An egg is a single cell, produced by the female, with the capacity to develop into a new individual.
Development may take place inside the mother’s body (as in most mammals) or outside, in which case the egg has a protective covering such as a shell. Egg yolk nourishes the growing young. Eggs developing inside the mother generally have little yolk, because the young are nourished from her body. Eggs developing outside may also have little yolk if they are produced by animals whose young go through a larval stage (such as a caterpillar) that feeds itself while developing into the adult form. The shelled eggs of birds and reptiles contain enough yolk to sustain the young until it hatches into a juvenile version of the adult.
