Interior of heart

The single chambered heart soon gives rise to the formation of the endocardial cushion with still a wide communication between atria and ventricles. Septum primum is formed at the roof of the right atrium which approaches the cushion and forms the ostium primum between it and the cushion. It then becomes detached from the top with the formation of the ostium secundum behind it and obliteration of ostium primum as it joins the endocardial cushion.. The ostium secundum then becomes the foramen ovale at the mature 6 weeks heart. With formation of the atrioventricular valves between the atria and ventricle, the heart becomes 4 chambered with only the foramen ovale as opening between the atria.