Gill slits are found in adult fish, such as sharks. They are openings to the gill arches. In early embryos, there is no gill arches and no gill slits. There are instead pharyngeal arches. Between these arches there are grooves, which are called pharyngeal pouches. The pouches or grooves will later develop into gills in fishes. These same pouches are also found in embryos of other vertebrates, such as amphibians, reptiles, birds and mammals. These pouches do not become gills in adult amphibians, reptiles birds or mammals. Instead these pouches develop into parts of the neck and jaws. Therefore, just because early embryos have pharyngeal pouches does not mean that these embryos will become fish.