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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.

Lymphopoiesis in fetal thymuses of mice and rabbits was studied to try to correlate heterogeneity of lymphocytes with heterogeneity of embryonic origin. The thymuses of fetal mice of 11 to 14 days gestation and of fetal rabbits of 18 and 19 days gestation were examined by electron microscopy. At the time of earliest appearance of lymphoid cells in both species desmosomes were seen on both epithelial and lymphoid cells. These desmosomes persisted for several days, then disappeared from completely differentiated lymphocytes in both species. The similarity between epithelial and lymphoid cells in their earliest developmental stages suggests a common precursor cell for both.