Ontogeny (also ontogenesis or morphogenesis) describes the origin and the development of an organism from the fertilized egg to its mature form. Ontogeny is studied in developmental biology.
The idea that ontogeny recapitulates phylogeny, that is, that the development of an organism exactly mirrors the evolutionary development of the species, is discredited today. However the phenomenon of recapitulation, in which a developing organism will for a time show a similar trait or attribute to that of an ancestral species, only to have it disappear at a later stage is well documented. For example, embryos of the baleen whale still develop teeth at certain embryonic stages, only to later disappear. A more general example is the emergence of pharyngeal gill pouches of lower vertebrates in almost all mammalian embryos at early stages of development.