Yolk sac is then infiltrated with a system of blood vessels, carrying nutrients from the yolk back to the embryo. Other extra-embryonic folds expand up over the embryo to form the amniotic folds. The ...

Yolk sac is then infiltrated with a system of blood vessels, carrying nutrients from the yolk back to the embryo. Other extra-embryonic folds expand up over the embryo to form the amniotic folds. The ...