(second opening), which allows blood to flow freely from the right atrium to the left.

When the right of the atrium expands due to the incorporation of the pole of the sinus, a new fold appears, called the septum secundum. At its right side it is fused with the left venous valve and the septum spurium. A free opening will then appear, called the foramen ovale. The remains of the upper septum primum, will become the valves of the foramen ovale.