Comparative Embryology

Haeckel (1834-1919) formulated that 'ontogeny recapitulates phylogeny'. This means that the developmental stages that an organism goes through repeat the evolutionary history of the group to which the organism belongs.

Stages in Embryological Development

Though it does not explain the mechanism of evolution, the study of the embryos and their development reveal that at the embryonic stages there are features that are absent in the adult form. These features are common to a group of organisms. For example, At a particular stage, all the vertebrate embryos possess a single circulation with two-chambered heart showing no separation into right and left halves.

As the development progresses, the embryos change according to group they belong to - fish, amphibian, reptile, bird or mammal. The above situation is retained only in the fish and all other vertebrates deviate from this.

Given below is another example of the embryological evidence that certain animals have a common ancestor:

Embryological Evidence