Structure of a hen's egg — shell, albumen, yolk
A hen's egg looks simple, but inside it is very well organised — each layer has a job to do. We explain what is what and why, from the hard shell all the way to the tiny germinal disc on the yolk.
verifiedFrom the team that has organised work on poultry farms for years.
A hen's egg is one of nature's best-designed "packages". Everything inside — water, protein, fat, vitamins — is there to nourish a developing embryo for 21 days. Before reaching the hatchery, the egg must survive transport and storage, and each layer plays its part in this.
How to use this guide
We describe egg structure from the outside in — the same order in which the hen builds it in the oviduct. Each layer gets its own card below. If you want to know how a hen produces an egg from scratch, see the guide on how an egg is formed. If you want to know whether an egg is suitable for hatching, read about egg quality assessment and Haugh units.
Egg structure — from shell to germinal disc
Six layers, each made of different material with a different function. Starting from the outside.
Shell
The hard outer casing, made mainly of calcium carbonate — the same mineral as chalk and marble. The shell is brittle but very resistant to compression. It is covered with thousands of tiny holes called pores through which the egg "breathes" — exchanging gases with the surroundings. This is not a flaw; it is deliberate: carbon dioxide and water vapour escape through the pores, while oxygen enters for the developing embryo.
Cuticle (bloom)
The cuticle (also called the bloom) is a thin coating on the surface of the shell, invisible to the naked eye. The hen applies it as the last step, just before laying. The cuticle plugs the pores and protects the interior from bacteria — it acts like a natural varnish. That is why hatching eggs should not be washed aggressively: excessive washing removes the cuticle and opens the way for bacteria. More on handling eggs before incubation in the guide on poultry egg incubation.
Shell membranes
Just inside the shell are two thin shell membranes — outer and inner. They look like delicate tissue paper. At the blunt end of the egg the two membranes separate to form a gap: the air cell. The chick uses this air cell just before hatching — it pierces the inner membrane with its beak and takes its first breath. The older the egg, the larger the air cell, because water evaporates through the pores over time.
Albumen (white)
The albumen is the clear, gel-like mass inside the egg. It comes in two fractions: dense albumen (closer to the yolk, firmer) and thin albumen (near the membranes, looser). The more dense albumen, the fresher and better quality the egg — this is what is measured in Haugh units. The albumen provides water and protein, and cushions the yolk against impact.
Chalazae (twisted cords)
Chalazae are two whitish, twisted protein cords that hold the yolk precisely in the centre of the egg, like springs. They are made of condensed albumen. The firmer and more pronounced the chalazae, the fresher the egg — they lose their tension with age and the yolk begins to drift towards the membrane. Chalazae are completely edible and safe.
Yolk and germinal disc
The yolk is a store of fat, vitamins and pigments — food for the developing embryo. It is surrounded by the vitelline membrane. On the surface of the yolk sits the germinal disc — a small, pale disc-shaped spot. In an unfertilised egg it is just a cluster of cells. In a fertilised egg, the embryo develops from it, hatching into a chick after 21 days. The colour of the yolk depends on the hen's diet: carotenoids from maize and paprika give a deep orange-yellow colour.
Function of each layer of the egg
Each part of the egg plays a specific role — protective, nutritive or structural. A brief summary.
Shell: mechanical protection and gas exchange
The hard shell protects the egg from crushing and damage. Thousands of pores allow gas exchange — oxygen in, carbon dioxide and water vapour out. The hen builds the shell over 18–20 hours, drawing calcium from feed and bones. This is why layer nutrition directly affects shell thickness and hardness — more in the guide on layer nutrition standards.
Cuticle and membranes: bacterial barrier
The cuticle is the first line of defence — it seals the pores and prevents bacteria from entering. The shell membranes are the second barrier: they stop microorganisms that nonetheless penetrate the shell. Together they protect the egg interior and enable long-term storage.
Air cell: first breath of the chick
The air cell grows over time as water evaporates through the pores. In a fresh egg it is small (a few millimetres); in an older egg it is larger. Just before hatching the chick pierces the inner membrane and brings air into its lungs — its first real breath, before it even breaks through the outer shell.
Albumen: water, protein and cushioning
The albumen provides the embryo with water and structural proteins. The denser fraction (near the yolk) forms a springy cushion protecting the yolk from impact. The thinner fraction (near the membranes) acts as a thermal buffer. Albumen quality — measured in Haugh units — is an important criterion when selecting eggs for hatching.
Chalazae: yolk suspension
The chalazae keep the yolk exactly in the centre of the egg, which is critical during incubation: the embryo should always be on top of the yolk (as it is lighter than the rest). Good chalazal suspension means that even when eggs are turned in the incubator, the embryo returns to the correct position.
Yolk: fuel for the embryo
The yolk is the full energy store — fats, vitamins A, D, E, K and yellow pigment (xanthophyll). The embryo feeds on the yolk for all 21 days. Before hatching it absorbs the remaining yolk into the yolk sac, which means the chick arrives with a food reserve for its first hours of life — that is why chicks can wait a few hours after hatching before they reach the feeder.
Common myths about egg structure and freshness
A few things often said about eggs that are either untrue or only part of the truth.
Myth: an egg with no blood spots is always better
Blood or meat spots in the albumen are not dangerous — they are small ruptures of blood vessels or tissue fragments that entered the egg during formation. They do not indicate a bad egg or a sick hen. In commercial production they are removed by candling for appearance reasons, but they have no impact on food safety.
Myth: a large air cell means the egg is rotten
A large air cell means the egg is older — that is true. But older does not mean spoiled. An egg stored in a cold store for 3–4 weeks may have a large air cell but still be safe to eat. Smell and appearance after cracking are more reliable freshness indicators than air cell size alone.
Myth: a deep yellow yolk is healthier than a pale one
Yolk colour depends entirely on the hen's diet — carotenoids in feed (maize, paprika, algae). A deep orange colour looks appealing but is not evidence of higher vitamin content. A hen fed wheat will produce a pale yolk with similar nutritional value to a maize-fed hen.
Myth: washing hatching eggs improves cleanliness and hatchability
Aggressive washing with water and detergents destroys the cuticle and removes the natural bacterial barrier. A wet shell absorbs microorganisms faster than a dry one. Professional hatcheries sanitise eggs by fumigation (glutaraldehyde or hydrogen peroxide in gas phase) or dry cleaning — not by scrubbing with hot water. Avoid washing hatching eggs with water.
Frequently asked questions about egg structure
What is the cuticle and why is it on the shell?add
The cuticle (bloom) is a thin, invisible coating that the hen applies to the shell as the final layer just before laying. It plugs the tiny pores in the shell and forms a barrier against bacteria. That is why hatching eggs should not be washed aggressively — heavy washing removes the cuticle and opens the door for microorganisms.
What is the air cell in an egg and what is it for?add
The air cell (air space) is a gap between the two shell membranes at the blunt end of the egg. It forms when the egg cools after laying — the contents contract and the membranes separate. The chick uses this cell just before hatching: it pierces the inner membrane and takes its first breath. The air cell grows with the age of the egg — the larger it is, the older the egg.
What are chalazae and are they edible?add
Chalazae are two twisted, whitish cords of dense protein that hold the yolk precisely in the centre of the egg. They are completely edible and safe. The firmer the chalazae, the fresher the egg — they lose tension over time and the yolk begins to shift. In the food industry chalazae are removed mechanically when producing liquid egg products, as they affect product uniformity.
What is the germinal disc on the yolk?add
The germinal disc is a small, pale spot on the surface of the yolk. In an unfertilised egg it is just a cluster of ova cells. In a fertilised egg it is the starting point from which the embryo develops over 21 days of incubation. The presence of a germinal disc does not affect the taste or nutritional value of the egg — fertilised and unfertilised eggs taste the same.
How can you tell a fresh egg from an old one by looking at it?add
The simplest test is to place the egg in water: a fresh egg sinks to the bottom (small air cell), an older egg tilts at one end, and a very old one floats. After cracking: a fresh egg has a firm, domed yolk and thick albumen — the yolk stands up and the white does not spread widely. A flat, pale yolk and watery albumen indicate an old egg or one stored in poor conditions.
Why does the shell have pores — is that not a risk?add
The pores are deliberate and necessary. Through them the egg exchanges gases: oxygen enters (needed by the developing embryo) and carbon dioxide and water vapour exit. Without pores the embryo would suffocate. The risk arises only when the cuticle is damaged — for example by washing — because then the pores become an open gateway for bacteria.
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