As already described, the early heart tube consists , of endocardium, myocardium and epicardium.
The myocardium differentiates into an outer compact layer of muscle and an inner spongy layer, the loosely arranged trabeculae of which project into the heart cavity. The endocardium dips into the spaces between the trabeculae and lines them. Initially the spongy layer has a much greater thickness than the compact layer. However in later development the cardiac wall becomes mainly compact. The original trabeculae remains in the atria and auricles as musculi pectinati and in the ventricles as trabeculae carneae.
The musculature of the ventricles is more developed than that of the atria. However the thicker wall of the left ventricle is mainly acquired after birth as a result of the harder work performed by the left ventricle.
Initially the myocardium of the atria and ventricles is continuous with each other. However, development of connective tissue in the region of atrioventricular canal separates the atrial muscle from the ventricular muscle and only a small muscular link remains. This connecting muscular band differentiates to form the atrioventricular bundle. It is specialized to conduct the cardiac impulse from atria to ventricles. The sinuatrial node, the atrioventricular node, and the Purkinje fibers develop
as a local differentiation of cardiac muscle.