At the onset of heart formation, we have six aortic arches instead of three. Supposively the three that we lose were from our evolutionary history. The three arches to go away during development are arches one, two and five. In the first five stages of cardiac development the heart is a tube that has four bulges. The tube forms by the fusion of lateral plate mesoderm from two distinct cardiac fields. The heart then forms an "s" shape by factors that develops into the heart structure that we are more familiar with. One bulge is called the bulbus cordis (truncus arteriosis). This bulge continues on to form a ventricle. Another bulge called the primitive ventricle becomes the other ventricle chamber. The last two bulges are the sinus venosus and primitive atrium. The sinus venosus becomes incorporated into the wall of the right atrium, and the primitive atrium becomes the two atria. The factors that cause the "s" shape cause the heart to loop and this causes the structures that need to be posterior to move to their location, and the structures that need to be anterior move to the anterior.