When formation of the aorticopulmonary septum has been completed, the semilunar valves begin to develop at the orifices of the aorta and pulmonary trunk. The primordium of each semilunar valve appears as three swellings of endocardial tissue at the orifices of both the aorta and pulmonary trunk. The swellings become hollowed out on their upper surfaces to form three thin-walled cusps of the semilunar valve. In addition to the local tissue, the neural crest cells also contribute to the formation of the semilunar valves.