In the avian blastoderm four (hemi) concentrically disposed, deep yolk-rich “extraembryonic” structures (Rauber’s sickle and sickle horns, caudo-lateral marginal zone, area opaca, perigerminal zone) are successively involved in the early formation of the coelomo-cardiovascular system. Our experiments indicate that two hemangioblastic foci exist in the young avian blastoderm. The caudal (median) part of Rauber’s sickle induces the formation of the area vasculosa caudalis, giving rise to the aorta dorsalis and the arteriae vitellinae. The sickle horn material by direct induction gives rise bilaterally to the formation of the primary heart tubes, pericard and associated venae vitellinae. After partial or total removal of the area opaca no regeneration occurs, indicating mosaicism. The spatial relationship between the inducing Rauber’s sickle material (у−ooplasm surrounding the area centralis) and the blood islands in the caudal marginal zone giving rise to the early area vasculosa can be clearly visualized by histochemical staining (Unna) in toto. The further horse-shoe shaped expansion of the area vasculosa occurs in the lateral marginal zone and not in the area opaca (as was formerly assumed). However for the further expansion an intact anchoring on the area opaca must be present. In culture in vitro the area opaca (expanding spherically around the caudal, lateral and rostral marginal zones) is characterized by enlarging vacuolized zones and finally forms the area vitellina interna. The perigerminal zone plays a role in the development of the area vitellina externa and further expansion of the blastoderm.