A chicken egg is a self-contained reproductive cell produced by a hen, designed to nourish a developing chick but most commonly consumed as food. It consists of a protective calcium carbonate shell, a protein-rich white, and a nutrient-dense yolk, all assembled layer by layer inside the hen over roughly 24 to 26 hours. Whether fertilized or not, the egg that lands on your plate is the same biological structure, one of the most efficient packages of nutrition in the human diet.
How an Egg Forms Inside the Hen
A hen’s reproductive tract builds an egg in stages, each part of the tract adding a specific layer. It starts at the ovary, where a cluster of yolk-filled follicles develops at different sizes. When the largest follicle matures, it releases into the first section of the oviduct, a funnel-shaped opening where it spends about 15 minutes. If sperm is present, fertilization happens here.
The yolk then moves into a longer section where the egg white is deposited around it over about three hours. Next, two tough, transparent membranes wrap around the white in just over an hour. These membranes are made partly of keratin, the same protein found in human hair, and they serve as a barrier against bacteria.
The egg spends the bulk of its formation time, roughly 20 to 21 hours, in the shell gland. Here, water and minerals pass through the membranes to inflate the egg into its familiar oval shape. Calcium is then deposited as crystals that harden into the shell. A final protective coating called the cuticle seals the outside before the egg rotates so the large end emerges first and is laid through the cloaca. The entire process takes about a day, which is why most hens lay one egg per day at peak production.
Anatomy of an Egg
When you crack an egg open, you’re looking at components that each serve a distinct biological purpose.
The shell accounts for 9 to 12 percent of the egg’s total weight and is made almost entirely of calcium carbonate. It contains roughly 17,000 tiny pores that allow air and moisture to pass through, which is essential for a developing embryo but also the reason eggs eventually go stale. The cuticle, a thin outermost coating, blocks many of those pores to slow moisture loss and keep bacteria out.
Beneath the shell sit two transparent shell membranes that act as a second line of defense against contamination. As the egg ages, air seeps between these membranes at the wide end, forming the air cell that grows larger over time. This is actually one way producers test freshness: a larger air cell means an older egg.
The egg white (albumen) makes up about 67 percent of the egg’s liquid weight and contains more than half its total protein. It’s arranged in four alternating layers of thick and thin albumen holding roughly 40 different proteins. Some of these proteins serve as natural antimicrobials. Lysozyme, which makes up about 3.5 percent of egg white protein, breaks down bacterial cell walls and can even form complexes with viral DNA. This chemical defense system helps keep the yolk sterile.
The chalazae are the opaque, rope-like strands you sometimes notice when you crack an egg. They anchor the yolk to the shell membranes, keeping it centered. More prominent chalazae actually indicate a fresher egg.
The yolk sits at the center, enclosed by a thin, clear membrane that grows more fragile with age. When calculated on interior contents alone, the yolk makes up about 35 percent of the egg while the white accounts for 65 percent. Despite being smaller, the yolk is far more calorie-dense: roughly 32 percent fat, 16 percent protein, and 50 percent solids overall. It contains most of the egg’s vitamins and minerals, including iron, vitamins A and D, phosphorus, calcium, and B vitamins.
What Determines Shell Color
Shell color is genetic, not nutritional. Brown eggs, white eggs, and blue eggs all have essentially the same interior. The difference comes down to pigments deposited in the shell gland during the final hours of formation.
Brown shells get their color from a pigment called protoporphyrin IX, which is laid on top of the shell late in the process. You can confirm this by looking at the inside of a brown egg: it’s white, because the pigment only coats the exterior. Blue shells work differently. A pigment called biliverdin is deposited throughout the shell itself, so a blue egg is blue all the way through. Green and olive eggs result from hens that produce both pigments, layering brown on top of a blue base. The breed of chicken determines which pigments the shell gland produces and in what quantity.
Nutritional Profile of a Large Egg
A large egg weighs about 50 grams and delivers roughly 6 to 7 grams of protein split between the white and yolk. The white is nearly fat-free and provides more than half the total protein. The yolk carries all the fat (about 5 grams), most of the vitamins, and is one of the few natural food sources of vitamin D, with USDA sampling showing about 82 IU per 100 grams. Cholesterol clocks in at about 186 milligrams per large egg (based on a computed mean of 372 mg per 100 grams).
Yolk color ranges from pale yellow to deep orange depending on the hen’s diet and breed. Hens that eat more pigment-rich feeds like marigold, corn, or leafy greens produce darker yolks, but color alone doesn’t reliably indicate higher nutrient content.
Size and Grade Classifications
In the United States, eggs are sorted by weight per dozen, not by the dimensions of individual eggs. The USDA recognizes six weight classes:
- Peewee: 15 ounces per dozen
- Small: 18 ounces per dozen
- Medium: 21 ounces per dozen
- Large: 24 ounces per dozen
- Extra large: 27 ounces per dozen
- Jumbo: 30 ounces per dozen
Most recipes assume large eggs. If you’re substituting extra large or medium, the difference of about 3 ounces per dozen (roughly a quarter ounce per egg) can matter in baking but is negligible for scrambled eggs or omelets.
Grades reflect interior quality, not size. Grade AA eggs have the firmest whites and tallest, most rounded yolks, with the smallest air cells. Grade A is slightly less firm but still good for frying or poaching. Grade B eggs have thinner whites and flatter yolks, making them better suited for scrambling or commercial products. None of these grades indicate a difference in safety or nutrition.
Storing Eggs Safely
In the United States, commercial eggs are washed and sanitized before sale, which removes the natural cuticle that seals the shell’s pores. Without that coating, refrigeration becomes essential. The FDA recommends storing eggs at 40°F or below and using them within three weeks for best quality. Keep them in their original carton rather than the door shelf of your fridge, since the carton insulates against temperature swings and prevents the eggs from absorbing odors through their porous shells.
In many other countries, eggs are sold unwashed with the cuticle intact, which is why you’ll see them stored at room temperature in European grocery stores. The biology is the same; the handling protocol is different.