development
development The complex process of growth and maturation that occurs in living organisms. Cell division and differentiation are important processes in development. In vertebrate animals there are three developmental stages: (1) cleavage, in which the zygote divides to form a ball of cells, the blastula; (2) gastrulation, in which the cells become arranged in three primary germ layers (see gastrula); (3) organogenesis (or organogeny), in which further cell division and differentiation results in the formation of organs. The development of many invertebrates (e.g. insects) and amphibians involves the process of metamorphosis. In all organisms, development is directed by the coordinated expression of various genes in time and space, regulated by the complex interplay of the gene products (see morphogen). Early development may be influenced by maternal genes (see maternal effect genes) as well as embryonic genes. Genes involved in development are often highly conserved across very diverse organisms, as exemplified by the Hox genes that determine the structures along the head-to-tail axis of most animals. See also homeotic genes; morphogenesis; primary growth.
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development