BMJ Engineering | Anatomy of an egg
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Anatomy of an egg

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Anatomy of an egg

The internal quality of an egg is determined by the appearance and consistency of the contents inside the shell, which you can examine by breaking a fresh egg into a dish or by cutting a hard cooked egg in half. Look closely and you will discover that an egg consists of more than just simple white and yolk.


For starters, the egg white, or albumen — which accounts for most of the egg’s liquid weight — consists of four layers:

  • The outer thin is a narrow layer of watery white lying next to the shell membrane. It repels bacteria by virtue of its alkalinity and its lack of nutrients that would otherwise feed bacteria.
  • The outer thick, or firm, white, is a gel that makes up the greater portion of the albumen.
  • The inner thin is a watery layer separating the inner thick from the outer thick layer.
  • The inner thick, or chalaziferous, layer surrounds the yolk. It cushions the yolk and contains defenses against bacteria.

But what are those small white blobs that appear next to the yolk, when a fresh egg is cracked into a dish?

“As an egg is formed within a hen, normal rotation causes the ends of chalaziferous layer to twist together to form a fibrous cord, or chalaza, on two opposite sides of the yolk,” explains poultry expert. “These cords anchor the chalaziferous layer and protect the yolk by centering it within the shell. Some people mistake these small, but rather visible, whitish cords for a developing baby chick.

“A baby chick actually begins formation on a tiny round, white spot on top of the yolk, called the germinal disc. One of the functions of the chalaziferous layer is to keep this germinal disc oriented upward, to preserve its ability to develop into a chick. Of course, for a chick to develop, the egg must be fertile, and for that you need a rooster.”

Is there a way to tell whether or not an egg is fertile?

“Yes, you can tell by cracking the egg and examining the yolk,” Ms. Damerow explains. “If the egg is not fertile, the germinal disc has an irregular shape. When an egg has been fertilized, the spot organizes into a set of tiny rings, one inside the other. If you look closely, you can see this tiny bullseye on top of the yolk.

“An important note is that just because an egg has been fertilized doesn’t mean it will immediately begin developing into a chick. A fertilized egg remains dormant unless it undergoes incubation in a mechanical incubator or under a hen.”

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