4. Primitive blood circulation A t each contraction of the primitive heart, the blood is pumped through ventral aortae in the aortic arches aortic arches run within branchial arches and open into the dorsal aortae (paired cranially), from which the precursors of the internal carotid artery run forwards to supply the head on the left as well as on the right side In the mid-cervical region, the dorsal aortae fuse in one common trunk - unpaired dorsal aorta

5. The dorsal aorta sends off branches of 3 types: - intersegmental arteries - run between developing somites - vitelline arteries - (several pairs) - run to the yolk sac - umbilical arteries - one pair that run to the villous chorion (chorion frondosum ) and conduct deoxygenated blood from the embryo to the placenta to the heart the b lood returns through superior cardinal veins (left and right) from the cranial portion of the embryonic body and through inferior cardinal veins from the caudal part of the embryo n ear the heart , both veins they join at each side and form common cardinal vein f rom the villous chorion, blood returns at first via paired umbilical veins , from which the left vein persists and brings oxygenated blood to the embryo ) from the yolk sac , b lood return s to the embryo through vitelline veins (several pairs) Ventral aorta

15. the septum secundum ( t he second septum ) then begins to grow down on the right hand side of the septum primum f rom the beginning , the septum has semilunar shape and its border delineates oval foramen - the foramen ovale as the ostium secundum and oval foramen lie in different levels, the blood may pass from the right atrium into the left atrium in the fetal period through the oval foramen into the gap between both septa e and through the ostium secundum

19. 5 Development of the valves Cardiac valves are duplicatures of the endocardium

24. The great arteries in the adult