By the end of 4th week, a thin, sickle-shape membrane, called septum primum,grows from the roof of the common' atrium downwards into the lumen. The free edge of the septum .primum grows toward the septum intermedium formed by the fusion of the endocardia! cushions. The temporary gap between the septum intermedium and crescentic inferior border of the septum primum is called foramen primum (also called osteum primum). This foramen allows passage of blood form the right to' the left atrium. The osteum primum becomes progressively smaller and finally disappears as the inferior margin of the septum primum fuses with the septum intermedium. However, before the closure of the foramen primum, programmed cell death creates perforations in the upper central part of the septum primum. These perforations coalesce to form another opening called foramen secundum (also called osteum secundum). After the closure of the foramen primum, the foramen secundum allows the blood to be shunted from the right atrium to the left atrium.