If a chicken egg is a single cell, why doesn't it become two by mitosis and continue like regular human eggs?

A chicken egg is a single cell inside a hard shell. If the egg has been fertilised, it will become two cells, but still inside the shell. Then it will become 4, 8, etc., and eventually will become a chicken. Since all this happens inside the shell, the cells must get smaller as they divide, because no extra matter is coming in from outside.

And, in most cases, eggs that make it to sale are not fertilised, so they are still single cells. Every now and then you will find a fertilised one (anything from a small black-red dot to something more resembling a very small foetus), which is clearly multiple cells.

From
http://www.howstuffworks.com
part of what I got after I did a search on 'chicken egg':

"The fertilized egg is a highly complex reproductive cell and is potentially a small center of new life. The germinal disc from which new life develops is attached to the yolk of the egg. Surrounding and protecting the germ cell and yolk is the white, or albumen, consisting of several layers."

It sounds as though the chicken egg has one cell, the germ cell, attached to the yolk, and implies that the yolk and the albumen are not cells and not made of cells. Can't find anywhere that says it in so many words.