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Lymphopoiesis was studied by electron microscopy in the palatine tonsil of the rabbit from 18 days gestation to 5 days after birth. At 18 days tonsils formed as mounds of mesenchyma covered with epithelium. At 19 days the basal epithelial cells started to increase in number, eventually forming ‘buds‘ which projected into the mesenchyme. Simultaneously, lymphocytes appeared nearthe epithelium or buds. There was marked resemblance between the basal epithelial cells and the lymphocytes. Budding slowed down after the 25th day, but individual basal cells continued to migrate into the mesenchyme and lymphocytes increased in number. Ultrastructure wassimilar in both types of cells, and differentfrom mesenchymal cells. At 29 days lymphocytes were found in the basal epithelial layer behind an intact basement membrane. The evidence indicated that lymphocytes were derived from epithelium.