Separation of the blood stream into aortic and pulmonary streams causes the two ventricles to become functionally separated. The expanding walls of the ventricles come together to form the interventricular septum (pars muscularis), which first is produced passively and only later grows actively. The septum is horseshoe-shaped and is concave at the cranial end. Its posterior branch reaches up to the dorsal endocardial cushion of the AV canal, whereas its anterior branch terminates near the ventral endocardial cushion. There is initially an aperture called the interventricular foramen (secondary interventricular foramen) between the left and right ventricle. This foramen is closed through growth of the conotruncal ridges and the dorsal endocardial cushion (pars membranacea of the interventricular septum). However, the passage from the left ventricle into the aortic part of the partitioned conus cordis (primary interventricular foramen) remains open and becomes the aortic vestibule.