A chicken egg takes about 25 hours to form inside a hen’s body and, if fertilized and incubated, another 21 days to develop into a chick ready to hatch. That process involves two distinct phases: the egg’s physical construction in the hen’s reproductive tract, and the embryo’s growth from a single cell into a fully formed bird. Both are remarkably precise.
How the Egg Forms Inside the Hen
A hen’s reproductive tract is essentially an assembly line. The yolk, which is a mature ovum released from the ovary, travels through four specialized regions over roughly 25 hours. Each region adds a new layer.
The first stop is the infundibulum, a funnel-shaped opening where the yolk is captured. It stays here for only about 15 minutes, but this is the only window for fertilization. If sperm are present, they meet the yolk here. Whether fertilization happens or not, the egg continues down the line.
Next comes the magnum, where the egg white (albumen) is deposited over about three hours. Two layers go on: a thick inner layer rich in protein and riboflavin, followed by a thinner outer layer. The magnum also adds the chalazae, those twisted rope-like strands you see when you crack an egg open. They anchor the yolk in the center and keep it from sloshing around.
The egg then passes through the isthmus in just over an hour. Here, two thin but tough shell membranes wrap around the albumen. These membranes act as a barrier against bacteria and help give the egg structural integrity before the hard shell is added.
The final and longest stage happens in the shell gland (also called the uterus), where the egg spends 20 to 21 hours. Water and minerals pass through the membranes to plump the egg into its familiar oval shape. Then the shell gland deposits calcium in the form of calcite crystals, building up the hard outer shell layer by layer. Just before the egg is laid, a thin protective coating called the cuticle seals the surface. This waxy layer helps block bacteria from entering through the shell’s microscopic pores.
What a Fertilized Egg Needs to Develop
A fertilized egg won’t develop unless it’s kept warm and turned regularly. In nature, a brooding hen provides both. In artificial incubation, the target temperature is 100°F in a forced-air incubator (102°F in a still-air model, since heat stratifies without a fan). Humidity stays at 58 to 60 percent for most of the 21-day incubation period, then gets bumped up to 65 percent or higher for the final three days to keep the membranes from drying out and trapping the chick.
Turning is just as critical. Commercial incubators rotate eggs 24 times per day through the first 18 days. This prevents the embryo from sticking to the inner shell membrane, which can be fatal. Dropping the turning frequency to 12, 6, or 3 times per day significantly reduces hatch rates. In the last three days, turning stops so the chick can orient itself for hatching.
Week by Week: Embryo Growth
Development moves fast. By day 3, the embryo already has a beating heart, and blood vessels are clearly visible spreading across the yolk. By day 5, the beginnings of elbows and knees appear. Feather tracts, the tracks along which feathers will eventually grow, are visible by day 8, along with the upper and lower beak taking shape at equal length.
The second week brings rapid physical maturation. Toes are fully formed by day 12, and the first visible feathers emerge. By day 13, scales appear on the legs and a light covering of feathers spreads across the body. By day 16, feathers cover the chick completely, and nearly all of the albumen has been consumed.
The final days are about preparation for life outside the shell. The chick absorbs the remaining yolk sac into its abdomen, which will serve as its sole nutrition source for the first day or two after hatching. It shifts position so its head is tucked under its right wing, with its beak pointed toward the air cell at the blunt end of the egg.
How the Embryo Eats and Breathes
A developing chick has no mouth-to-food connection for most of incubation. Instead, it relies on two routes to pull nutrients from the yolk. The yolk sac membrane, a heavily blood-vessel-rich tissue surrounding the yolk, breaks down fats through enzymatic digestion and releases the products into the embryo’s bloodstream. This is the primary nutrient highway for most of development. The second route is the yolk stalk, a narrow tube connecting the yolk sac directly to the embryo’s intestine. Around day 18, this stalk becomes the dominant pathway, and large amounts of yolk content start flowing directly into the gut.
Albumen gets used too, though less obviously. During the last week of incubation, albumen flows into the fluid-filled cavity surrounding the embryo. The chick actually swallows this fluid, and some of the albumen proteins end up absorbed through the yolk sac as well. Nothing goes to waste.
For breathing, the embryo develops a specialized membrane called the chorioallantoic membrane, which begins forming around day 5 and eventually lines the entire inner surface of the shell. This membrane is packed with tiny blood vessels that sit just beneath the shell’s pores, allowing oxygen to diffuse in and carbon dioxide to diffuse out. It functions exactly like a lung, just spread flat against the inside of the egg.
How the Embryo Builds Its Skeleton
The chick’s bones need calcium, and the biggest source is the shell itself. The same chorioallantoic membrane that handles gas exchange also dissolves calcium from the inner surface of the eggshell. Specialized cells in the membrane pump acid toward the shell, creating a locally acidic environment that dissolves the calcite crystals. The freed calcium ions are then picked up by a different set of cells and shuttled into the embryo’s bloodstream through the membrane’s capillary network.
This process primarily targets the mammillary layer, the innermost knobby region of the shell where it connects to the shell membranes. Over time, these knobs visibly erode, and the shell membranes begin to detach. This is one reason eggshells become noticeably thinner and more fragile toward the end of incubation.
Pipping and Hatching
Hatching is a two-stage breakout. First comes “internal pipping,” when the chick pushes its beak through the inner membrane and into the air cell at the blunt end of the egg. This is the chick’s first breath of air, and its lungs begin to take over from the chorioallantoic membrane. The transition isn’t instant. Between 12 and 36 hours pass between this internal pip and the moment the chick breaks through the outer shell, called “external pipping.”
To crack through the shell, the chick uses a small, hard calcium deposit on the tip of its upper beak called the egg tooth. This tiny point concentrates force on a single spot, letting the chick punch a hole from the inside. The egg tooth was first described by the ornithologist William Yarrell in 1826, who watched hens and ducks hatch and noticed the sharp little structure pressing against the shell from within.
Once the first hole appears, the chick slowly rotates inside the egg, chipping a line of fractures around the circumference of the shell. This is called “unzipping.” Eventually the chick pushes the cap off and tumbles out, wet and exhausted. The egg tooth, no longer needed, falls off or is reabsorbed within a few days of hatching.