Eggs
A self-contained life-support system.
An egg is a container holding an embryo and everything it needs to develop. The amniotic egg, with its protective membranes, was the innovation that let vertebrates reproduce away from water and colonise dry land — it is what separates reptiles, birds and mammals from amphibians.
Parts and functions
| Part | Function |
|---|---|
| Shell | Protection; porous, so gases exchange through it |
| Shell membranes | Two layers guarding against bacteria |
| Air cell | Forms as contents cool and contract; the first air a hatching chick breathes |
| Albumen | Water and protein; cushions and supplies moisture |
| Chalazae | Twisted cords holding the yolk centred |
| Yolk | Concentrated food store |
| Germinal disc | Where the embryo develops |
Why eggs are that shape
The asymmetric oval is stronger than a sphere against pressure applied at the ends, which is how a brooding parent's weight arrives. It also rolls in a tight circle rather than a straight line — a real advantage for cliff-nesting birds such as guillemots, whose eggs are markedly pointed. Species nesting in enclosed burrows tend to lay much rounder eggs.
Shell chemistry
A bird eggshell is about 95% calcium carbonate, the same compound as limestone and chalk. Thousands of microscopic pores allow oxygen in and carbon dioxide and water vapour out — the shell is a barrier to bacteria but deliberately not to gas.