By the third week of development, the endocardial tubes have developed a swelling in the part closest to the heart. The swelling is known as the bulbus cordis and the upper part of this swelling develops into the truncus arteriosus. The structure is ultimately mesodermal in origin. During development of the heart, the heart tissues undergo folding, and the truncus arteriosus is exposed to what will eventually be both the left and right ventricles. As a septum develops between the two ventricles of the heart, two bulges form on either side of the truncus arteriosus. These progressively enlarge until the trunk splits into the aorta and pulmonary arteries.