The Science and Anatomy of an egg

Most people see it as a simple oval form, we eat it for brekfast, poached, boiled, scrambled or fried, to us it seems like an everyday thing, but an egg is more than that. It is a fascinating structure, and also contains the potential of life.
An egg is protected by a porus shell made up of calcite, a christalline form of calcium carbonate. It is semipermeable, meaning water and air can pass through via the thousands of tiny pores. The egg has a very outermost layer called the cuticle which protects ftom bacteria. On the inside of the shell there are two membranes. The inner and the outer membranes are surprisingly tough. They are made up partly of keratin, which is found in hair, skin and nails, and act as a bacteria barrier.
When the egg is freshly laid it is warm, but once it cools the contents shrink, leaving an air space between the two membranes. The air space grows larger as the egg ages, which is why incubated eggs have large air gaps when shone through with a candling lamp.
Inside these two membranes there is the albumen or egg white which is made up of water and many different liquid proteins. The ‘chalazae’ are twisted ‘ropes’ of protein holding the yolk and also acting as shock absorbers. This structure can be seen on a fertilized, unincubated egg. The viteline membrane encases and supports the yolk.
The yolk contains protein, fat, water (to a lesser extent) and vitamins and minerals. This is where the chick obtains nutrition. The yolk ranges from pale yellow to deep golden depending on breed and management. Chickens that roam freely and eat lots of green generally have golden egg colour.
The eggs colour ranges from white to dark brown, with green and blue hues as well. The pigment is the last thing that is added during its formation in the ovary and depends on breeding and genetics.
When the egg is laid it comes out of the vent. Both eggs and droppings share this opening but the set up and placing of various organs inside the hen ensures that the two never come in contact. There is also no urinary opening to complicate matters, a chickens urine is not liquid, it is the white part of the droppings, called urates. The sunshine that a plant absorbs makes it green, while the sunshine that the hen eats when she eats the plant makes her eggs yolk yellow. Battery eggs are have pale yolks, because the only sunshine the hen gets is that that her food contains.
The meat spot is a small deposit of blood sometimes found in the yolk. It gives no indiction that the egg is fertile, it is simply caused by the rupture of a bood vessel during the eggs formation. It is safe to eat but can be removed before cooking with the tip of a kife.
Occasionally a hen may lay an egg that has no yolk or a double yolk, this is the result of an unsynchronyzed production cycle an usually occurs at the begining or end of the laying period. A double yolked egg should not be used for breeding so if the egg is extra large don’t be tempted to use it.

Even when she is young, a hen has more egg cells than she will ever lay. These are rudimentary or germ cells. The egg ripens and a yolk forms around it in layers. It travels down the oviduct, and at this point would recieve sperm from the male. The chalazae are formed and a membrane covers the yolk.  Then, liquid protein called the albumen, or egg white is added in layers, the last one being firm and spongy as a shock absorber. Then, the shell is formed and in the last three hours pigment is added. During incubation the chalazae break, so the hen turns the egg occasionally to keep it centred.

I hope this article has expanded your knowledge, if you are interested in more science subjects, take a look at our The science behind a chickens eyes page and our chicken genetics page.

Egg Laying in Chickens


Even as a chick a hen has all the eggs she will ever lay in her ovaries, in an immature form called  rudimentary egg cells (yolks- to- be ).
How quickly this supply is used up depends on breed, feeding, housing and hygiene.
If you use an artificial light to keep your hens laying through the winter their store of rudimentary cells will be used up faster. In her first year, a hen lays the largest amount of eggs that she will ever lay in one year, although the total weight of the eggs she lays stays roughly in her first and second year because in her second year, although the individual number decreases, the size of the eggs increases.

The size of the hen barely affects that of her egg. Take Wyandottes and Dutch
bantams for an example. In the picture the egg above is that of a Wyandotte
while the one below is from a Dutch bantam and Wyandottes are many times
the size of a Dutch bantam.

Colour also varies greatly from white to dark brown with beige, cream, blueish,
Greenish and pinkish at various ends of the spectrum!
In the formation of the egg the pigment is added last.
A yolk is released from the ovary and the albumen (egg white) forms around it, covered with a membrane.
The egg rotates through the body, giving it its form and then the shell is formed.
Last but not least the pigment is added.

There is a time in the year when hens will take a break from laying. Some people switch to cheeper  food because  the hens arent laying, but that is the wrong thing to do. By taking a break, the hens are recharging their batteries, so if anything, they should be fed better food!

You must have an adequate nestbox, or the hens will go and find their own place to lay, sometimes in a bush, or in a hedge or, more frustratingly, under the shed! Nestboxes should have a ‘lip’ at the front to stop the litter spilling out. They should be about 30cm wide and 40cm long. they should have 40cm walls to give the hen privacy, because, hens like quiet, peaceful, dark places to lay.

If you collect eggs daily, your hens shouldn’t get a chance to taste an egg but if they do they will probably start pecking open freshly laid eggs. A dark nestbox helps to prevent this behavior. Once they acquire this habit they are unlikely to stop unless you interfere, you can (most of the time) stop this behaviour by making a hole in an egg, spilling its contents on the nest box floor and adding non-toxic foul tasting liquid like mustard and chilli pepper.

Reduce stress, dont have bright lights around the nestbox. Chickens may also eat eggs because they are not getting enough calcium, so it is worth providing a calcium supplement, in liquid form to add to water, or as oyster shell grit. If this doesn’t work you can make a double bottomed nest box with sloping floor where the egg rolls down a crack to the bottom floor where soft material breaks its fall, so that it is out of the chickens reach.

To find out more about eggs and their anatomy and science click here.