The phrase "ontogeny recapitulates phylogeny" refers to a scientific theory that an organism's fetal development follows the species' previous evolutionary forms. This theory has been disproven.
According to this theory, fetal development resembles ancestral forms and goes from one form to the next in chronological order. The youngest fetus resembles the oldest ancestor, and so on. However, this is not the case. While the fetuses of air-breathing animals retain gills until a certain stage, the gill stage does not necessarily resemble the adult form of any evolutionary ancestor. Instead, fetal development often resembles the developmental forms of its ancestors; the embryos of both chickens and humans resemble the embryos of fish and lizards.