ABSTRACT
In the mesoderm of the yolk sac walls, the blood islands give rise to vitelline blood vessels. Simultaneously, blood islands in the embryonic disk give rise to the cardiogenic tube and dorsal aortae that are connected by aortic arches. Originally, bilateral endocardial tubes fuse in the midline to form the heart tube. Specific regions of the heart tube can be identified. Starting from the inflow tract that receives blood from the yolk sac, the embryonic disk and the placenta are the sinus venosus, the atrium, the ventricle and the outflow tract. Differentiation of the heart tube commences at inflow and progresses to outflow. The tube undergoes remolding and septation. The remolding separates atria from ventricles. Septation separates the right side from left side of the heart.
