HEYA!!!!
Under the shell are two thin membranes. The first is just under the shell and provides some additional support to the shell. The second surrounds the white of the egg and as the egg ages the two membranes separate. The fact there are two membranes and that they split as the egg ages is why fresh eggs are difficult to peel the shell from when boiled.
Yolk & Chalaza
To hold the yolk in a central position there is a connection of tissue from the membrane that surrounds the yolk to the top and bottom of the second membrane that is technically known as the chalaza. If you crack an egg then you may see a whitish string in the egg white, this is the chalaza.
In a duck egg, there tends to be more yolk in ratio to white.
The white provides cushioning and additional nutrition for the developing chick and contains less nutrition than the yolk.
Hope it helps you.
:)