AbstractThe appendices and tonsils iliaca of 198 embryonic and neonatal rabbits between 26 days of gestation and 18 days after birth have been examined. Special cytological and histochemical techniques and light and phase microscopy have been employed in order to determine the origin of the lymphocytes in these lympho‐epithelial organs.Sequential developmental and cytological changes in the appendices and tonsils iliaca of the embryonic and neonatal rabbit indicate that the first lymphocytes are of small and medium size and appear by the twenty‐ninth day of development, viz., two to three days prior to birth. Lymphoblasts are not evident until a short time after birth when the lymphocytic nature of these organs has been established. The first lymphocytes in the appendix and tonsil iliaca appear to developin situby the direct transformation of reticular or mesenchymal cells. Lymphocytes tend to migrate toward the mesenchymal condensation of the so‐called domed elevations of the appendix and tonsil iliaca or remain in the connective tissue beneath these domed structures. In addition, lymphocytes enter the appendix and tonsil iliaca via the blood stream at or immediately after birth. These lymphocytes intermix with other lymphocytes in these organs and undergo proliferation within the lamina propria and developing lymphocytic nodules.Morphological evidence strongly suggests a dual origin or source of the lymphocytes in the developing appendix and tonsil iliaca of the rabbit; a population of lymphocytes derived originally from reticular cells and a population of lymphocytes derived extra‐appendiceally possibly of thymic (epithelial) origin. Neither the origin of lymphocytes or lymphocytic precursors from appendiceal epithelial cells nor the derivation of lymphocytic nodules from appendiceal crypts could be substantiated in this investigation. The mode of origin of lymphocytes and subsequent manner of lymphocytic development in the appendix and tonsil iliaca of the embryonic and neonatal rabbit differ markedly from that observed in the embryonic thymus and bursa of Fa