This document discusses the anatomy and development of the atrial septum. It begins by describing how the heart tube forms in the embryo. It then explains how the atrial septum develops from three sources: the septum primum, septum intermedium, and septum secundum. The foramen ovale allows blood to pass from the right to left atrium in fetuses. After birth, pressures change and the foramen ovale closes. Failure of this closure results in an atrial septal defect. The document provides details on the normal features and types of congenital septal defects.