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SummaryA survey of the ultrastructure of the stromal cell in histologically normal endometrial biopsies from 41 women at various stages of the menstrual cycle was made. The nucleus and nucleolus showed structural changes which could be correlated with the metabolic activity of the cell at various stages of the cycle. The latter stages of the cycle were characterized by the differentiation of the stromal cells into two distinct cell lines ; the predecidual cell and the endometrial granulocyte. The cyclical changes were compared with those occurring in the endometrial glandular cell and the autonomy of the stromal cell is discussed.