Bone marrow as a major lymphoid organ in Rana.

The bone marrow of adult frogs (Rana pipiens and R. catesbeiana) contains leukocytes equivalent in number to those present in the spleen. As in the spleen, a high percentage of these cells are morphologically identified as lymphocytes, and the bulk of these can be categorized as small or medium lymphocytes. In young frogs (R. catesbeiana), the ratio of marrow to spleen leukocytes is considerably greater than in adults, and in limited comparisons a high percentage of lymphocytes are found. A comparison of adult marrow and spleen anti-TNP (trinitrophenyl) plaque-forming cell responses at various days early in the response indicates that in the majority of animals marrow normally produces a larger total primary response.