As already described earlier, formation of the septum intermedium results in division of the atrioventricular canal into right and left atrioventricular orifices. Each orifice soon becomes surrounded by swellings produced by proliferation of mesenchyme under the endocardium of the atrioventricular canals. These swellings are excavated on their ventricular sides to form valvular cusps. The mesenchymal core of each cusp differentiates into dense fibrous connective tissue. Three cusps are formed at the right atrioventricular orifice and constitute the tricuspid valve.
Two valvular cusps are formed at the left atrioventricular orifice, constituting the bicuspid valve. The cusps remain connected to the ventricular wall by muscular strands. The proximal parts of these strands retain muscular tissue and become papillary muscles. However, the, distal parts of these strands, which are attached to valves cusps, undergo fibrous degeneration to form the chordae tendineae.