Von Baer's laws (embryology)

In developmental biology, von Baer's laws of embryology (or laws of development) are four rules proposed by Karl Ernst von Baer to explain the observed pattern of embryonic development in different species.^[1]

von Baer formulated the laws in his book On the Developmental History of Animals (German: Über Entwickelungsgeschichte der Thiere), published in 1828, while working at the University of Königsberg. He specifically intended to rebut Johann Friedrich Meckel's 1808 recapitulation theory. According to that theory, embryos pass through successive stages that represent the adult forms of less complex organisms in the course of development, and that ultimately reflects scala naturae (the great chain of being).^[2] von Baer believed that such linear development is impossible. He posited that instead of linear progression, embryos started from one or a few basic forms that are similar in different animals, and then developed in a branching pattern into increasingly different organisms. Defending his ideas, he was also opposed to Charles Darwin's 1859 theory of common ancestry and descent with modification, and particularly to Ernst Haeckel's revised recapitulation theory with its slogan "ontogeny recapitulates phylogeny".^[3][4] Darwin was however broadly supportive of von Baer's view of the relationship between embryology and evolution.