How An Egg Forms In A Chicken: A Step-By-Step Guide

Eggs are one of nature’s most incredible creations. The process an egg goes through in a hen’s body is complicated, but it works well to make sure the egg has all the nutrients and protection a chick needs as it grows. This article will show you how an egg grows inside a chicken, step by step.

An Overview of Egg Formation

Egg formation begins in the hen’s ovary, where yolks are released and travel to the oviduct to be surrounded by albumen, membranes, and a shell. It takes approximately 24-26 hours for a yolk to develop into a fully-formed egg and be laid by the hen.

The key stages of development are:

• Release of yolk from ovary
• Entry into oviduct
• Addition of albumen
• Addition of shell membranes
• Formation of the shell
• Egg laying

While the process follows a set sequence, many factors can influence egg quality and production rate, including breed, diet, age, lighting, and stress. Now, let’s look at each stage in closer detail.

Step 1: Ovulation and Yolk Release

Formation starts when a yolk ovulates from the hen’s ovary. Hens have two ovaries, but as adults, only the left one works.

The ovary contains many small yolks, each surrounded by a follicle. When a follicle matures, it ruptures and releases the yolk into the oviduct in a process called ovulation.

There are a lot of nutrients in the yolk, like fats, proteins, vitamins, and minerals, which will help the chick embryo grow. Pigments in the hen’s food, especially plant pigments that are yellow or orange, give the yolk its color.

Step 2: Entry into the Oviduct

When the egg yolk is released, finger-like structures called fimbriae grab it at the opening of the oviduct. It goes into the infundibulum, which is the first part of the oviduct.

This is where fertilization happens if sperm are present. In egg farm and backyard flocks, hens are not mated so the yolk continues unfertilized.

Step 3: Adding the Albumen in the Magnum

From the infundibulum, the yolk travels to the magnum portion of the oviduct. Here it spends 2-3 hours while the albumen, or egg white, forms around it.

The albumen consists mostly of water and proteins secreted by cells in the oviduct lining. Thin albumen forms first, followed by thick albumen closest to the yolk which keeps the yolk centered.

The albumen provides hydration, nutrition and shock absorption for the growing embryo.

Step 4: Forming the Shell Membranes

After the albumen layers are complete, the egg moves to the isthmus. Here the shell membranes are added. The inner and outer membranes are made of thin protein sheets.

These membranes cushion the egg, allow gas exchange, and provide an optimal environment for the developing chick. They also act as a barrier to keep out bacteria.

Step 5: Creating the Eggshell

The last major stage is the formation of the shell in the shell gland pouch. This takes approximately 20 hours, during which calcium carbonate is deposited on the membranes to build the hard shell.

The shell is made of crystalline calcium carbonate primarily sourced from the hen’s feed. A cuticle also forms on the surface of the shell to seal pores and block bacteria.

The pigments that give shells their speckled brown or blue color are added as the last layers. When complete, the shell comprises about 10% of the egg’s total weight.

Step 6: Egg Laying

In the final stretch, the egg travels down to the vagina where it rotates slightly to position itself blunt end first. Powerful muscular contractions push the egg outward, and the hen lays the egg through the cloaca.

The cloaca is a shared exit for the digestive and reproductive tracts. As the egg is laid, it is coated with a protective bacterial barrier called the bloom.

The Cyclic Nature of Egg Formation

A hen’s body is primed to start forming the next yolk almost immediately after laying an egg. Ovulation occurs 30 minutes to 24 hours after laying, meaning the cycle repeats about once a day.

Egg production is an energy intensive process. For commercial breeds, peak lay occurs around 30-40 weeks of age. As hens mature, egg quality and production gradually decline.

Factors Influencing Egg Quality

While egg formation follows a consistent biological process, many factors influence the quantity and quality of eggs produced.

Breed

Some breeds are genetically better egg producers. Breeds like Leghorns and Australorps are optimized for high egg yields versus meat production.

Feed and Nutrition

Balanced nutrition provides the building blocks for quality eggs. Key nutrients include calcium for strong shells, protein for albumen, and pigments like xanthophylls for vibrant yolk color.

Age and Molting

Younger hens lay more frequently. Production declines as hens enter their second year and molting becomes more frequent.

Day Length

Increasing day length in spring triggers an increase in ovulation and egg production. Supplemental lighting helps extend the productive laying period.

Stress

Heat, overcrowding, noise and disease cause stress than can suppress ovulation and egg production. Minimizing stressors supports sustained, healthy egg yields.

Overall Health

Good general health and freedom from diseases like infectious bronchitis directly impact egg numbers and quality.

Appreciating the Incredible Egg

The intricate sequence of events inside the hen’s body produces one of nature’s most perfectly packaged foods. Each egg is uniquely equipped to nourish and sustain life. Understanding the physiology behind this 24-hour production cycle can deepen our appreciation for the humble egg.

So the next time you crack open an egg, take a moment to marvel at the biological perfection before you!