The spiral septum that separates the truncus arteriosus into a ventral pulmonary trunk and the dorsal aorta[MP]. The aorticopulmonary septum is developmentally formed from neural crest, specifically the cardiac neural crest, and actively separates the aorta and pulmonary arteries and fuses with the interventricular septum within the heart during development. The actual mechanism of septation of the outflow tract is poorly understood, but is recognized as a dynamic process with contributions from contractile, hemodynamic, and extracellular matrix interactions. [ http://en.wikipedia.org/wiki/Aorticopulmonary_septum ]