Transmission electron microscopic investigations were carried out on the early development of the dorsal aorta in chick embryos during the period ranging from the 6-somite stage to the 16-somite stage. Special attention was paid to the formation of the vascular lumen. The initial angioblasts tended to aggregate into a cluster with a narrow intercellular space. Only a few punctate thickenings of the plasma membrane were found at junctional sites of the angioblasts. The vascular cavity appeared to be produced by expansion of the intercellular space and possibly also by enclosure of the extracellular space with attenuated vasoformative cells. The wall of the early vessels so formed was frequently observed to be open to the extravascular space through clefts originating probably from incomplete closure of the early endothelium. No specific ultrastructural features characterizing the angioblasts or early endothelial cells were recognized.