How did the Egg get into the Shell?


You know the question “Which came first, the chicken or the egg?” Well, have you ever wondered which came first…the egg or the shell?

It turns out that the egg, or at least the inside of the egg, the yolk and the white come first, and the shell just sort of forms around the soft interior before it leaves the hen’s body.  Pretty amazing!

Nature figured out a way to surround each tender little egg with a strong, seamless, perfectly shaped, hard coat while it is inside the hen, so it could survive outside of her body – with or without a little chick inside. How does this happen? As with so many things in nature, we don’t have a full explanation. We know that it happens, and where it happens, but not too much about why it happens…except to help little chicks survive.

The eggshell is amazing in that air and moisture can pass through it, but it doesn’t get wet or soggy. It has a little coating on the very outside that keeps out bacteria and dust. The shell is actually made from little tiny columns of a calcium-like material called CALCITE that comes from the hen’s diet, or from her own body, which can actually pull the calcium from her bones in order to provide for the shell. There are two membranes around the yolk and white of the egg. The outermost membrane has little spots that sprout these calcite columns. The columns grow and interlock to form the hard shell. If you have seen pictures of crystals growing, it seems to be a little like that. The egg essentially grows its own shell.

When the hen herself is born, she has all the eggs (or ova) she will have for her lifetime. That could be tens of thousands of potential eggs! Obviously, she will never use them all. When she is mature enough, her body begins to release one at a time. This is called ovulation. The chicken’s reproductive system is light sensitive and responds to the number of hours of light per day.  Ovulation usually occurs during the day and rarely after 3pm.  Once the egg has been released, it takes about 25 hours to transform the ovum (singular form of ova) to a fully developed egg and lay that egg. Many hens lay 5-7 eggs per week.

The egg is released from the ovary into a long reproductive tract. It stays in the first section for about 15 minutes. If the egg is to be fertilized, it will happen here. Whether or not it is fertilized, the egg will continue to mature and move down the reproductive tract. The next section is about 13 inches long and the egg stays there for three hours while the white part (the albumen) of the egg forms. The next section is only four inches long. This is where the inner and outer shell membranes form. This takes about 75 minutes. The inner membrane is that fine white membrane we see when we crack open an egg. It holds the contents of the egg together, which is amazing, since it’s so thin! The outer protective shell is separate.

Then comes a section called the SHELL GLAND, which is four to five inches long. The hard part of the shell is manufactured here. This is when and where the most calcium is required and the hen’s body can automatically harvest calcium from her bones if needed to create that shell.  The egg stays here for at least 20 hours. The color of the egg is determined during this stage. In the last section of the reproductive tract, strong muscle tissue pushes the egg out into the world.

How Does a Chicken’s Egg Get its Shell?

Watch the video below to get a visual on how this process happens:

Which came first — the chicken or the egg? Watch as BrainStuff gives the technical answer — and takes a look at the science behind eggshell formation.