Anatomy of an Egg: Every Part from Shell to Chalaza, Explained

<p>An egg is a self-contained system for developing an embryo, and every structure inside it has a job. This page labels each part of an egg from the shell inward, explains what it does, and gives the chemical composition by weight. It sits under the site's <a href="https://theultimateegg.com/science">science section</a>.</p><figure class="egg-diagram"><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 700 440" role="img" aria-labelledby="egg-anatomy-title egg-anatomy-desc" width="100%"><title id="egg-anatomy-title">Labeled anatomy of a chicken egg</title><desc id="egg-anatomy-desc">A cross-section showing the shell, outer and inner shell membranes, air cell, thick and thin albumen, chalazae, vitelline membrane, yolk, and germinal disc.</desc><path d="M70,220 C70,130 150,80 300,80 C430,80 520,150 520,220 C520,290 430,360 300,360 C150,360 70,310 70,220 Z" fill="#efe7d6" stroke="#111111" stroke-width="3"/><path d="M88,220 C88,145 158,98 300,98 C420,98 502,158 502,220 C502,282 420,342 300,342 C158,342 88,295 88,220 Z" fill="#f8f3e6" stroke="#111111" stroke-width="1"/><ellipse cx="300" cy="220" rx="198" ry="112" fill="#fbf6da"/><ellipse cx="330" cy="220" rx="132" ry="94" fill="#f2ecbf"/><path d="M96,168 C128,182 128,258 96,272 C86,238 86,202 96,168 Z" fill="#ffffff" stroke="#111111" stroke-width="1"/><path d="M300,220 q-14,-12 -26,0 q-14,12 -26,0 q-14,-12 -26,0 q-14,12 -26,0" fill="none" stroke="#111111" stroke-width="3"/><path d="M392,220 q14,-12 26,0 q14,12 26,0 q14,-12 26,0" fill="none" stroke="#111111" stroke-width="3"/><circle cx="345" cy="220" r="66" fill="none" stroke="#111111" stroke-width="1"/><circle cx="345" cy="220" r="62" fill="#f4b400" stroke="#c98a00" stroke-width="1"/><circle cx="398" cy="196" r="8" fill="#fff6da" stroke="#111111" stroke-width="1"/><line x1="190" y1="112" x2="150" y2="48" stroke="#111111" stroke-width="1"/><text x="145" y="44" text-anchor="end" font-family="Arial, sans-serif" font-size="15" fill="#111111">Shell</text><line x1="335" y1="106" x2="362" y2="46" stroke="#111111" stroke-width="1"/><text x="366" y="42" text-anchor="start" font-family="Arial, sans-serif" font-size="15" fill="#111111">Shell membranes (outer &amp; inner)</text><line x1="98" y1="215" x2="44" y2="192" stroke="#111111" stroke-width="1"/><text x="38" y="188" text-anchor="end" font-family="Arial, sans-serif" font-size="15" fill="#111111">Air cell</text><line x1="180" y1="300" x2="128" y2="388" stroke="#111111" stroke-width="1"/><text x="122" y="404" text-anchor="end" font-family="Arial, sans-serif" font-size="15" fill="#111111">Thin albumen</text><line x1="320" y1="308" x2="320" y2="392" stroke="#111111" stroke-width="1"/><text x="320" y="410" text-anchor="middle" font-family="Arial, sans-serif" font-size="15" fill="#111111">Thick albumen</text><line x1="430" y1="245" x2="500" y2="392" stroke="#111111" stroke-width="1"/><text x="505" y="408" text-anchor="start" font-family="Arial, sans-serif" font-size="15" fill="#111111">Chalazae</text><line x1="410" y1="220" x2="560" y2="255" stroke="#111111" stroke-width="1"/><text x="565" y="259" text-anchor="start" font-family="Arial, sans-serif" font-size="15" fill="#111111">Vitelline membrane</text><line x1="395" y1="205" x2="600" y2="150" stroke="#111111" stroke-width="1"/><text x="605" y="150" text-anchor="start" font-family="Arial, sans-serif" font-size="15" fill="#111111">Yolk</text><line x1="404" y1="190" x2="500" y2="70" stroke="#111111" stroke-width="1"/><text x="505" y="66" text-anchor="start" font-family="Arial, sans-serif" font-size="15" fill="#111111">Germinal disc</text></svg><figcaption>Cross-section of a chicken egg with each anatomical part labeled.</figcaption></figure><div class="ad-unit" data-ad-format="in-article"><!-- AdSense in-article unit --></div><h2>The parts of an egg</h2><h3>Shell</h3><p>The shell is the rigid outer layer, about 95% calcium carbonate laid over a protein matrix. It carries thousands of microscopic pores that let oxygen in and let carbon dioxide and moisture out, so the shell is porous rather than sealed. A thin outer coating called the cuticle, or bloom, covers the pores and slows bacterial entry.</p><h3>Outer and inner shell membranes</h3><p>Two thin protein membranes sit between the shell and the white and act as a second barrier against bacteria. At the wide end of the egg the two membranes separate, and the space between them becomes the air cell.</p><h3>Air cell</h3><p>The air cell is the pocket of air trapped where the two membranes pull apart at the blunt end. A freshly laid egg has almost none; the cell forms as the egg cools after laying and grows over time as moisture and gas escape through the shell. Because of that, the size of the air cell is a rough gauge of an egg's age, which is why a very old egg floats.</p><h3>Albumen (egg white), thick and thin</h3><p>The albumen is the egg white, arranged in alternating layers of thin and thick fluid: a thin layer next to the membranes, a surrounding thick layer, an inner thin layer, and a dense inner layer that anchors the yolk. It is roughly 90% water and 10% protein, and the dominant protein is ovalbumin, which is what lets a white whip into a stable foam. See <a href="https://theultimateegg.com/fun-facts/how-egg-whites-foam-the-science-of-whipping-ovalbumin">how egg whites foam</a> for the mechanism, and <a href="https://theultimateegg.com/fun-facts/avidin-biotin-and-raw-egg-whites-why-cooking-solves-the-problem">avidin, biotin, and raw egg whites</a> for a look at the white's other proteins.</p><h3>Chalazae</h3><p>The chalazae are the two twisted, rope-like strands of dense albumen that run from opposite sides of the yolk and anchor it near the center of the egg. A single strand is a chalaza; the pair are chalazae. They are not an imperfection and do not need to be removed for cooking. Prominent, well-defined chalazae are a sign of a fresh egg. For the structural detail, see <a href="https://theultimateegg.com/fun-facts/what-are-chalazae-the-structural-engineering-inside-an-egg">what the chalazae actually do</a>.</p><h3>Vitelline membrane</h3><p>The vitelline membrane is the clear, thin envelope that holds the yolk together and keeps it separate from the white. It is strong in a fresh egg and weakens with age, which is why the yolk of an old egg breaks more easily when cracked.</p><h3>Yolk</h3><p>The yolk is the egg's nutrient store, the food supply for a developing chick. It concentrates the egg's fat and fat-soluble vitamins along with a share of its protein. Its color, from pale yellow to deep orange, comes from carotenoid pigments in the hen's feed rather than from any difference in nutrition.</p><h3>Germinal disc (blastodisc)</h3><p>The germinal disc is the small, pale spot on the surface of the yolk. It holds the hen's genetic material and is the point where fertilization occurs. In a fertilized egg it develops into the embryo; in the unfertilized eggs sold for food it stays a faint dot.</p><h2>Chemical composition of an egg</h2><p>By weight, a whole chicken egg (the edible shell-free portion) breaks down roughly as follows. The white and yolk differ sharply: the white is almost entirely water and protein, while the yolk holds nearly all of the fat.</p><table><thead><tr><th>Component</th><th>Approx. % by weight (whole egg)</th></tr></thead><tbody><tr><td>Water</td><td>~75%</td></tr><tr><td>Protein</td><td>~12.5%</td></tr><tr><td>Fat (lipids)</td><td>~10%</td></tr><tr><td>Minerals (ash)</td><td>~1%</td></tr><tr><td>Carbohydrate</td><td>~1%</td></tr></tbody></table><p>The shell adds separately about 9 to 12% of the egg's total weight and is roughly 95% calcium carbonate. These figures are averages; exact values shift with hen breed, age, and diet.</p><p>Together, these components of an egg (shell, membranes, air cell, albumen, chalazae, vitelline membrane, yolk, and germinal disc) make up its full anatomy.</p>