Endocardial cushions project into the atrial 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[Gray's via WP]. [ http://amigo.geneontology.org/amigo/term/GO:0003282 http://en.wikipedia.org/wiki/Septum_intermedium ]