These eggs were laid 21 days ago

and this one is just about to hatch.

If you listen you can hear the chick

pecking at the inside of its shell.

Soon it will break through

and take its first breath of fresh air.

It's a dramatic moment, that first breath

- one shared by so many creatures -

including ... us.

But hold on, think about this: When you were in the womb

you got oxygen from your mother through your umbilical cord.

But for the last 21 days this chick has been cut off from its mother - sealed inside an egg.

So how does it get oxygen?

An egg seems like a perfectly self-contained system.

The yolk and the white contain all the nutrients you need

to build a baby chick.

As with a human baby, all this construction requires oxygen

and that's the one thing that isn't stored inside the egg.

So where does it come from?

Well take a look at this.

When you magnify an egg's shell a thousand times

you can see the calcium carbonate crystals that make up the shell

and here and there -- tiny holes.

One thousandth of an inch across.

And these tiny holes let outside air filter in.

So oxygen can pass through the shell but the chick growing inside

doesn't have working lungs yet.

How does it get that oxygen into its bloodstream?

Well, a few days after an egg is laid

something amazing happens.

When you hold a fertilized egg up in front of a bright light, you can see it:

a delicate network of blood vessels that grows out of the embryo's abdomen

and presses up against a membrane just inside the shell.

Oxygen from the air comes in through the tiny holes in the shell

then diffuses into the embryo's blood.

And the growing chick gets rid of carbon dioxide at the same time.

It all looks remarkably similar to your early days in the womb

There was a yolk sac, at least at first,

and a network of blood vessels growing out from

the place where your belly button now is.

But instead of pressing up against the edge of a shell

your blood vessels reached the wall of the womb

where they joined with an outer membrane to form the placenta.

In the placenta, oxygen from your mother diffused into your bloodstream.

It really is an exact mirror of what's going on in the eggs of birds and reptiles.

While all this is happening, lungs are developing.

We humans don't fill those lungs with air until after we're born.

But chicks get a head start.

That's because the whole time oxygen is coming in through the shell.

Moisture is slowly evaporating out.

That creates an empty space that gradually fills with air.

A day or so before the chick is ready to hatch,

it starts to move.

It punctures that air pocket and fills its lungs.

It then has just enough oxygen to battle out of the egg

and take its first breath of fresh air.

This is Skunk Bear, NPR's science show

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