The developmental history of an organism from its origin to maturity. It starts with fertilization and ends with the attainment of an adult state, usually expressed in terms of both maximal body size and sexual maturity. Fertilization is the joining of haploid gametes (a spermatozoon and an ovum, each bearing half the number of chromosomes typical for the species) to forma diploid zygote (with a full chromosome number), a new unicellular living being. The gametes are the link between one generation and the next: the fusion of male and female gametes is the onset of a new ontogenetic cycle. Many organisms die shortly after sexual reproduction, whereas others live longer and generations are overlapped. Species are usually perceived as consisting mostly of Q1 adults, but in most cases the majority of their representation in the environment is as intermediate ontogenetic stages. See FERTILIZATION; REPRODUCTION (ANIMAL).