What is the primary function of the vitelline veins in an embryo?

The primary function of the vitelline veins in an embryo is to return blood from the yolk sac and branch out to the liver. During early embryonic development, the yolk sac serves as a nutritional source for the developing embryo before the formation of the placenta. The vitelline veins collect nutrient-rich blood from the yolk sac and transport it to the embryo, specifically directing it towards the developing liver, which plays a crucial role in metabolism and nutrient processing. This circulation system is vital for supplying the necessary nutrients and oxygen at this stage of development before the establishment of other circulatory pathways such as the umbilical circulation.

The other options focus on different aspects of embryonic or adult circulation, such as the placenta or kidneys, which are served by separate vascular structures. Thus, understanding the specific role of the vitelline veins highlights their unique importance in early embryogenesis.