Embryological support for evolution was also proposed by Ernst Heckel based upon the observation of certain features during 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 slit just behind the head but it is a functional organ only in fish and not found in any other adult vertebrates. However, this proposal was disapproved on a careful study performed by Karl Ernst von Baer. He noted that embryos never pass through the adult stages of other animals.