Von Baer's laws (embryology)

von Baer's laws of embryology (or laws of development) is a set of four rules discovered by Karl Ernst von Baer to explain the observed pattern of embryonic development in different species.

von Baer formulated the laws in the book Über Entwickelungsgeschichte der Thiere ("On the Developmental History of Animals"), 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). 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".

von Baer's law is a series of statements generally summarised into four points. As translated by Thomas Henry Huxley in his Scientific Memoirs:

von Baer discovered the blastula (the early hollow ball stage of an embryo) and the development of the (the stiffening rod along the back of all chordates, that forms after the blastula and gastrula stages). From his observations of these stages in different vertebrates that he realised that Meckel's recapitulation theory must be wrong. For example, he noticed that the yolk sac is found in birds, but not in frogs. According to the recapitulation theory, such structure should invariably be present in frogs because they were assumed to be at a lower level in the evolutionary tree. von Baer concluded that while structures like the notochord are recapitulated during embryogenesis, whole organisms are not. He asserted that (as translated):

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