Whether you’re raising chickens or simply curious about the miracle of egg production, understanding how eggs are formed will deepen your appreciation for your flock and the work they do each day.

The Hen’s Reproductive Anatomy

Only female chickens, or hens, lay eggs. Their reproductive system is made up of a single functional ovary and an oviduct, a long, coiled tube where the egg takes shape. Although female chicks are born with two ovaries, only the left one becomes active in adulthood.

The ovary contains thousands of tiny yolks called ova. These are undeveloped eggs, and only one is released at a time during the laying cycle.

Step 1: Yolk Release from the Ovary

The process begins when a mature yolk (ovum) is released from the ovary. This is called ovulation. The yolk is rich in nutrients and surrounded by a thin membrane. It is bright yellow due to the pigments from the hen’s diet, especially if she consumes greens or corn.

Ovulation typically occurs 15 to 45 minutes after the previous egg is laid. Light exposure, especially in the morning, helps stimulate this step.

Step 2: The Infundibulum – Where Fertilization Can Occur

Once the yolk is released, it enters the first section of the oviduct called the infundibulum. This funnel-shaped part catches the yolk and is the site where fertilization would happen if the hen has mated with a rooster.

Fertilization is not necessary for egg-laying. Hens lay eggs regardless of whether they have mated. If no rooster is present, the yolk continues its journey unfertilized.

The yolk stays in the infundibulum for about 15 to 20 minutes.

Step 3: The Magnum – Forming the Egg White

Next, the yolk enters the magnum, the longest section of the oviduct. Over the course of three hours, the egg white, or albumen, begins to form. This layer is mostly made of water and proteins and serves to cushion and protect the yolk.

The thick and thin layers of albumen are secreted here, helping to give the egg its structure and internal balance.

Step 4: The Isthmus – Adding Membranes

The forming egg moves into the isthmus, a narrower section where the inner and outer shell membranes are added. These membranes serve as a barrier against bacteria and help the egg hold its shape. This step takes about 1 to 2 hours.

You might notice these membranes when peeling a hard-boiled egg—the thin skin between the white and the shell is a product of the isthmus.

Step 5: The Shell Gland (Uterus) – Shell Formation

The most time-consuming step happens in the uterus, also called the shell gland. Here, the egg spends about 20 hours while a hard shell made of calcium carbonate forms around it.

The shell starts out as a soft covering and gradually hardens as layers of calcium are deposited. This is also the stage when pigmentation is added. Hens that lay brown eggs deposit pigments like protoporphyrin onto the shell, while blue-egg layers produce a pigment called oocyanin.

The shell’s color is determined by the hen’s breed and genetics. Regardless of color, all shells are composed primarily of calcium and contain thousands of tiny pores that allow air exchange.

Step 6: The Vagina and Laying the Egg

Once the shell is complete, the egg is passed into the vagina. This part of the oviduct does not contribute to the egg's structure, but it helps position the egg correctly—rounded end first—for laying.

When the hen is ready, a muscle contraction pushes the egg out of her body through the cloaca, a shared exit for both the digestive and reproductive systems. This final stage takes only a few minutes.

The result is a fully formed egg, warm and fresh, ready to be collected from the nest.

The Cycle Begins Again

After laying an egg, the hen’s reproductive system rests for a short period. Depending on the breed, environment, and diet, she may lay another egg within 24 to 30 hours. High-production breeds like Leghorns and Rhode Island Reds often lay six to seven eggs per week.

Age, daylight hours, and health can affect the frequency and quality of egg production. As hens get older, they may lay fewer eggs or eggs with thinner shells.

A Natural Wonder

The process of egg formation is one of the most efficient and finely tuned biological systems in nature. With no outside assistance, a hen’s body turns nutrients from her diet into a complete, self-contained life-support system. Whether fertilized or not, every egg is a testament to nature’s ingenuity—and to the hardworking hen who laid it.