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Experiments have been performed on rats bearing heterolo, nous tumor transplants to determine the effect of thymectomy and of thymic extracts on the growth of the tumor.The hypothesis of an association between the function of the thymus and the pathogenesis of leukemia, at least in mice, has attracted everincreasing interest since the discovery by FURTH (2) that thymectomy in young AK mice reduced the incidence of spontaneous leukemia from a very high to a low level.The results of GROSS (3) and LEVINTHAL (7) are very similar, and GROSS (3) concluded that thymectomy, either preceding or following inoculation of leukemic filtrate, inhibits development of leitkemia in otherwise susceptible mice. Subsequently, KAPLAN (4) and LAW and MILLER (6) reported a similar influence of thymectomy on lymphoma development under other cxperimental conditions. FURTH (2) and KAPLAN (5) attributed this phenomenon simply to removal of the tissues of maximal susceptibility. However, LAW and MILLER (6) reported that the incidence of lymphoid tumors could be restored to essentially normal levels in thymectomized mice by autologous or homologous thymic implants, suggesting that the thymus may be involved in the pathogenesis of the disease in an additional role, possibly of endocrine nature.COMSA and BEZSSONOFF (1) state that their extract was able to prevent the disturbances induced by thymectomy in infantile guinea pigs. These thymic extracts were specific in their action, since the extracts of liver, lungs and muscle, prepared by the same method, produced negative results and showed no detectable activity in any of the bioassay tests.NAKAMOTO (11) observed that an acid extract of the thymus of the young calf caused an increase in lymphocytes and mitochondria in the lymphocytes within 48 hours following injection into young rabbits. This was Zenrralblatt f u r Veterinarmediiiii, Bd. VI11. Heft 1 2 18 PAW, VENZKE and HOLZAEPFEL observed only following injections of lipoid fractions of the thymus, and no such effects were obtained with acid extracts made from alcohol-insoluble thymus substance or extracts prepared from thymi which showed fatty degeneration.NAKAMOTO (12) found that injection of the lipoid fraction of the spleen produced low counts of both lymphocytes and mitochondria in the peripheral blood. Therefore, the stimulating action on lymphocyte formation IS specific for the thymus and is not a common property of lymphatic tissues.METCALF (8) reported the thymus has been shown to produce a lymphocytosis-stimulating factor (LSF) and that this factor is involved in the maintenance of normal lymphocyte homeostasis, both by its direct stimulation of lymphopoietic cells and indirectly throlugh the suppression of lymphocyte production by adrenal corticoids. The author believes the LSF is a true hormone which is detectable only in the human and mouse thymus. MOLNAR et al. (9) found that thymic extract increases the growth and malignancy of implanted Brown-Pearce carcinoma in rabbits. It was observed that an activation of the hypophyseal...

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Summary and Conclusions In tissue culture of thymus the P‐V bodies from reticular cells and medium sized lymphocytes are precursors of small lymphocytes. The P‐V bodies maturate through growth into small lymphocytes. Small lymphocytes multiply through amitotic division. The amitotically divided small lymphocytes maturate through growth into medium sized lymphocytes. Medium sized lymphocytes through a chromatin diminution process differentiate into reticular cells. Reticular cells multiply through mitotic division and are only a differential stage in the development of the lymphocyte. Reticular cells differentiate through an increase in protoplasm into fibroblasts. Zusammenfassung und Schlufßfolgerungen Beobachtungen an zellulären Thymusbestandteilen in Gewebekulturen In Gewebekulturen vom Thymus sind die von den retikulären Zellen und den mittelgroßen Lymphozyten stammenden P‐V‐Körperchen die Vorläufer der kleinen Lymphozyten. Die P‐V‐Körperchen reifen durch Wachstum zu kleinen Lymphozyten. Kleine Lymphozyten vermehren sich durch amitotische Teilung. Die amitotisch geteilten kleinen Lymphozyten reifen durch Wachstum zu mittelgroßen Lymphozyten. Mittelgroße Lymphozyten differenzieren sich durch einen Vorgang der Chromatinverminderung zu retikulären Zellen. Retikuläre Zellen vermehren sich durch mitotische Teilung und sind nur ein Differenzierungsstadium in der Entwicklung des Lymphozyten. Retikuläre Zellen differenzieren sich durch eine Vermehrung des Protoplasmas zu Fibroblasten. Résumé Observations sur les éléments cellulaires du thymus en culture de tissu Dans les cultures de cellules du thymus les corpuscules P‐V issus des éléments réticulaires et des lymphocytes moyens sont les précurseurs de petits lymphocytes. Les corpuscules P‐V murissent par croissance et deviennent des petits lymphocytes. Les petits lymphocytes se multiplient par division amitotique. Les petits lymphocytes divisés par amitose atteignent par croissance leur maturité et deviennent des lymphocytes moyens. Les lymphocytes moyens se différencient par un appauvrissement en chromatine et deviennent des réticulocytes. Les réticulocytes se multiplient par mitose et ne sont qu'un stade de différenciation dans le développement des lymphocytes. Des réticulocytes se différencient par augmentation du cytoplasme et deviennent des fibroblastes. Resumen Observaciones realizadas en los componentes tímicos celulares en histocultivos En los histocultivos de timo, los corpúsculos P‐V procedentes de las células reticulares y de los linfocitos de tamaño mediano son los precursores de los pequeños linfocitos. Los corpúsculos P‐V, por medio de crecimiento maduran a pequeños linfocitos. Los pequeños linfocitos se reproducen mediante división amitótica. Mediante crecimiento, los pequeños linfocitos divididos amitóticamente maduran a linfocitos de tamaño mediano. Por el fenómeno de reducción cromática, los linfocitos de tamaño mediano se diferencian a células reticulares. Las células reticulares se reproducen por división mitótica y sólo son un estadi...