What is an example of a vestigial structure observed in vertebrate embryos according to Haeckel?

Step-by-step solution

Haeckel observed that vertebrate embryos develop a row of vestigial gill slits, which are functional in fish but not in other adult vertebrates. Embryological support for evolution was also proposed by Ernst Heckel based upon the observation of certain features during the embryonic stage common to all vertebrates that are absent in adulthood. For example, the embryos of all vertebrates including human develop a row of vestigial gill slit just behind the head but it is a functional organ only in fish and not found i