The atrioventricular canal (AV canal) has been situated on the left until now, connecting the left atrium with the left ventricle. At S14, it shifts into the middle of the heart. This allows blood to flow into the right ventricle as well as into the left ventricle. A mesenchymal ring forms in the AV canal, which differentiates into four endocardial cushions (one ventral, one dorsal and two lateral). The ventral (upper) and dorsal (lower) endocardial cushions grow toward each other and fuse to form the AV septum or septum intermedium, which divides the AV canal into a right and a left channel (S18).