Subdivision and internal differentiation of the heart
The transition from a serial to a parallel blood stream is accompanied by the transversal and longitudinal subdivision of the loop-shaped tubular heart. The transversal subdivision (formation of cardiac valves) causes subdivision into the atrium, ventricle and outflow tract. The longitudinal subdivision (formation of the septa) causes subdivision into the left and right atria, left and right ventricles, as well as into the aorta and the pulmonary trunk.