Turtle (Emys orbicularis L.) embryo thymuses grafted in the somatopleure or onto the quail embryo chorioallantoic membrane developed in these heterotopic sites for 2-12 days. When the thymus was removed from embryos at early stages such that no thymocytes were yet present during normal development, epithelial cells with mitoses were observed in the explants but no turtle thymocytes developed whatever the duration of explantation. An extrinsic origin of lymphoid precursor cells can explain such results. Quail lymphoid-like cells distinguishable from turtle cells by their nuclear structure began entering the explants 5 days after grafting. Their number increased progressively until the 12th day, when all the grafts were retrieved. When an already lymphoid thymus was removed from embryos at later stages, turtle thymocytes remained fairly abundant in the explants until 5 days. Beyond this period, they gradually disappeared. After 11-12 days, none were left and only epithelial tissue from the turtle was present, showing excellent development. Quail lymphoid-like cells entered this more mature thymus following the same time course as they did in the early rudiment. Thus we observed no difference between attraction of quail lymphoid cells by the precolonization thymic epithelium or by the lymphoid thymus.