EVIDENCES : COMPARATIVE EMBRYOLOGY

Comparative Embryology: It is observed that in the development of an embryo, general traits appear before the more specialized, that the embryos of different species are more alike than the adults and depart progressively from each other during ontogeny.

Vertebrate embryos show many similarities, for which the most reasonable explanation is their common ancestry. The gill arches and the gill slits in the mammalian embryo are similar to those of a fish embryo at a comparable stage of development. They then differentiate into structures quite different from those in the fish. All the gill slits close and disappear except the one that forms the Eustachian tube, which connects the pharynx at the back of the mouth to the middle ear. The gill arches themselves have a variety of fates. The obvious question is why should there be a stage in the mammalian embryo where gills and gill arches, which never function as such, are nevertheless present, even though they differentiate into quite different adult structure. The obvious answer is that the mammals are descended from fishlike ancestors and that in the course of evolution modifications have occurred in its development.

The similarities which still persist in the ontogeny of fish and mammals are indicative of a fundamental similarity in their genotypes due to their common ancestry. The evidence indicates that evolution must operate within the framework and limitations imposed by existing patterns of development. Embryos still carry the clues to their common ancestry.