Abstract
The development of lymph follicles of the bursa of Fabricius of chick and quail embryos was studied by light and electron microscopes. The epithelial cells of the inner (or medullary) zone were transformed both into reticular and lymphocytic series. There was a stage where a close contact between epithelial and mesenchymal cells through the disruption of the epithelial basement mem-brane of the primordial lymph follicle (bud). It was most plausible that cells of lymphocytic series not only in the outer but also in the inner zone are of dual origin: epithelial and mesenchymal. The inner (or medullary) zone of the bursal follicle consisted of epithelial cell reticulum and lymphocytic series without the contribution of mesenchymay cells and blood vessels. The outer (or cortical) zone consisted of mesenchymal cell reticulum and lymphocytic series with the partial contribution of epithelial cells and blood vessels. Lymphocytic cells of the bursa showed several developmental stages from blastic types to medium-sized lymphocytes. But they tended to have larger cytoplasmic diameters with stronger basophilia and more abundant free ribosomes in their cytoplasm than those of the thymus.
