ABSTRACT
Blood islands are identifiable prior to the development of the chick embryo’s earliest circulatory arc through which blood might flow. In fact, once the blood islands have been established, the onset of circulation can be considered the next definitive ontogenetic step in erythropoiesis. F. R. Sabin has drawn an illustration of a histologic section through the marginal sinus of an 18 somite chick depicting the origin of blood cells from the endothelial surface of the vessel. It is universally agreed that yolk sac erythropoiesis is intravascular; a concept that has been time-honored since earliest studies of the genesis of erythropoiesis in birds and other vertebrates. The earliest groupings of cells in the splanchnic mesoderm in the area vasculosa have been recognized as angioblasts because they give rise to the earliest yolk sac vasculature. The earliest chick embryo is seen as a thin, whitish, disc-shaped cell mass sitting on top of the yolk.
