An eggshell may look solid and impermeable, but it has about 8,000 pores that are large enough for oxygen to flow in and carbon dioxide to flow out.
It was John Davy of Edinburgh, Scotland, who proved this in 1863 by pumping pressurized air into an underwater egg and watching thousands of tiny bubbles float to the surface.
So yes, an unborn baby chick can breathe just fine inside the egg.
