Quantification of T-cell progenitors in the blood, paraaortic foci, bone marow, and spleen during embryogenesis. Frequencies of T-cell progenitors were evaluated by quantification of ov thymocyte chimerism and limiting dilution experiments (Fig 1 and Table 1). Total numbers of T-cell progenitors were then calculated using the number of cells determined in each organ (Fig 3 and Table2).