The thymus, spleen and lymph node at immunized or X-ray treated rats have been cxplanted to tissue cultures in order to study mast-cell and plasmacytc reaction in normal and heparinized media. Mast eells are produced hy the thymus in the first place. by the lymph nodes if necessary, and never by the spleen. The principal Organ of plasmacyte production is the spleen; the lymph nodes produce them only in eases of necessity. It is suggested on the evidence of the present and earlier experiments that the so-called "lymphatic system" consists of mnrphnlogically similar but functionally different organs. An analysis of (he different funct ions leads to the conclusion that the thymus is principally involved in states where the correlation between the tissues is disturbed: it produces acid mucopolysaccharides and mast cells. The spleen, on the other bund, has an important function in states of tissue immunity: it produces immune substances and plasma cells.
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