How does a chicken grow inside an egg?

Unscramble the fascinating fertilisation process of bird eggs

Whether you boil them, scramble them or whip them into a prize-winning soufflé, eggs are one of the most versatile ingredients in the kitchen. But have you ever wondered about the ones that don’t make it onto the plate? These little capsules happen to be some of the most wondrous things in the natural world!

Laid by birds and reptiles, and sporting all shapes and sizes, each egg has a similar makeup – a brittle shell protects a gloopy inner of the familiar ‘yolk’ and ‘white’. The yolk is released as the chicken ovulates; it can then be fertilised, and continues to travel through the hen’s reproductive tract. The white of the egg is comprised of various different layers of albumin, structural fibres and membrane, which surround the yolk as it travels through. Finally, the eggs are ‘shelled’ and laid by the hen usually 24 hours later.

The fertilised yolk contains all of the genetic information needed to create a newborn chick. To support the chick’s development, eggs are high in fat and protein – the more fat in the yolk, the darker the colour. Read on to find out about the development from fertilised egg to chick.

Stages of development

The embryo begins to develop at one side of the yolk – this is held in place in the centre of the egg white by a protein cord called the chalaza.

Blood vessels are present, and the embryo has a heartbeat. After five days, there is substantial growth and the tiny chick has an eye. The embryo feeds on nutrients from the yolk through the blood vessels.

The embryo’s neck has lengthened and its brain is developing. Claws, legs and wings begin to show and lengthen, and there are also feather follicles forming as more blood vessels draw sustenance from the yolk.

The embryo now fully resembles a chick, down has covered its body, and days 15 and 16 are spent growing. By day 17, the egg white is used up, and the chick starts to get into hatching position.

After 20 days, the white and yolk have been absorbed and the chick is fully formed. It has rotated within the egg so that it can break the shell using its egg tooth – the hardened end of its tiny beak.


This article was originally published in How It Works issue 87, written by Ella Carter


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