Model of ontogeny of T-cell and B-cell progenitors in the murine fetuses. The progenitors restricted to T, NK, and DC lineages (p-T/NK/DC), which are generated in FL, may be preferentially released in FB during 11 to 14 dpc. The B-cell lineage–restricted progenitors (p-Bs) are also generated in FL but most of them do not circulate in the blood stream. At earlier stages (10.0-11 dpc), the p-T/NK/DCs may mainly be derived from the AGM region. The p-T/NK/DCs lose NK and DC potential during intrathymic development to become unipotent T-cell progenitors (p-T_uni's). We propose that the circulating Lin^–c-kit⁺IL-7R⁺ p-T/NK/DCs identified in the present study are the major source of thymic T cells. The p-T/NK/DCs in FB can also be a source of extrathymic T cells, NK cells, and DCs in developing lymphoid organs.