AbstractThe present morphological investigation of the developing thymus of the rabbit during early embryogenesis differs with our previous studies concerning thymus lymphopoiesis in the chick and hamster which suggested that embryologically the first lymphocytes appearing in the thymus arisein situby the direct transformation of undifferentiated epithelial cells comprising the organ parenchyma. Significantly, in the rabbit and in contrast with the chick and hamster, a few cells morphologically resembling but not completely identical to medium‐size and large lymphocytes were evident in the mesenchyme near the thymus at a time just prior to the appearance of lymphocytes or lymphocytic precursors within the thymus parenchyma (twelve and one‐half days of gestation). Within a few hours after the appearance of these extrathymic lymphocytic cells both medium‐size and large lymphocytic cells were observed both in the thymus parenchyma and in the mesenchyme (thirteen and one‐half days of gestation); these cells remain more numerous in the mesenchyme than in the thymus during the next several days. In some instances, only a thin basement membrane separated the lymphocytic cells within the thymus from the surrounding mesenchyme. In contrast with our earlier studies, several examples have been encountered between the fifteenth and seventeenth days of gestation of lymphocytic cells extending through the basement membrane seperating the thymus parenchyma from the surrounding capsular connective tissue. Morphologically cellular modulations between thymus undifferentiated epithelial cells and lymphocytic precursors have not been as convincing in the developing rabbit thymus as in the chick and hamster. In contrast with thymus development in the chick few mature or developing granulocytes are found near the embryonic rabbit thymus indicating that the primitive extrathymic lymphocytic cells are not myeloblasts.The morphological evidence of this investigation is consistent with the thesis that the first lymphocytes present in the embryonic thymus probably are derived from the invasion of extrathymic lymphocytic cells into the organ parenchyma rather than formingin situfrom modulating undifferentiated epithelial cells. The presence of rare lymphocytic cells observed in the small vascular channels near the time of appearance of the lymphocytic cell population in the mesenchyme and within the thymus primordium would be consistent with the view that these primitive lymphocytic cells immigrate to the region of the thymus via the blood vascular