Science Questions

Ventilating an egg

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? Michael, Austria

Diana -   So how do chicks breathe before they hatch?

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.

July 2009