In the embryonic development of vertebrates, pharyngeal pouches form on the endodermal side between the pharyngeal arches. The pharyngeal grooves (or clefts) form the lateral ectodermal surface of the neck region to separate the arches.

The pouches line up with the clefts, and these thin segments become gills in fish.

Copy from https://en.wikipedia.org/wiki/Pharyngeal_pouch_(embryology)