Eggshells of all Textures
Eggshells vary widely in surface texture. Most eggs are smooth but some, like those of the
Cormorant, can be quite rough. As well, eggs of cormorants are chalky, rather than shiny, as in tinamous. Duck eggs are oily and waterproof, and cassowary eggs are
very heavily pitted.
Eggs Breathe
Regardless of how smooth an egg may feel, all eggshells have tiny holes or pores. The domestic hen's egg, for example, may have 7500 pores. Most of these pores are at the blunt end of the egg. Pores connect the egg with its surroundings. Respiratory gases as well as water vapour travel through these channels and enable the egg to breathe.
Why an eggshell?
We normally think of eggshells as being very fragile. However, eggshells
are remarkably strong and durable. They provide the embryo with good
protection from predators as well as from soil invertebrates and harmful
bacteria. The eggs that we eat from the domestic hen are thinner and
more fragile that those of its ancestor, the Red Junglefowl. The thinning
has occurred because chickens have been bred to produce larger eggs with
the same amount of shell material. The resulting egg would not likely
survive incubation in the wild.
Eggshell Components
Eggshells are made of calcium and magnesium salts within a fibrous network. Calcite is the main salt used as building material for the shell. As the embryo develops, calcium is transferred from the shell to the growing bones of the embryo. After a chick hatches, the mother sometimes eats the shell to recover some of the calcium lost in the making of an egg.
The Cuticle
The outer surface of the eggshell is covered by the cuticle. The cuticle is a thin layer made of proteins. The cuticle adds strength to the shell, gives the shell its texture and provides a barrier against bacteria.
