The common atrium is divided into right and left atria by the forjpation of the interatrial septum. This is a complex septum formed by the fusion of two embryonic septa called septum primum and septum secundum.
Before the details of the development of the interatrial septum are described, it is important to remember that during the embryonic and fetal life, oxygenated blood from the placenta is being returned to the right atrium (through the inferior vena cava). It is essential that major part of this blood should pass to the left atrium through the interatrial septum, so that it may enter the left ventricle through the atrioventricular orifice. From the left ventricle the oxygenated blood is pushed into the systemic circulation.
The above given description makes it dear that, throughout the intrauterine life, the interatrial septum is not a complete partition and in various stages of its development there is always an arrangement to allow the passage of blood from the right atrium to the left atrium. However, after birth the interatrial septum becomes a complete partition between the two atria.