The primitive atrium is divided in two by joining of several structures. From the roof of the primitive atrium descends the septum primum, which grows towards the endocardial cushions within the atrial canal. Right before the septum primum fuses with the endocardial cushions there's a temporary space called the foramen primum. Once they fuse a new opening forms in the middle of the septum primum called the ostium secundum or "foramen secundum". To the right of the septum primum and also coming down from the roof of the primitive atrium, descends a semilunar-shaped partition called the septum secundum. The free edges of the septum secundum produce an orifice called "foramen ovale", which closes after birth when the septum primum and secundum fuse to each other completing the formation of the atrial septum.