Here's why:

* Yolk is packed with nutrients: The yolk is a rich source of proteins, fats, and vitamins that provide the developing embryo with the building blocks and energy it needs to grow.

* Energy storage: The yolk acts as a food reserve, providing the embryo with a constant supply of energy throughout its development.

* Other parts of the egg: The egg white (albumen) primarily provides water and protein, but not the primary energy source. The shell provides protection.