Birds' eggs differ in size and colour, but in other ways they are similar. Inside a hard, oval shell is the white, the fluid that protects the yolk. The yolk is food for the growing chick. The chick starts as a tiny disc on the top of the yolk. The shell lets in air so that the chick can breathe.

The egg must be kept warm all the time to grow properly. When the chick is ready, it makes a hole in the shell with its BEAK and crawls out.

http://rrp.blogfa.com/