Egg Anatomy: A Masterclass in Biological Engineering
The egg is to cuisine what the article is to speech.
The Science of Eggs
From shell to yolk, the egg is a marvel of natural engineering. Explore the anatomy, chemistry, and biology of one of nature's most perfect packages.
Egg Anatomy
Click or hover over any label to learn more about each part of the egg.
Shell
The outermost layer, composed primarily of calcium carbonate (CaCO3). A single eggshell contains roughly 7,000-17,000 tiny pores that allow gas exchange — oxygen in, carbon dioxide and moisture out. Despite feeling solid, the shell is a semi-permeable, breathable structure. Shell color (white, brown, blue, green) is determined by the hen's breed and has no effect on nutrition or flavor.
Shell Membranes
Two thin, translucent protein layers line the inside of the shell. The outer membrane adheres to the shell; the inner membrane surrounds the albumen. Together they act as a bacterial defense barrier. When you peel a hard-boiled egg and struggle with that clingy layer — that's the inner membrane doing its job.
Albumen (Egg White)
The clear liquid surrounding the yolk, comprising about 60% of the egg's total weight. It consists of two layers: a thick, gel-like inner albumen that cushions the yolk, and a thinner outer albumen. Composed of roughly 90% water and 10% protein (primarily ovalbumin), it contains no fat and is one of nature's most complete protein sources. When cooked, the proteins denature and coagulate, turning opaque white.
Yolk
The golden center — the egg's nutrient powerhouse. Contains all of the egg's fat, nearly half its protein, and most vitamins (A, D, E, K, B12). The yolk's color ranges from pale yellow to deep orange depending on the hen's diet — more carotenoids (from plants, insects, marigold) produce a richer color. In a fertilized egg, the yolk provides nutrition for the developing embryo.
Chalaza
Two twisted, rope-like strands of protein that anchor the yolk in the center of the albumen, like biological shock absorbers. More prominent chalazae indicate a fresher egg. They're entirely edible and dissolve during cooking. In development, they keep the yolk centered so the embryo has equal access to nutrients from all sides.
Air Cell
A small pocket of air between the inner and outer membranes, usually at the egg's wider end. It forms as the egg cools after being laid — the contents contract, pulling the membranes apart. The air cell grows larger as the egg ages and loses moisture through the shell pores. This is why older eggs float in water — the enlarged air cell provides buoyancy. The float test is a reliable freshness indicator.
Select a part of the egg to explore its anatomy.