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.
![CVS is the first system to function in the embryo. As the embryo grows the development of the cardiovascular system become essential to the developing embryo so cardiovascular system develops during the third week. The cardiac progenitor cells lie in the epiblast. From there they migrate through the streak. Cells destined to form the cranial segments of the heart migrate first, and cells forming more caudal portions ( right ventricle, left ventricle sinus venosus respectively migrate in a sequential order. ,[object Object]](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)