Endocardial Tube

The primordial endocardial tube constitutes a pair of longitudinal, endothelial-lined channels, which form via vasculogenesis during early embryonic development, from the cardiogenic mesoderm. Later, the endocardial tubes fuse to form part of the primordial heart tube (with the myocardium). The non-fused posterior portions of the endocardium become the openings of the vitelline veins, which feed into the heart, via the sinus venosus.