The development of the embryo is primarily concerned with the formation of the four extra-embryonic membranes [yolk sac,amnion,chorion,allantois] and placenta.As the human egg is devoid of yolk , the process of formation of  yolk via nutrients deposited in the oocyte is called as Vitellogenesis.During the embryo developent to throughout the pregnancy , nutrients and blood is transported to the sac wall by primitive aortae and returns back via vitelline veins into the embryo's tubular heart.