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'Rhailly, R., and Müller, F.. Developmental Stages in Human Embryos, Carnegie Institution of Washington, Publication 637, 1987.]

[adapted from figure 13-6]