The aortic stream and the pulmonary stream entwine to form a spiral in the outflow tract. From S14-S16, the spiral conotruncal ridges grow towards each other and fuse together between these two blood streams. The truncal ridges give rise to the aorticopulmonary septum. The conal ridges separate out the parts of the outflow tract inside the heart: conus arteriosus in the right ventricle, vestibulum aortae in the left ventricle (animation: development of the endocardial tube).

Atrioventricular valves (Valvae atrioventriculares)

After partitioning of the AV canal by the AV septum, the tissue in the two ostia develops into the tricuspid valve on the right and the bicuspid valve on the left. Parts of the cuspides (sails), the chordae tendineae and the papillary muscles are formed from the endocardial cushions. The other part originates from the wall of the ventricle. From S16 onwards, the ventricle is enlarged by trabeculation. The endocardium becomes undermined; the musculature forms into a tendon-like tissue in the region of the cusps and chordae tendineae. This process continues until week 22 of development.