Fig. 1: Pictorial timeline of the development of the inflow tract.

Dorsal views of embryonic hearts are shown between CS10 and CS14, with panels a–e emphasizing endocardial continuity and panels f–j the myocardial coat. The panels are aligned relative to the position of the pulmonary vein (black horizontal line). Note that the distance between the arterial and venous poles of the heart loop does not change during cardiac looping (CS10-CS12;^52,55). The appearance of a myocardial wall indicates the formation of that compartment in the inflow tract. Myocardium appears in the wall of the atrium at CS10 and in the wall of the systemic venous sinus at CS12. The pulmonary vein, along with its flanking atrial ridges, also begins to form at CS12. The sinus node becomes recognizable as a separate structure at CS13. The left and right atriums are already distinguishable at CS10, but the sinuatrial junction does not become a right-sided structure until CS13. The atrial septum appears at CS14. It is identifiable as the “empty” space between left and right atriums in panel e. The hepatocardiac veins are the only source of venous blood for the heart until CS12, when the initially small common cardinal veins appear. All images are also available as preset views in the corresponding 3D-PDFs.