Heart has a nervous system , the conduction system, in addition to spontaneous contractility: early heart development. At days 19-21, cardiogenic cords of the splanchnic mesoderm give rise to two endocardial tubes, embryonic folding allows apposition and fusion to form heart tube. Heart tube is surrounded by ventrally located pericardial sac. The cardiac mesentery is the dorsal mesocardium; it disintegrates, leaving heart tube suspended and forming transverse pericardial sinus. Heart tube consists of endothelium, cardiac jelly, myocardium, epicardium (inner to outer) In week 3, the heart tube loops and fetal blood circulation begins. 4 divisions of the heart tube: bulbus cordis, primitive ventricle, primitive atrium, Primitive atrium pectinate muscle in right auricle. Pulmonary veins sprout, branch, and head toward lungs. Moon-shaped septum primum separates atria; right-left shunt first provided by foramen primum, then by foramen secundum. Septum secundum forms with foramen ovale shunt (later becomes fossa.