At about the time when foramen secundum is being formed, another septum grows down from the atrial roof of the right side of the septum primum. This new septum, which is thick and muscular is called septum This is also a crescentic septum and, as it grows down, it gradually overlaps
the foramen secundum in the septum primum. The free concave edge of the septum secundum grows posteroinferiorly, but it halts before it reaches the septum intermedium, leaving a large opening near the floor of the right atrium; this opening is called foramen ovale.
The uppermost part of the septum primum (initially attached to the roof of the left atrium) gradually disappears. The remaining part of the septum primum (attached inferiorly to the septum intermedium) forms the flaplike valve of the foramen ovale. Throughout rest of the fetal development, the foramen ovale allows most of the blood entering the right atrium to pass into the left atrium. However, passage of blood in the opposite direction (ie, from the left to the right atrium) is prevented because the thin and membranous septum primum closes against the relatively thicker septum secundum.
After birth the right to left shunt of the Blood stop because the increased pressure of blood in the left atrium pushes -the flexible septum primum against the more rigid septum secundum. The two septa become fused to each other to form the definitive interatrial septum which becomes a complete partition between the atria. In about 20% of the people the septum primum and secundum do not become actually fused and there remains a narrow oblique cleft between the two atria. This condition, called probe patency of foramen ovale, is
quite harmless because it does "not allow intracardiac shunting of blood,
In the adult heart the lower edge of the septum secundum can be seen in the right atrium as limbus fossae ovalis. The depression below this (representing , the embryonic foramen ovale) is known as fossa ovalis.