Development of the cardiac loop (Cor sigmoideum, S11)

The intermediate part of the heart between the inflow and the outflow tract shifts to the right and ventrally. The proximal part of the heart containing the inflow tract bends towards the left, dorsally and cranially. This creates a right-oriented loop (D-loop, with D meaning dexter). The distal part continues along a left-convex curve into the aortic sac, which is anchored to the mesenchyme of the first pharyngeal arch. At S11, the sinus venosus comes to lie dorsally due to the D-loop formation. It is followed cranially by a dilated part: the right atrium and left atrium. The left atrium connects to the ventricle (the future left ventricle) through a narrow slit (atrioventricular canal, AV canal). Blood reaches the future right ventricle through a narrow opening (externally: sulcus primus or bulbo-ventricular sulcus), and from there it flows through the outflow tract (conus cordis and truncus arteriosus, termed together as conotruncus) into the aorta (animation: development of the endocardial tube).
The mesocardium becomes perforated during loop formation and finally breaks down. It is now only at the arterial and venous ends, where pericardial reflections remain (transverse sinus and oblique sinus in the adult).