Ever wondered how chicks get oxygen inside their egg shell?

All living things need oxygen to breathe. Even human babies get their share of oxygen inside the mother’s womb through the umbilical cord. So how about a chick that is growing inside an egg? You will be surprised to know that eggs are quite sophisticated in their packaging. Even though eggs look fragile, they are in fact, biological wonders.

The eggs of birds and also other reptiles contain albumen, also called the egg white along with yolk. The fertilized embryo develops inside the egg shell, feeding off the yolk and the egg white. So, you can see that the little bird growing inside has got everything in place, like food and shelter. The only thing that is not visibly present is probably fresh air. But, this is taken care of by the membranes present inside the egg, along with the tiny pores present on the egg shell.

When the eggs are laid by the mother, they are very warm, when compared to the outside conditions. Slowly, after sometime, the eggs cool down. The material inside the egg shrinks a little bit. Now, under the egg shell, there are two membranes present, which are stuck to each other.  When the eggs cool down and the material inside shrinks, the two membranes are pulled apart a little to create a small sack, called the air sac. This air sac is filled with oxygen. The oxygen gets into the air sac through a process called diffusion. This happens through the many tiny pores present on the egg shell.

The oxygen is used by the growing chick in a chemical reaction called metabolism. In this process, some amount of carbon dioxide is also released, which has to be expelled out. Through the same process of diffusion, this carbon dioxide also is expelled out, through the tiny pores on the egg shell. As this gas leaves out, the air sac is again filled with oxygen. These tiny pores also help in letting in the moisture into the egg. This keeps the developing bird and all other egg parts from drying out.

Have you ever noticed that hard-boiled eggs always feel a little heavier than raw ones? The same concept of diffusion is applicable here. When you boil eggs, the pores in the shells of the chicken eggs allow materials to cross back and forth between the inside of the egg and the outside environment. Here, in this case, the air is replaced by water!