How an egg is formed — the journey through the hen's oviduct
The egg on your table is the result of nearly a full day's work by the hen's body. Yolk, white, membranes and shell — each layer forms in a different part of the oviduct. We explain it step by step and show why layer nutrition matters so much for egg quality.
verifiedFrom the team that has organised work on poultry farms for years.
A laying hen produces roughly one egg every 24–25 hours. To make this possible, her body works continuously: the yolk matures in the ovary and then travels through the long oviduct, where albumen, shell membranes and the hard shell are deposited in sequence. Understanding this process helps you manage your flock better.
Why know the structure of the oviduct?
Each layer of the egg forms in a different part of the oviduct. If something is wrong with the shell, albumen or yolk, knowing the stages lets you pinpoint the cause — in nutrition, hen health or the environment. Details on the finished egg are in the guide on egg structure. Layer nutrition principles are covered in layer nutrition — standards.
How long does it take to form an egg?
The whole egg takes approximately 24–25 hours to form. The longest stage is shell formation in the uterus (shell gland) — about 20 hours. That is why hens need calcium especially in the evening and overnight, when the shell is being built. This is the direct reason why layer diets must be well balanced for calcium.
How an egg passes through the oviduct — 6 stages
- 1
Ovary — yolk maturation
In the ovary, follicles mature (each containing a yolk — the egg cell with a large food reserve). When a follicle is ready it ruptures and releases the yolk — this is called ovulation. The hen has only one active ovary (the left one). Yolk maturation takes several days; ovulation itself takes moments.
- 2
Infundibulum (funnel) — entry to the oviduct and possible fertilisation
Immediately after ovulation the yolk falls into the infundibulum — the wide opening of the oviduct. If a cockerel is present, fertilisation can occur here. The infundibulum is active for only a short window (approximately 15–30 minutes) — if the yolk misses the infundibulum it falls into the body cavity and is reabsorbed. Transit through the infundibulum takes about 15–30 minutes.
- 3
Magnum — albumen deposition
In the magnum — the longest section of the oviduct — most of the egg white is deposited around the yolk. The yolk rotates as it travels, so the albumen wraps around it evenly. The chalazae — twisted protein strands that anchor the yolk in the centre of the egg — also form here. Transit time in the magnum is approximately 3 hours.
- 4
Isthmus — shell membranes
In the isthmus two thin shell membranes (outer and inner) form around the albumen and yolk like a soft pouch. Some water and minerals are also added under the membranes at the end of this section. Transit time is approximately 1–1.5 hours. The shell membranes are clearly visible when you peel a hard-boiled egg — the thin film just under the shell.
- 5
Uterus (shell gland) — shell and colour
This is the longest stage and the most important for the farmer. Calcium carbonate is deposited on the shell membranes to form the hard shell. Near the end, pigment is deposited giving the shell its colour (white, brown or speckled, depending on breed). Transit time is approximately 20 hours. This is why hens need plenty of calcium in their diet — especially in the evening and overnight, when the shell is actively built. How much calcium and when to feed it is covered in layer nutrition — standards.
- 6
Vagina and cloaca — laying
The finished egg moves into the vagina and cloaca, from where it is laid. The hen lays the egg blunt-end first (the blunt end exits first). At the moment of laying the hen has already had a new ovulation — a new yolk has entered the infundibulum and the whole cycle starts again. This is why a healthy layer can lay one egg approximately every 24 hours for many months.
What affects egg quality — nutrition, health and environment
Each stage of the oviduct has its own requirements. Below are the key factors a farmer can control.
Dietary calcium — the foundation of a good shell
The shell is over 90% calcium carbonate. A hen uses several grams of calcium to build one shell — and repeats this almost every day. Calcium deficiency or poor absorption (e.g. from vitamin D3 deficiency) leads to thin, fragile or misshapen shells. More in the guide on layer nutrition — standards.
Protein and energy — yolk and albumen quality
Yolk and albumen are mainly proteins and fats. A diet low in protein or energy results in smaller yolks, weaker albumen and poorer internal egg quality. Hens at peak lay have very high amino acid requirements — especially methionine and cystine.
Hen health and reproductive tract disease
Bacterial infections (e.g. salpingitis, peritonitis) and viral diseases (e.g. infectious bronchitis, IB) can disrupt any stage of egg formation — causing shell-less eggs, wrinkled or rough shells, cloudy albumen or abnormal shape. Egg quality assessment (Haugh units and other indicators) is described in the guide on egg quality assessment — Haugh units.
Interesting facts about egg formation
A few things worth knowing to better understand laying performance and egg quality.
Hens have one active ovary
Hens (and almost all birds) have only one functional ovary — the left one. The right ovary regresses early in embryonic development. In flying birds, carrying two fully active ovaries would be too heavy. Chickens don't fly, but evolution left them with this economical arrangement.
Shell colour comes from pigment, not breed as such
Brown shell colour comes from a pigment called protoporphyrin IX, deposited in the final stage of shell formation in the uterus. White means no pigment. Shell colour has no effect on nutritional value — it is simply a breed and genetics trait.
The new yolk enters the infundibulum almost immediately after laying
Ovulation typically occurs soon after the previous egg is laid — often within 30 minutes. This is why a productive layer cycles roughly every 24 hours and can lay nearly every day for many months. Day length (lighting programme) strongly influences regulation of this cycle.
Common myths and mistakes about egg formation
A few widespread misconceptions worth knowing.
Myth: brown eggs are healthier than white
Shell colour has nothing to do with nutritional value. Brown colour is just pigment in the shell, determined by breed. Egg quality depends on nutrition, hen age and housing conditions — not shell colour.
Myth: an egg without a cockerel is "inferior"
An unfertilised egg (from a hen kept without a cockerel) is identical in every way to a fertilised one — same taste, same nutritional value. The difference is purely biological: a fertilised egg can develop into an embryo if incubated.
Mistake: feeding extra calcium in the evening alone is enough
Evening calcium is indeed important because that is when the shell gland is most active. However, the whole diet must be balanced throughout the day — calcium without adequate vitamin D3 and protein is poorly absorbed. One-sided calcium supplementation without correcting the whole diet rarely solves the problem of fragile shells.
Mistake: an older hen lays as well as a young one
As hens age, egg quality declines — shells become thinner, yolks less vivid, and the proportion of defective eggs rises. This is a natural consequence of ageing in the oviduct and ovary. More on identifying a good layer in the guide on how to recognise a good laying hen.
Frequently asked questions about egg formation
How long does it take to form one egg?add
Approximately 24–25 hours. The longest stage is shell formation in the uterus (shell gland) — about 20 hours. The other stages (infundibulum, magnum, isthmus) take a few hours in total.
Why do laying hens need so much calcium?add
The shell is over 90% calcium carbonate. To build one shell, a hen uses several grams of calcium — and does this almost every day. Calcium deficiency or vitamin D3 deficiency (needed for absorption) leads to thin, fragile shells or shell-less eggs. Layer feed is rich in calcium, provided especially in the evening and overnight.
Where does shell colour come from?add
Shell colour comes from a pigment (protoporphyrin IX) deposited in the final stage of shell formation in the uterus. White shell = no pigment. Brown, cream or speckled = pigment, depending on breed. Colour has no effect on nutritional value.
What are chalazae in an egg?add
Chalazae are two twisted protein cords that hold the yolk in the centre of the egg, attached to the membranes on both sides. They form in the magnum as the yolk rotates during its journey through the oviduct. Prominent chalazae in a fresh egg are a good sign of quality.
Does a hen need a cockerel to lay eggs?add
No. Hens lay eggs without a cockerel — unfertilised eggs. A cockerel is only needed when you want to hatch chicks. In terms of taste and nutritional value, fertilised and unfertilised eggs are identical.
Why do hens lay eggs blunt-end first?add
This is determined by the structure of the oviduct and the shape of the egg. In the vagina and cloaca the egg is oriented with the blunt end towards the exit. The blunt end contains the air cell — after laying, air enters through this chamber and will be the chick's first breath. More on egg structure in the guide on egg structure.
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