Bifurcation into paired dorsal aortae. We will follow this complex twisting again when we study the development of the hindgut. Start with F6. Oblique section through both umbilical veins in body wall. Follow them to E6.
Umbilical veins enter liver, along with vitelline.
Most venous blood drains through the liver region into the sinus venosus, which nestles in the cranial surface of the growing liver. This is the site of the caudal attachment of the free heart to the body mesenchyme. Entry of venous blood into sinus venosus. Note smaller left horn of sinus venosus, and location of sinus venosus caudal to atria. Drainage of right horn of sinus venosus into right atrium.