For lack of a better term (my ignorance), the yolk is similar to a placenta. It feeds the embryo while in the egg and for up to 3 days after hatch. The embryo is not inside the yolk. They are attached to one another. If you cut the yolk of a newly laid egg it will look like this:
p.s. I believe most of this information is interchangeable for ducks and chickens besides the length of incubation and other things I am ignorant of.