In biology, a law stating that the earlier stages of embryos of species advanced in the evolutionary process, such as humans, resemble the embryos of ancestral species, such as fish. The law refers only to embryonic development and not to adult stages; as development proceeds, the embryos of different species become more and more dissimilar. An early form of the law was devised by the 19th-century Estonian zoologist Karl Ernst von Baer, who observed that embryos resemble the embryos, but not the adults, of other species. A later, but incorrect, theory of the 19th-century German zoologist Ernst Heinrich Haeckel states that the embryonic development (ontogeny) of an animal recapitulates the evolutionary development of the animal's ancestors (phylogeny). Main article: Life There are many universal units and common processes that are fundamental to the known forms of life. ... Categories: Biology stubs | Developmental biology ... Zoology (Greek zoon = animal and logos = word) is the biological discipline which involves the study of animals. ... Karl Ernst von Baer (February 17, 1792 - November 26, 1876) was a German-Estonian biologist and the founding father of embryology. ... Ernst Haeckel Ernst Heinrich Philipp August Haeckel (February 16, 1834 - August 8, 1919) was a German biologist and philosopher who popularized Charles Darwins work in Germany. ... Ontogeny (also ontogenesis or morphogenesis) describes the origin and the development of an organism from the fertilized egg to its mature form. ... In biology, Phylogenetics (Greek: phylon = race and genetic = birth) is the taxonomical classification of organisms based on how closely they are related in terms of evolutionary differences. ...