Endocardial cushions

The lumen of the atrial canal is reduced to a transverse slit, and two thickenings appear, one on its dorsal and another on its ventral wall. These thickenings, or endocardial cushions as they are termed, project into the canal, and, meeting in the middle line, unite to form the septum intermedium which divides the canal into two channels, the future right and left atrioventricular orifices.