyte and follicle population during the immature period lead to a marked reduction in the total number of oocytes and a simultaneous numerical increase in follicles at different stages of development. Factors influencing oocyte atresia and follicle development in the immature mouse are discussed.In the time interval between birth and maturity the ovary develops from a uniform fairly simple organ to a multiform, highly differentiated one. During the infant and juvenile period the ovary is not a dormant organ, but one in which constant growth, differentiation and degeneration occur.The ovary at birth consists mainly of 2 types of cells: 1) the highly specia¬ lized oocytes and 2) the multipotential 'stroma' cells; multipotential, because it must be from this pool of morphologically undistinguished cells that later granulosa cells, theca cells, lutein cells and others arise. In the time interval of about 5 weeks -between birth and beginning maturity -the ovary does not only grow in size but undergoes a remarkable development that leads among other things to the formation of complicated bodies like large follicles which

Summary. Follicular

The morphology and growth pattern of human ovarian follicles has been studied between birth and 9 years of age. Follicles have been classified according to their morphology, diameter, the diameter of the oocyte and the number of granulosa cells in the widest cross-section. Nine major classes offollicle were recognized. The smallest, Class B follicles, contained a non-growing oocyte and were surrounded by a single layer of flattened granulosa cells. The largest, Class F follicles, which were up to 6 mm in diameter, contained an oocyte which had completed growth (80 \g=m\m) and a large fluid-filled antrum. The range of follicles and the pattern of oocyte growth in relation to follicle growth found in the ovary was independent of age during childhood.Follicular growth and atresia are discussed in the light of current concepts of gonadal and pituitary function during infancy and childhood.

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