Delayed addition experiment on yolk sac cells and day-6 in vitro differentiation-induced ES cells. The numbers of erythroid colonies from day-10.5 yolk sac cells (A) and day-6 in vitro differentiation-induced ES cells (B) are shown. ▵, ○, and □ show the numbers of colonies when the culture started with none, Epo alone, and TPO alone, respectively. The data at day 0 and “C” (control) show results of the culture that contained both Epo and TPO from day 0 and that which did not receive the supplement of the cytokines, respectively. Refer to text for details. Data represent mean ± SE of quadruplicate cultures.