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The origin of plasma cells in the thymus as well as in the lymphoid organs and sites of inflammation is still being discussed. Moore and Schoenberg believed that in experimentally induced inflammation they developed from both small and medium lymphocytes. Nossal and Keuning concluded that plasma cells originated from a primitive lymphocyte in lymph nodes. In the thymus they are absent during the first weeks of age, but from the sixth week on they are easily recognized even under the light microscope, and their number appears to increase during the following weeks.In the present work we examined the corticomedullary junction area of the thymus in albino rats at progressive ages, starting at an hour after birth, and then at weekly intervals until the twelfth week. The appearance of the thymus during the first weeks is rather simple; it consists mainly of islets of undifferentiated epithelial reticular cells, surrounded by small and medium sized lymphocytes.

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