Thymus: primary site of lymphopoiesis

The thymus of the leopard frog is the primary, if not the sole, source of progenitor lymphocytic cells that normally colonize the spleen. Following embryonal parabiosis, a high level of cell chimerism exists in the thymus glands of each member of the parabiotic pair. Evidently, there can be an appreciable amount of cross-circulation of thymic cells between thymus glands.