How Does an Egg Develop Inside a Chicken?

The eggs you collect from your chickens develop through a process that is nothing short of miraculous. A hen’s reproductive system consists of two main parts: an ovary and an oviduct. A young female chicken, or pullet, starts life with two ovaries. As she matures, the right ovary remains undeveloped and only the left one becomes fully functional. The functioning ovary contains all the undeveloped yolks the pullet was born with, which represent the maximum number of eggs she could lay within her lifetime.

When a pullet reaches laying age, one by one the tiny yolks mature, so at any given time during her productive life, the bird’s body contains yolks at various stages of development. Each yolk receives nutrients by means of a network of blood vessels that surround the yolk. Within about two weeks, a tiny yolk grows to a diameter of about one inch. The network of vessels then ruptures to release the yolk from the ovary. Sometimes the rupture isn’t perfect and a harmless spot of blood is left on the yolk.

Approximately every 25 hours, one yolk is mature enough to be released into the funnel of the oviduct, a process called ovulation. Ovulation typically occurs within one hour after the hen laid her previous egg.

The two-foot long oviduct consists of five compartments, each with a different function. If fertilization takes place, it will occur as the egg passes through the oviduct’s funnel. The yolk then spends the next three hours spiraling through the oviduct’s magnum, where it becomes surrounded in egg white. Next in spends about one hour in the isthmus, where it is covered in two thin, protective membranes.

Now comes the longest, and most fascinating, part of the egg’s journey. The membrane-enclosed egg enters the shell gland, where it spends the next twenty hours. First it is plumped up with fluid until it achieves the approximate shape you would recognize as being an egg. Then it is sealed within a hard shell consisting of calcium carbonate crystals. Here something occasionally goes wrong and the hen lays a soft shelled or shell-less egg. But, under normal conditions, the properly hardened egg shell is next enveloped in a fast-drying protein solution called the bloom, or cuticle, that seals tiny pores occurring between the calcium crystals making up the shell.

At this point the egg is oriented to lead with its pointed end. Just before it is laid, the egg enters the hen’s vagina, where it rotates so the blunt end comes out first. An egg that plops into the nest on the blunt end is much less likely to crack than if it were to fall pointy end first.

When you find an egg right after it was laid, it feels warm. That’s because a hen’s body temperature is about 106⁰F. When the warm egg enters the world and cools to ambient temperature, an air space begins to develop between the two shell membranes. If you put a fresh egg into a bowl of water, it will sink. The older the egg is, the larger its air space becomes, until eventually the egg will float — a neat trick for determining whether or not an egg is fresh.

The entire egg development process takes about 25 hours, which causes a hen to lay her egg about an hour later each day. Since a hen’s reproductive system slows down during the night, eventually she’ll skip a day and start a new multiple-day laying cycle the following morning, which explains why a hen doesn’t lay an egg every day. Considering what an amazing feat it is to lay an egg, don’t you agree the hen is entitled to take an occasional day off?

And that’s today’s news from the Cackle coop.

Gail Damerow, author, Storey’s Guide to Raising Chickens
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