Hey,
So this is part of extraembryonic development. The Chorion, which is the outer membrane of the embryo (which is used for gas exchange), later becomes (with maternal tissue) becomes the placenta.
The Allantois, which stores waste products and also plays a roll in gas exchange, forms the umbilical cord, which functions in transporting gasses, nutrients, and water between the embryo and the placenta.
hope that helps.