Anatomy of the Chicken Egg

Cayuga Duck

Fun Fact

Cayugas can lay black eggs! Early in the season they sometimes lay eggs with a black pigment on the shells, this washes off easily and as the laying season progresses the pigment on the eggs becomes less intense

Have you ever been frying an egg and wondered how the yolk stays separate from the white, why there seems to be a thicker white nearer the yolk and a thinner one further out, why is the white cloudy sometimes and what is that stringy bit attached to the yolk? Here you will find everything you ever wanted to know about the anatomy of a chicken egg!

The Egg Shell

The shell is made of calcium carbonate with thousands of invisible pores which allow air and moisture to pass through. The shell is covered in a coating called the bloom, this is a protective coating which block harmful bacteria for entering the egg and reduces excess moisture loss. This bloom is the reason why you should never wash eggs, by doing so you are removing an important barrier which prevents bacteria from entering the egg, wipe soiled eggs with a dry kitchen towel instead.

Chicken egg shells come in a huge range of colours, from snow white to blues and greens, from the lightest brown to the darkest mahogany. Some people think that there is a difference in taste between brown and white eggs and just as many people think there is no difference at all. What do you think, leave a comment below with your thoughts!

Just inside the egg shell are two clear membranes called the inner and outer membranes which are made of keratin.

Outer Membrane

The outer membrane is against the inside of the shell, when an egg is laid it starts to cool and the inner and outer membranes separate, this in turn produces the air cell at the blunt end of the egg. The outer membrane acts as another barrier to bacteria and helps in the exchange of oxygen and carbon dioxide to and from the egg. The air cell actually makes peeling a hard boiled egg easier as it creates a cavity between the shell and the blunt end of the egg.

Inner Membrane

The inner membrane surrounds the albumen or egg white. It is also the most important and strongest barrier to bacteria in the egg.

The air cell 

This bubble of air develops between the inner and outer membrane at the rounded end of the egg as the freshly laid egg cools, the older the egg the bigger the air cell will be. You can see the air cell inside the shell on the left of the image below. A hatching chick breaks through to the air cell first and gets its first breaths of air from there, it then works its way around the shell in a circle chipping away until it is free.

Albumen (Egg white)

The name albumen comes from the Latin word albus which mean white. The clear glossy egg white surrounds the yolk and is 90% water with the remaining 10% being protein. You may be surprised that there are four different layers of albumen inside an egg. Nearest the yolk is the chalaziferous white, or inner thick white, it stabilises the yolks movement. Next is the inner thin white, then the outer thick white and the final layer, the outer thin white. The purpose of all these layers is to protect the yolk from movement and damage.


The chalaze are fibrous stringy structures that appear on two ends of the yolk, they work along with the albumen to support and balance the yolk in the middle of the egg. The freshest egg will have the most visible chalaze.

Vitelline Membrane

This is the layer that surrounds the yolk and the annoying layer that breaks when you are trying to fry the perfect sunny side up egg! It keeps the yolk separate from the albumen.

The Egg Yolk

Finally we have arrived at the heart of the egg. The role of the yolk is to provide all the nutrition that a growing embryo requires. There can be a lot of variety in the colour of egg yolks and this can be due to the breed of the hen who laid it as well as her diet. Hens fed yellow and orange feed like corn and unprocessed wheat and barley will produce darker orange yolks.

Some young hens produce eggs with two yolks, this usually does not last very long and they revert to normal single yolks but occasionally a hen will lay double yolk eggs for her whole egg laying life.

The egg yolk contains vitamins A, D, E, B6, B12, Iron, Calcium, Phosphorus, Lutein, Zeaxanthin, Choline and Protein. With such a power house of essential vitamins and minerals, eggs truly deserve the title of a SUPERFOOD

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