Ontogeny is the entire sequence of events involved in the development of an individual organism. It begins with the changes in the egg at the time of fertilization and includes developmental events to the time of birth or hatching and afterward—growth, remolding of body shape. Ontogeny pertains to the developmental history of an organism within its own lifetime, as distinct from phylogeny, which refers to the evolutionary history of a species. Ontogeny, embryology and developmental biology are closely related studies and the terms are sometimes used interchangeably.