By contrast to heterochrony, heterotopy is a spatial amendment in embryonic development. Like heterochrony, heterotopy changes bodily shapes and sizes.
German zoologist Ernst Haeckel introduced the ideas of heterochrony and heterotopy as facets of morphological innovation. Haeckel cited gonads – created by an evolutionary adjustment in the positioning of the germ layer – as an example of heterotopy. Haeckel’s encompassing idea behind heterochrony and heterotopy was his disproven hypothesis “ontogeny recapitulates phylogeny”: that embryonic development was a recapitulation of evolutionary stages from an animal’s remote ancestors (phylogeny) during gestation (ontogeny).
Allometry involves adaptive scaling in organisms and its commensurate effects. Dinosaurs were an upscaling from reptilian ancestors, whereas birds were a downsizing from dinosaurs.