a septum dividing the embryonic bulbus cordis and truncus arteriosus into pulmonary and aortic outflow tracts from the developing heart; the distal part of the septum is derived from the right and left endocardial cushions and so separates the pulmonary and aortic orifices; the proximal part of the septum is that portion of the septum incorporated into the membranous part of the interventricular septum. SYN: spiral bulbar septum, spiral septum.