Well, after about 25 days following fertilization, we have something like this. Basically what happens is now we no longer have a temporary connection between the mother's blood vessels and the coriane portion of the embryo. Now we have a direct connection between the Coryon and the uterine blood vessels. And so what that means is we have this continual source of nutrition and oxygen that comes from the mother. We also form the embryonic stock that eventually becomes that umbilical cord. And this connection between the corian and the blood vessels of the mother eventually forms the placenta.