n., pl. -nies.

The origin and development of an individual organism from embryo to adult. Also called ontogenesis.

ontogenetic on'to·ge·net'ic (ŏn'tə-jə-nĕt'ĭk) or on'to·gen'ic (-jĕn'ĭk) adj.
ontogenetically on'to·ge·net'i·cal·ly or on'to·gen'i·cal·ly adv.

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 form a diploid zygote (with a full chromosome number), a new unicellular living being which will grow through a series of asexual reproductions. 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 conceived as adults, but in most cases the majority of their representation in the environment is as intermediate ontogenetic stages. See also Reproduction (animal).

In unicellular organisms, each asexual reproduction leads to the formation of new individuals, the cells deriving from a first sexually derived individual forming a clone of genetically identical individuals. In multicellular organisms, the products of the asexual reproductions starting with the first division of the zygote remain connected, and the clone they form is a single individual. Clonation of individuals occurs even in humans, when the first results of asexual reproduction of the zygote separate from each other, leading to twin formation.

The ontogeny of a multicellular organism involves segmentation (or cleavage): the zygote divides into two, four, etc., cells which continue to divide. These cells are initially similar to the zygote, although smaller in size. They soon start to differentiate from their ancestors, acquiring special features, and forming specific tissue layers and, eventually, organs. These processes lead to the formation and growth of an embryo. Embryos can develop freely, within egg shells, or within the body of one parent; they can grow directly into juveniles (as in humans) or into larvae (with an indirect development, as in insects).

Juveniles are similar to adults but are smaller in size and not sexually mature. Their ontogeny continues until they reach a maximal size and reproductive ability. Larvae have different morphology, physiology, and ecology from adults; they become juveniles through a metamorphosis (that is, an abrupt change). Usually ontogeny is interrupted at adulthood, but some organisms can grow throughout their life, so that ontogeny ends with their death. See also Ecology.

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(ontoj′ənē)
n

The natural life cycle of an individual as contrasted with the natural life cycle of the race (phylogeny).

The general development of a race or other group of people.

The developmental history of an individual.

the development or course of development of an individual organism (compare phylogeny)

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