The septum secundum forms to the right of the septum primum, and grows in a rostro-ventral to dorso-caudal direction, i.e. perpendicular to the direction of growth of the septum primum (S18 to S21). An oval aperture (foramen ovale) remains between the free border of the septum secundum and the edge of the foramen secundum. It is only by S23 that the foramen secundum becomes completely covered by the septum secundum. Due to the pressure difference between the two atria, blood flows from the right to the left atrium, causing the septum primum to open to the left; due to this, a functional foramen ovale remains. (animation: septation of the atrium).
At S13, the sinu-atrial node together with the sinus venosus becomes incorporated in the dorsal wall of the right atrium. The left atrium absorbs the pulmonary veins, which give rise to the smooth posterior wall of the left atrium.