EMBRYOLOGICAL evidence: Embryological support for evolution was also proposed by Ernst Haeckel based upon the observation of certain features during the embryonic stage common to all vertebrates that are absent in adults. For example, the embryos of all vertebrates including humans develop a row of vestigial gill slits behind the head, but it is a functional organ only in fish and not found in any other adult vertebrate. According to Ernst Hackel, ontogeny (development of the embryo) is the recapitulation of phylogeny (the ancestral sequence). This view is summarized by his biogenetic law: Ontogeny recapitulates phylogeny. This proposal was disapproved of a careful study performed by Karl Ernst von Baer. He noted that embryos never pass through the adult stages of other animals.