Ontogeny is the origination and development of an organism, usually from the time of fertilization of the egg to adult. The term can also be used to refer to the study of the entirety of an organism's lifespan.
Parts of a human embryo
Process of zygote to gastrula in development
Germ layers and what tissues they become in humans
Ernst Haeckel, Anthropogenie.
The theory of recapitulation, also called the biogenetic law or embryological parallelism—often expressed using Ernst Haeckel's phrase "ontogeny recapitulates phylogeny"—is an historical hypothesis that the development of the embryo of an animal, from fertilization to gestation or hatching (ontogeny), goes through stages resembling or representing successive adult stages in the evolution of the animal's remote ancestors (phylogeny). It was formulated in the 1820s by Étienne Serres based on the work of Johann Friedrich Meckel, after whom it is also known as Meckel–Serres law.
George Romanes's 1892 copy of Ernst Haeckel's controversial embryo drawings
Drawing by Wilhelm His of chick brain compared to folded rubber tube, 1874. Ag (Anlage) = Optic lobes, matching bulges in rubber tube.