||P.889 bottom paragraph: "Sixty rats varying in body weight from 65-300 gr. were sacrificed and the wet weights of the spleen, mesenteric node, and thymus were determined. The total amount of DNA in each tissue was determined by the colorimetric method (Ceriotti, 1952). The DNA content per cell was determined for TDL (thoracic-duct lymph) lymphocytes collected by a Bollman fistula. Assuming the DNA content to be constant and comparable for cells in each of the various tissues, the total number of nucleated cells per organ was calculated. The number of cells per mg. wet weight for each organ was GMM (mesenteric lymph node) = 1.1×10^6, spleen = 2.0×10^6, and thymus = 3.6×10^6."