Question of the Week

Ventilating an egg

Sun, 12th Jul 2009

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Michael, Austria asked:

Eating a wonderful Thai chicken, I got curious about the development of the chicken within its egg. How does the oxygen come in and the CO2 go out?


Diana -   So how do chicks breathe before they hatch?

Eggs, illustration by Adolphe Millot from Nouveau Larousse Illustré [1897-1904]Kirsty -   My name is Kirsty Peck and I work as a wildlife advisor for the RSPB.  Well an egg is a very complicated structure.  And as you can imagine, it’s got all the different life support systems for the chick in that tiny little package.  And the embryo itself lies right next to that huge yolk in the centre of the egg.  And as it develops, there are all kinds of membranes that will eventually form the different organs in the chick.  One of these membranes, they call it the chorion is one that it kind of envelopes part of the yolk sac and also it runs along the outside of the shell in part of the egg.  The chorion has got the very thin walled network of blood vessels all along it and in the same way, the blood vessel that you will find in that along would be picking up oxygen from the air inside the lung.  The chorion will pick up oxygen through the shell because you’ve got to remember, the shell of an egg is porous so oxygen can come in and at the same time any carbon dioxide from the blood can be excreted out through the shell.

Diana -   The chorion acts as a lung tissue within the egg but what happens when the chick wants to escape?

Kirsty -   What happens of hatching is that as you probably know, an oval shaped egg in the wide end of it, there’s a little air pocket and at hatching, as the blood vessels in these membranes will wither away and drop off, then the chick will be relying on the air and not the air pocket as it’s starting to break through the shell which will give it kind of something like a scuba diver’s aqualung or something to give it a limited air supply while still it’s breaking out of the egg.


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Diffusion Madidus_Scientia, Tue, 7th Jul 2009

But diffusion, according to Einstein, follows an inverse square law, so it could take more than a year for a molecule to diffuse the length of an egg...

chris, Tue, 7th Jul 2009

Isnt a chicken slightly porous which allows air molecules to be transmitted through the shell at much faster rates than traditional diffusion? orlandosingles, Tue, 7th Jul 2009

The egg shell has pores which allow the diffusion of air. A typical chicken egg has 7000 pores. Within the shell are two membranes (one within the other) between which an air sack forms, at the rounded end of the shell, once the egg has been laid. These membranes separate the shell from the albumin.

Now for my supposition. This air sack acts as a 'lung' and the lung is ventilated by the movement of the developing chick. This cannot happen until the chick begins to develop and even then, may not be sufficient for good air exchange, but the incubating parent bird will from time to time move the eggs. This has always been seen as the bird ensuring even temperature during incubation, but I would suggest that it has another, and more important, purpose. The movement of the egg causes the albumin to slop around, thus putting pressure on the air sack and releasing the pressure as it passes over the internal membrane. In this way, the albumin acts like a diaphragm.

Within the albumin is a fibrous layer separating the inner albumin from the outer albumin and another such layer surrounding the yolk. These two layers are connected to the inner membrane of the shell by a similarly fibrous 'chalaza'. All of this appears to be the means by which the yolk is protected and cushioned against the hard shell. But I wonder if this fiberous material is also the means by which oxygen/CO2 exchange between the blastoderm and the air sack is dealt with, making it the 'blood' or 'umbilical' of the egg. Don_1, Fri, 10th Jul 2009

It's always up to me to save this site by providing the true and correct answer !

Most eggs come with their own air con and ventilation systems.

here..i have shown you on this true and accurate non interfered with actual photo of a real egg

A Real Egg Laid Earlier Today

neilep, Fri, 10th Jul 2009

Not quite got the hang of Photoshop™ yet, have you sheepy??? Don_1, Fri, 10th Jul 2009

What !..surely ewe jest !  neilep, Fri, 10th Jul 2009

And where would the power supply come from to power the air con?........ (altogether, now....)

"Is it a battery chicken?"

John Chapman, Fri, 10th Jul 2009

The other purpose of the air sack is to help the developed chick practise breathing before it hatches. The chick pecks through the first membrane and takes some breathes of air before it pecks its way out.
You can enjoy the pores in the egg shell by placing an onion next to your egg in the fridge. Leave it for a week and then cook the egg and eat it. Your egg will be onoion flavoured, yum.
Make it Lady, Sun, 12th Jul 2009

Why will the egg reduce its size in water? I would have thought that the egg would be a more concentrated environment than water and hence would favour the movement of water into the egg making it larger not smaller...? chris, Sun, 12th Jul 2009

I Meant to put water with lots of sugar dissolved in it but got distracted, sorry. This comes of trying to post when the cats need feeding. I will be more attentive in future. Make it Lady, Mon, 13th Jul 2009

Okay, thanks for the clarification.

Incidentally, forum fans might like to listen to the podcast this week to see who gets mentioned in the answer to this's also a lovely answer.

The show's coming out tonight...

chris chris, Tue, 14th Jul 2009

Well now sheepy, it looks like you are now famous......... or should that be infamous?! But your ability on Photoshop™ still sucks!
Don_1, Wed, 15th Jul 2009

Also, make a naked egg by putting it into colourless pickling vinegar for a few days. Scrub off the shell, gently with a nail brush. You should be left with the insides of the egg, trapped inside the membrane. The egg will be slightly swollen as some of the vinegar will have entered the egg. Place the naked egg into water. after a few days it will have reduced in size. Good old Ozzy Mosis. Make it Lady, Wed, 15th Jul 2009

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