Egg Anatomy: What's Inside that EggShell Anyway?

Most of you probably eat eggs regularly. They are nutritious and an inexpensive, versatile protein source. You probably crack the shell and let the insides plop into your fry pan or mixing bowl without a second thought. But did you ever wonder what exactly is inside that eggshell? Or sometimes see red spots or squiggly white strands inside and wonder what they are? Well, wonder no more.

An egg is comprised of several components: the bloom, the shell, the membrane, the white, and the yolk. It's pretty amazing that a chicken manages to lay an egg roughly every 26 hours and within each egg is a self-contained beginning of new life, if the egg is fertile (more on that later), that contains every nutrient necessary for life except Vitamin C.

Commercially sold eggs are candled before heading to grocery stores, they're generally not all that fresh, and they aren't fertilized, so you might never see eggs with blood spots, chalazae or bulls' eyes until you start raising your own chickens and eating fresh backyard eggs.  But its kind of fascinating to learn the different components.  So what exactly is the function of each part of the egg? And what's really inside ?


As the last step in the laying process, a thin nearly invisible layer is applied to the eggshell called the 'bloom' (sometimes also called the 'cuticle'). This covering seals the shell to help protect the egg from air and bacteria entering through the tens of thousands of tiny pores in the eggshell and also reduces the moisture loss from within the egg. Eggs should not be washed until just before using to help preserve the bloom and to help keep the egg fresh.


Just under the eggshell is a pair of thin whitish membranes. They help to keep air out of the egg.  Once an egg is laid, an air pocket begins to form between the two membranes at the blunt end of the egg. This air sac will continue to grow as the egg ages.  Part of the difficulty in peeling fresh hard-boiled eggs stems from the lack of space between the two membranes. Older eggs peel easily because the air between the membranes has begun to separate the egg contents from the shell. [To learn the secret to peeling very fresh eggs, read HERE]


The shell is the hard outer covering of the egg and accounts for about 10% of the total egg weight depending on the size of the egg. The shell, along with the bloom, is the egg's best line of defense against contamination from bacteria and germs. The shell is mostly made of calcium carbonate, with small amounts of magnesium carbonate, calcium phosphate and protein.

Shell thickness is directly proportionate to egg size. The same amount of shell material that covers a smaller egg must be "stretched" to cover a larger one, so the larger the egg, the thinner the shell.

All egg shells start out white and then blue and/or brown pigment is applied during the laying process. The blue is applied earlier (in breeds who carry the blue gene) and does seep through to the inside of the shell, but if you notice, the inside of a brown eggshell is always white.  All eggs taste the same and contain  virtually the same nutrients regardless of shell color.  Egg taste is determined by the hen's overall diet and egg freshness, not breed.


The egg white, or albumen, accounts for more than 65% of the egg's total weight.  Most of the calories and nutrition are in the yolk, but the white of an egg contains 60% of the protein, which is about 10% of the USRDA, as well as niacin, riboflavin, magnesium and potassium. Eggs are considered a complete protein because they contain all eight essential amino acids. Protein-wise, one egg equals one ounce of other protein, such as lean meat, fish or poultry.

The white of a fresh egg will be cloudy and very thick. As the egg ages, the white will become nearly transparent and thin as air flows through the pores in the eggshell.


Each egg yolk is covered with a thin transparent membrane which keeps the yolk from breaking. This membrane becomes thinner and weaker as an egg ages, so fresh egg yolks will stand up taller and be less likely to break.

The egg yolk contains about 80% of the total calories (there are approximately 75 calories in a medium egg) and virtually all of the fat and cholesterol in the egg, along with the majority of the vitamins and minerals including choline, lutein, Vitamins A and B, calcium and folate. The color of the yolk is determined by the level of xyanthophyll in the foods a hen eats. Xyanthophyll is a carotenoid found in marigold petals, corn, alfalfa, basil and other foods. [read more about yolk color HERE]


The chalazae are ropy, twisted strands in the egg white that anchor the yolk in place in the center of the white. They are more prominent in fresh eggs and perfectly edible, although they don't break down with cooking, so removing them is recommended for smooth dishes such as puddings or custards.


Despite what you may have heard or read, red blood spots on an egg yolk is not an indication of fertility. They are merely ruptured blood vessels that have been damaged or broken during the laying process, during the travel down the oviduct, or by rough handling of the egg. As an egg ages, the yolk absorbs water from the egg white. This dilutes the blood spot, so a spot indicates that the egg is fresh.

The blood spots are edible, but for aesthetic purposes you may want to remove them before cooking the egg.  It is estimated that less than 1% of all eggs produced contain blood spots.

Similar to blood spots are 'meat' spots. They are a bit 'meatier' and more brownish and are bits of reproductive tissue that ends up in the egg. Again, perfectly edible but I tend to pick them out.


Once an egg has been fertilized, you will see a multi-ringed bull's eye on the yolk. It's very prominent and indicates that the egg (had it not been cracked into a bowl!) would likely hatch into a chick if incubated for 21 days under a hen or in an incubator. Fertilized eggs are perfectly edible and taste the same as non-fertilized eggs. The only difference is that they contain miniscule amounts of the male rooster's DNA in addition to the hen's DNA that all eggs contain.

So now that you have a better understanding about what you're seeing when you collect your eggs, I bet you'll consider each and every one even MORE of a miracle!

Because life is just better with Chickens!

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  1. Such a great breakdown! Thank you!

  2. Thank you so much for sharing this, it is so interesting. Going to explain all this to kids (and husband), and give my 'girls' an extra big hug - I had no idea they put so much work into each egg.
    I have never seen a blue egg. Which breeds lay them?

    1. Ameraucanas, Araucanas, some Easter Eggers and Cream Legbars all lay blue eggs. They are beautiful. We have some photos of ours on our fb page and here's an article about how they occur:

  3. This is probably the best step by step explanation of eggs I have read so far.