Tadpoles are the larval forms of frog and toads. They respire through gills whereas the adult form respire through the skin and lungs. Since, fishes also respires through gills. Hence, presence of gills in the tadpole indicates that frogs are evolved from gilled ancestors.
Related Theory
"Ontogeny recapitulates phylogeny" mean that the development of individual organisms (ontogeny) follows (recapitulates) the same phases of the evolution of larger ancestral groups of related organisms (phylogeny).