In recent years the thymus, like the pineal, has come into its own as an organ of physiologic significance. Many previously unrelated observations are being integrated, and old concepts once considered almost naive have taken on new meanings. For example, by the turn of the century, Beard' had already expressed the idea that the thymus was the "parent source" for the lymphoid structures of the body and that thymic lymphocytes originated from thymic e p i t h e l i~m .~.~ It is necessary to understand this histogenetic background in order to clarify the otherwise confusing lymphoepithelial patterns and complexes encountered in thymic turn or^.^ The occurrence of another tumor in the thymus, the seminoma or germinoma, which usually arises in the gonad,5 may also become understandable in view of the evidence showing that thymic elements affect immunologic responses.6 The localization of primordial germ cells in the thymus could conceivably result in the conveyance of genetic immunologic information.An antiquated and even interred concept, that of status thymicolymphaticus, was found to have validity in work showing both thymolymphatic hyperplasia and diminished resistance in suprarenalectomized a n i m a l~.~-l~Subsequently it was learned that pituitary hormone" and steroid products of adrenogenital origin, corticoids in particular, caused thymic involution and that this response of the thymus might even be used for bioassay.In most studies, the effects of these agents were evaluated by means of systemic administration. It seemed possible that the introduction of active materials directly into the gland might bring out cellular reactions of thymic tissue to high local concentrations of physiologically effective substances which might not otherwise be detected. Such alterations would almost certainly also reflect direct, rather than indirect, effects. As a matter of fact, in a report that testosterone affected the thymus less when given intraperitoneally than when administered subcutaneously, the suggestion was made by Browning and co-workers12 that it might be worthwhile evaluating the effects of mediastinal administration. It had also been hoped that we might be able to demonstrate zonal differences in thymolytic activity near adrenocortical tissue transplants. In the present study, adrenocortical, testicular, and ovarian tissues were transplanted to the thymus and pellets of estrogenic, androgenic, and corticoid hormones were placed in the gland. Two-to three-month-old albino rats derived from a Wistar strain supplied locally were employed throughout. The thymus was approached by dividing the upper sternum; the upper end of the gland was incised and the tissue fragment or pellet slipped into the defect.
Adrenal TransplantsA small cortical fragment, about 0.5 mm. in diameter (including the capsule), of the rat's own adrenal, was employed for transplantation. Successful growth of the implant was obtained in a higher percentage of animals subjected to bilateral adrenalectomy, whether done in either one or two stages,...
