An egg is a single cell , produced by female , with the capacity to develop into a individual . Development may take place inside the mother’sbody (as in most mammals ) or outside , in which case the egg has a protective covering such as shell . Eggs developing inside the mother generally have little yolk , because the young are nourised 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 juenile version of the adults .