Dusting off some old brain cells here. It's been a few years since I was an ag student. The white spot on the surface of the yolk is called the germinal spot, which once fertilized is the blastoderm, the early term for the cells that divide and become the embryo. The yellow part of the yolk is actually what feeds the growing embryo.
I may have some of the small details wrong, but I know that the yolk doesn't become the embryo, but feeds the embryo, with the bulk of it being absorbed into the chick's abdominal cavity just before hatching.
I may have some of the small details wrong, but I know that the yolk doesn't become the embryo, but feeds the embryo, with the bulk of it being absorbed into the chick's abdominal cavity just before hatching.