A sagittal section through the heart, in the plane of the atrioventricular canal. The drawing shows in simplified form the character of the cardiac wall and the difference between the atria and the ventricles. The cardiac mesenchyme ("jelly") in the myoendocardial interval persists as endocardial cushions that serve as an atrioventricular valve. Similarly it persists in the conus cordis and truncus arteriosus, aiding in preventing backflow from the aortic arches. Based on serial sections and a Born reconstruction of No. 836.
The above text and accompanying figure are taken from O'Rahilly and Müller, 1987. Adapted from figure 13-6.
Keywords: aorta, aortic arch 2, aortic arch 3, atrioventricular canal, cardiac jelly, cardiac mesenchyme, conus cordis, gelatinous reticulum, hepatocardiac vein, myocardium of atrium, right ventricle, sinu-atrial foramen, sinus venosus, truncus arteriosus, valvular lip
Source: The Virtual Human Embryo.