Fig. 1. The avian yolk sac chimera experiment. The experimental strategy for the generation and analysis of yolk sac chimeras. An embryonic day 2 (E2) quail embryo body (blue) is used to replace a chick embryo body on the chick blastoderm (presumptive yolk sac) before the circulation is established in the host or grafted embryo. After several days of in ovo development (E4-E13), the spleen and thymus tissues are examined for a natural marker that distinguishes quail cells from chick cells. Quail cells contain a characteristically large, irregular nucleus, with a large heterochromatic mass, whereas chick cells contain finely dispersed heterochromatin. The results show that the progeny of cells derived from the quail embryo body (and not the chick yolk sac) contribute to the hematopoietic cell population in these adult tissues.