The early embryonic vascular system permits circulation through a series of incompletely fused and developing channels. Before the heart is fully formed, the flow of circulation is diffuse rather than strongly unidirectional as it is in the atrial and venous systems of later fetal development. Movement of blood through the early embryonic vascular system begins as soon as the primitive heart tubes form and fuse. Contractions of the primitive heart begin early in development, as early as the initial fusion of the endothelial channels that fuse to form the heart.

Primitive blood plasma is almost indistinguishable from interstitial fluid and it contains few cells. The vascular system is essentially transparent until enough blood cells form within the yolk sac. As the cells take on hemoglobin, they gain color and the early embryonic system becomes visible.

At the end of the first month...