The atrioventricular node is apparently distinguishable. The atrioventricular canal is now directly inferior to the outflow tract. The atrioventricular cushions have fused in 15 of 38 specimens studied dividing the single atrioventricular canal into a left and right channel. The outflow tract cushions have fused in 27 of 38 specimens studied. The pleuropericardial folds have fused , closing the pleuropericardial canals on either of both sides in most embryos of this stage.
Myofibrils are visible in the myocardium of the left ventricle.
13 Vena cavae and atria. The right venous valve fuses with the interatrial septum and encircles the orifice of the inferior vena cava. It is also fused with the septum of the cavocoronary sinus whereby it forms the primordium of the valve of the inferior vena cava and the coronary sinus. The left venous valve gradually diminishes and becomes incorporated into the interatrial septum. By this stage the dorsal and ventral atrioventricular cushions are completely fused with little evidence of their former separate nature.