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Although the thymus gland has been known from antiquity, its role in the body economy has remained enigmatic until recently. During the past year, however, a flurry of reports have given the first indications of the role of this organ in developmental biology. In mammals the thymus develops from the third and fourth pharyngeal pouches in early embryonic life, reaches maximal relative size near the time of birth, and then undergoes a gradual involution. In man the thymus appears in the 10 mm embryo, reaches maximal relative size in the neonate, attains maximal absolute size in the 12-year-old child, and then ..... gradually decreases in relative and absolute size as maturity is reached (1).Our interest in the thymus developed in 1953 when we (2, 3) discovered that a patient with "acquired agammaglobulinemia" had developed a marked immunologic deficiency and an abnormality of the thymus--a benign thymoma--at about the same time. Removal of the thymoma, which was primarily an epithelial stromal overgrowth of the thymus pathologically, failed to alter either the protein abnormality or the immunologic defect. Since that time seven cases of the combined occurrence of these two disorders have been reported (4-8), and in no instance has removal of the thymic tumor restored immunologic function or the protein deficit. Nonetheless, it seems clear that the association of these two rare conditions has been far too frequent to be explained by chance alone.

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Lethally irradiated mice were injected with small numbers of normal spleen cells and then immunized with sheep erythrocytes. Antibody activity was found in their spleens in localized areas whose number corresponded to the number of spleen cells injected. When sheep and pig erythrocytes were injected together, antibody against each was found in separate areas. Each area may consist of the progeny of a single precursor cell, restricted to forming a single antibody.