8. CONT.. The crescent part of the horse shoe shaped area expands to form the out flow track of ventricular regions, thus the heart becomes continuous expanding tube consisting of an inner endothelial lining and outer myocardial layer. A mesothelial cells from the regions of the sinus vensus migrate over the heart to form epicardium thus the heart tube consist of three layers. Endocardium: forming internal endothelial lining of the heart. Myocardium: forming the muscular walls. Epicardium: covering the outside of the tube. This outer layer is responsible for formation of the coronary arteries including their endothelial lining and smooth muscles.