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In the preceding papers (2-4) it has been shown that, in rats thymectomized early in life, there is a partial or complete suppression of immune phenomena of the type attributed to delayed hypersensitivity, specifically of skin sensitization to tuberculin or to purified protein (bovine serum albumin), experimental allergic encephalomyelitis, and skin homograft rejection. Evidence is presented that humoral antibody formation may be diminished as well. The present paper deals with histologic changes found in the lymphoid tissues of the animals employed in these studies. The principal finding is an almost complete absence of small lymphocytes from their usual position in the spleen and lymph nodes. Methods and MaterialsThe material of the present study consisted of hematoxylin-and eosin-stained paraffin sections of various lymphoid tissues removed at autopsy from animals described in the preceding papers. Selected sections were also stained for reticulln. The animals were classified on the basis of the amount of residual thymus ("thymectomized" means no thymus or less than 100 mg residual thymus). In the bovine serum albumin (BSA) and experimental allergic encephalomyefifis (EAE) experiments, the thymectomized animals were further divided into those which reactcd, i.e. developed appreciable Arthus or delayed reactivity to BSA or developed EAE, and those which did not. In the tables and graphs the word "reacted" is employed to designate the former group.For the purposes of the present study, sections were examined of the liver, spleen, and various lymph nodes and, in occasional animals, the gastrointestinal tract (multiple levels) and femoral marrow. In animals which received adjuvant in the left hind foot-pad, with or

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