Developmental relationships between vitelline and intra-embryonic haemopoiesis studied in avian 'yolk sac chimaeras'.

Using the quail-chick marker technique, cells have been traced in the haemopoietic organs of 149 developing chimaeras composed of a quail embryo with a chick yolk sac. The existence of intraembryonic stem cell relaying yolk sac stem cells, previously demonstrated with this system, is confirmed. Thymus, bursa of Fabricius and bone marrow are preferentially populated by intraembryonic stem cells. The spleen shows a transitory phase of colonization by chick yolk sac stem cells at 11--12 days of incubation. From 7 days onwards the yolk sac receives quail stem cell emigrating from the embryo.