Ch 13 study guide The Cardiovascular System When does the cardiovascular system start to develop? When does the primordial heart start to beat? From what does the primordial vascular system develop? Where does the primordial myocardium come from? What are the four chambers of the heart primordium? What is the truncus arteriosus? What is it continuous with? What are the three systems of paired veins that drain into the primordial heart? Where do the pharyngeal arteries come from? What do they become? What is the critical period of heart development? What are CHDs? What are VSDs? True or false: fetal lungs oxygenate the fetal blood. What is patent foramen ovale? Patent ductus arteriosus? When does the lymphatic system begin to develop? How many lymph sacs develop? Where do lymph nodes develop? When do lymph nodules develop?