1. From the Beginning
When a hen is younger, a pullet, they have two ovaries. As she matures, the right ovary will begin to atrophy so that only the left ovary remains. In this single, functioning ovary are about 4000 ova, or undeveloped yolks.
2. Ready to Lay
When a pullet reaches an age at which they are ready to lay, the ova will mature one by one and at any given time the hen's body has ova at all different stages of development. Every 25 hours or so, one ovum will receive enough layers of yolk and be released into the oviduct in a process called ovulation.
3. Egg Creation
If an egg has already been laid, ovulation will begin about an hour later. The yolk will travel through the oviduct. If your hen has mated with a rooster, it will be fertilized. It will then continue to be wrapped in layers of albumin, or egg white, then wrapped in some thin membranes and finally encased in a shell. This whole process, leading up to the laying, takes about 25 hours.
4. The Laying process
When your hen has an egg they are ready to lay, they will begin another process, one that you can observe if you are quiet and still.
They will enter their nest, settle in for at least an hour and a half, sometimes more. They may appear to be resting, or even as though they are falling asleep. Eventually they will become excited, they may raise their tail and spread their bottom feathers. As these movements increase in frequency they will become closer to laying. Compare it to human contractions; a woman will feel them with greater frequency until it is time to push the baby out.
If you were to lift up your hen, and look under her tail and between the feathers you would see a small opening. It looks like a horizontal slit about an inch wide, surrounded by a ribbed rim. This is called the vent.
When your hen suddenly stands up with her feet wide apart, the vent opens a little and a red membrane begins to appear. Your hen will begin to lower her bottom and the vent will rapidly widen, stretching the rim. A pinkish dome will form, beneath this is the egg. The vent will open wide, comparable to female dilation of the cervix, and the rim is narrow and stretched. A pink layer of tissue with blood vessels will bulge through and out of this will slip the egg. It will appear as a light colored disk.
You hen will push and with each push the egg will come out a bit more. The pink membrane will form a collar around the widest point of the egg as it slips through. The egg will pop out and for a moment the membrane will hang out before being sucked inside again. The vent will close and the egg creation process will begin again.
Your hen will inspect her egg and then leave the nest, cackling proudly.
5. What Happens Inside
Inside your hen, the egg will be ready to lay. The area of the hen just inside the vent is called the cloaca and is a coming holding place for poo and eggs. When it is time to lay, the hen will naturally turn a portion of the oviduct and the cloaca inside out. This prevents the egg from becoming contaminated during laying.
Top 5 Egg Layers
Types of Chicken Eggs for Purchase
The Chicken or The Egg?
5 Fun Eggshell Crafts
How the Chicken Lays the Egg
Egg Laying Problems
What Is Brooding and Why Is My Hen Doing It?
Egg Candling: What It Is and How To Do It Correctly
Keeping Your Hens Laying Through the Fear
Setting Up the Proper Nest Box for Your Hens