The formation of the ventricular septum was studied in human embryos ranging from 3.6 to 25 mm CR-length. Before septation, two distinct chambers are present which will contribute to the adult ventricles. They are called the bulbus and the ventricle. The circular constriction between these chambers is the bulboventricular fold. The anterior portion of the ventricular septum develops from this bulboventricular fold. Posteriorly, it fuses with a second septum, which is of purely ventricular origin. This is the inlet septum. Another portion of the bulboventricular fold persists as the trabecula septomarginalis. The trabecula septomarginalis divides the normal right ventricle into an inlet portion, stemming from the embryonic ventricle, and an outlet portion which derives from the bulbus.