For example, Haeckel proposed that the pharyngeal grooves between the pharyngeal arches in the neck of the human embryo not only roughly resembled gill slits of fish, but directly represented an adult "fishlike" developmental stage, signifying a fishlike ancestor. Embryonic pharyngeal slits, which form in many animals when the thin branchial plates separating pharyngeal pouches and pharyngeal grooves perforate, open the pharynx to the outside. Pharyngeal arches appear in all tetrapod embryos: in mammals, the first pharyngeal arch develops into the lower jaw (Meckel's cartilage), the malleus and the stapes.