Wikipedia
The theory of recapitulation, also called the biogenetic law or embryological parallelism—often expressed in Ernst Haeckel's phrase "ontogeny recapitulates phylogeny"—is a largely discredited biological hypothesis that in developing from embryo to adult, animals go through stages resembling or representing successive stages in the evolution of their remote ancestors. Since embryos also evolve in different ways, within the field of developmental biology the theory of recapitulation is seen as a historical side-note rather than as dogma.
With different formulations, including the early Meckel-Serres Law, such ideas have been applied and extended to several fields and areas, including the origin of language, religion, biology, cognition and mental activities, anthropology, education theory and developmental psychology. Recapitulation theory is still considered plausible and is applied by some researchers in fields such as the study of the origin of language, cognitive development, and behavioral development in animal species.