The embryology or the developmental history of animals offers unique proof of evolution. All the multicellular sexually reproducing organisms start their life from fertilized eggs, which are single cells. With the progress of development, two and three germinal layers are produced in the embryos. From these germ layers, all the body organs and organ systems are derived.
Observing that all multicellualr organisms pass through double-layered (gastrula) condition and then become three layered. Haeckel (1811) postulated the ‘Recapitulation theory’ or ‘Biogenetic law’ for evolution. He meant that the embryos of highly evolved forms passed through the adult stages of their ancestral forms. For example, the double –layered stage is the adult condition of coelenterates.
Moreover, the frog embryo becomes a tadpole larva, which is much like a fish. The biogenetic law is stated as ‘ontogeny repeats phylogeny’. Ontogeny means the developmental history of a particular organism while phylogeny means the development history of the broad group to which the organisms belongs. Recapitulation theory is totally discarded now a days and its modified version that ‘embroys of the higher organisms recapitulate the embryonic history of their ancestors’ seems to be more viable.
From comparative study of embryo of the vertebrate group, the above statement becomes clear. It is seen that before the embryo of man manifests mammalian characters it passes through the embryonic conditions of a fish, then of a frog, then of a lizard in succession. Similar is the case for a bird’s embryo. Thus, one will note that the adults of two species of animals may resemble little but their embryos do so strongly if they are evolved from some common ancestors.
Von Baer’s Principle:
Von Baer postulated the following principle in support of embryological evidence:
(i) General characteristics appear during development first then appear the specialized characters.
(ii) From the more general, less general and finally the specialized characters appear.
(iii) An animal during development departs progressively from the structure of ancestral forms.
(iv) Young stages of an animal resemble the embryos of other groups of animals.
Sometimes embryology helps in establishing the true taxonomic position of animals whose adult forms have suffered retrogressive development.
For example ,tunicates like Herdmania has a degenerate adult form while its larva shows much advanced characters of urochordates. Similarly, Sacculina in its adult form is much degenerate sac-like parasitic form on the body of crabs. It does not show any of the crustacean characters that its nauplius and cypris larvae do.
Hence, study of embryology to ascertain the phylogeny of related groups of organism helps in a greater way to understand the path of evolution.