The first sign of ventricular septation is the bulboventricular septum, which separates the dilated portion of the bulbus cordis (primordial right ventricle) from the embryonic ventricle (primordial left ventricle). The bulboventricular septum exists as a muscular fold and forms the primitive interventricular septum. Continued growth results in enlargement of the ventricles and subsequent lengthening of the interventricular septum. The interventricular foramen finally closes as a consequence of differential cellular proliferation.