After the formation of the bulboventricular loop, the atrium opens into the primitive left ventricle by a narrow opening called atrioventricular canal. By the time atrium exhibits two sacculations (the future right and left atria), the atrioventricular canal becomes widened transversely and extends to the right. As a result, the two parts the common atrium, ie, the future right atrium and left atrium, become connected to the primitive right ventricle and left ventricle, respectively. Soon afterwards, the atrioventricular canal divides into right and left canals. This division takes place by the appearance of superior and inferior endocardial cushions (also called atrioventricular cushions). These cushions are small swellings formed by the proliferation of the endocardial tissue. As their name indicates, one of these cushions appears in the superior (ie, dorsal) wall, whereas the cither is formed in the inferior (ie, ventral) wall of lthe atrioventricular canal The superior and inferior endocardial cushions gradually grow toward each other. By the end of the 6th week, they meet and fuse to form a septum intermedium,
which divides the common atrioventricular canal into right and left atrioventricular canals