Eggs

Parts of an Egg: Labeled Diagram, What Each Part Shows and How USDA Grades Use Them

Field guide · Updated October 26, 2026 · about 7 min read

Crack an egg from your own hens into a dish and most of what you see has a name and a job: the ropy strands beside the yolk, the firm white that stands up around it, the pale spot on top of the yolk. This page puts the parts of a chicken egg in one labeled drawing and one table, mostly following the USDA Egg-Grading Manual, and explains how the same parts tell you how fresh an egg is.

A labeled cross-section

The USDA manual describes an average egg of about 57 grams (2 ounces) built from four main parts: yolk, white, shell membranes and shell. The drawing shows them as they sit in a laid egg, large end on the left.

Shell and cuticle: the shell is about 94% calcium carbonate; the cuticle is its outermost layer and fills the pores when the egg is laid.
Shell membranes: an outer and a thinner inner membrane, together about 0.006 mm thick.
Air cell: forms between the membranes at the large end as the egg cools.
Outer thin white: about 23% of the white, just inside the membranes.
Firm (thick) white: about 57% of the white, a jacket around the yolk.
Inner thin white and chalaziferous layer: a thin white around the yolk, and the very thin but firm layer directly on the yolk.
Chalazae: twisted strands that hold the yolk centered.
Vitelline membrane: the colorless membrane that contains the yolk.
Yolk: concentric rings of yolk material, with the latebra running to the center.
Germinal disc: the small white spot on the yolk.

Drawing by HenClass following the USDA Egg-Grading Manual, Virginia Tech and Illinois Extension descriptions cited on this page. Not to scale: membranes and cuticle are drawn far thicker than they are.

Each part and what it tells you

Part	What it is	What it shows you
Cuticle	Outermost layer of the shell, chemically similar to the shell membranes; it fills the pores at laying. Often called the "bloom," a name the USDA manual calls erroneous.	Washing and rough handling damage it; see egg handling and washing.
Shell	Three layers (mammillary, spongy, cuticle) crossed by pores. Virginia Tech counts about 7,000 pores in a chicken eggshell; carbon dioxide and moisture leave and oxygen enters through them.	Shell color "does not affect the quality of the egg" and is not a grading factor (USDA). Colors by breed: egg color chart.
Shell membranes	Two tough, fibrous layers of a protein similar to that in hair and feathers (USDA). They help keep bacteria out and slow moisture loss (Virginia Tech, Illinois).	The air cell forms between them.
Air cell	A freshly laid egg normally has none; it forms at the large end as the contents cool and contract more than the shell (USDA).	It grows as water evaporates, so a small air cell means a fresher egg. In a fertile egg it supplies the chick's first air shortly before hatching (Illinois); see the incubator chart.
Thin and thick white (albumen)	Four layers: chalaziferous 3%, inner thin 17%, firm 57%, outer thin 23% of the white (USDA).	A tall, firm white is a sign of quality; the white thins as the egg loses carbon dioxide and moisture.
Chalazae	Firm, twisted strands formed as the egg spirals through the oviduct (USDA); "The chalazae hold the yolk in the center of the egg" (Virginia Tech).	A normal part of the egg, not a defect. They can look like a spot when candled; see egg problems for real blood and meat spots.
Vitelline membrane	Colorless membrane around the yolk, mostly protein and fairly permeable to water (USDA).	As eggs age it stretches and weakens, and the yolk flattens.
Yolk	About 31% of the egg's weight, laid down in concentric rings; the latebra is a tube-like structure running to its center (USDA). Food for a developing embryo (Illinois).	Color comes from feed: yellow grains and green feeds give dark yellow to orange yolks (USDA).
Germinal disc	The small white spot on the yolk, "where the female's genetic material is found" (Virginia Tech).	Its shape shows whether the egg was fertile; see below.

What an egg is made of

By weight, the USDA manual puts the white at about 58% of the egg, the yolk at about 31% and the shell at about 11%. Its composition table:

Part	Water	Protein	Fat	Ash
Whole egg	65.5%	11.8%	11.0%	11.7%
White	88.0%	11.0%	0.2%	0.8%
Yolk	48.0%	17.5%	32.5%	2.0%

The shell itself is 94% calcium carbonate, 1% magnesium carbonate, 1% calcium phosphate and 4% organic matter, which is why laying hens need a steady calcium supply (see feeding laying hens).

How the parts change as an egg ages

The USDA manual says quality declines continuously from the moment of laying. The changes are easy to see once you know the parts:

Air cell: enlarges as water evaporates. How fast depends on age, shell texture, temperature and humidity.
White: thins as it loses carbon dioxide and moisture; its pH, 7 to 8.5 when fresh, tends to rise in storage. The yolk then moves more freely.
Yolk: absorbs water from the white and grows; the vitelline membrane stretches and weakens, so the yolk flattens on top.
Cloudy white: the manual notes that the white of a freshly laid egg is usually cloudy.

Refrigeration temperatures and storage times for backyard eggs, from USDA and FDA, are on the egg handling page.

How USDA grades read the same parts

Graders look at the shell, air cell, white and yolk, and an egg's grade is "based on the factor with the lowest rating" (USDA AMS). Grading is a voluntary, fee-based service; only eggs packed under a USDA grader's supervision carry the grademark (AMS). For eggs from a backyard flock, the standards are simply a useful yardstick.

Factor	Grade AA	Grade A	Grade B
Air cell depth (candled)	⅛ inch (3.2 mm) or less	3⁄16 inch (4.8 mm) or less	Over 3⁄16 inch
White	Clear and firm	Clear and reasonably firm	Weak and watery
Haugh units	72 or more	60 to under 72	Under 60
Yolk outline when candled	Slightly defined	Fairly well defined	Plainly visible as a dark shadow
Broken out	Covers a small area; thick white; round, upstanding yolk	Slightly larger area; somewhat thinner white	Spreads over a large area; yolk may be enlarged and flattened

Haugh units are calculated from the height of the thick white and the egg's weight. AMS says Grade AA or A is preferred for baking and Grade B for hard-boiling. Size is a separate scale, by minimum net weight per dozen: Jumbo 30 oz, Extra Large 27, Large 24, Medium 21, Small 18 and Peewee 15 (AMS).

Fertile or not: reading the germinal disc

An egg from a flock with a rooster may be fertile. UGA Extension's hatchery breakout guide tells the two apart by shape, size and color:

Fertile (blastoderm): round and symmetrical, often called a "doughnut": a white ring around a clear center, sometimes with a white dot in the middle. About one-quarter to one-third larger than an infertile disc, and a less intense white.
Infertile (blastodisc): a small, intense white spot, rarely round, with jagged edges, often with bubbles around its edge; it may look granular.
Finding it: the disc can rotate beneath the yolk; turning the yolk or pouring off some white brings it to the top. UGA notes that blood spots, meat spots and yolk mottling can be mistaken for fertility.

Break-out checklist

Crack the egg onto a flat plate and look at it from the side.
Thick white standing high around the yolk and a round, upstanding yolk point to an AA- or A-quality egg; a wide, watery spread points to B.
Two ropy strands at the sides of the yolk are the chalazae, a normal part of the egg.
Check the germinal disc if you keep a rooster.
Specks of blood or tissue are covered on the egg problems chart; terms are in the glossary.