The development of the interventricular septum begins around the 27th day of gestation. At this time, the ventricular cavities are small and communicate with each other by a narrow channel. The ventricles grow and enlarge rapidly while the communicating channel grows slowly. The result of this is that the inferior opposing walls of the ventricles give the appearance of an invagination between the ventricular chambers. The fusion of the opposing ventricular walls then gives rise to the muscular interventricular septum.
The interventricular septum develops from three embryonic components: the endocardial cushions, the conus cushions and the muscular septum. The endocardial cushions divide the inflow tracts from the ventricles. The conus cushions divide the outflow region of the ventricles. The truncus cushions divide the truncus arteriosus in the aorta and main pulmonary artery.