EMBRYOLOGICAL EVIDENCES -
(i) Gill clefts and notochord appear on the embryonic development of all vertebrates from fishes to mammals. (ii) The notochord is replaced by the vertebral column in all adult vertebrates.
(iii) Lungs in adult amphibians, reptiles and mammals replace gills.
(i) Similarly in early development: In all the multicellular animals the fertilized egg (zygote) under goes cleavage to produce morula (solid mass of cells), blastula (a single layered hollow structure) & gastrula.
(ii) Resemblance in vertebrate embryos: The embryos of all vertebrates resemble one another.
(iii) Heart of a mammal or bird is initially two - chambered as in fished and finally becomes four-chambered. (iv) Tadpole of frog resembles to fish shows "ontogeny repeats phylogeny".
Tadpole of frog