Blood flow
The gap below it is known as the ostium primum (), and becomes increasingly small. The septum primum eventually fuses with the endocardial cushion, closing the ostium primum off completely. Meanwhile, perforations appear in the superior part of the septum primum, forming the ostium secundum (). The septum primum will eventually form part of the fossa ovalis. Blood flow between atria will continue through the foramen ovale (heart).