Von Baer's laws (embryology)

Von Baer's laws are four principles of embryonic development set out by the naturalist Karl Ernst von Baer in his 1828 book On the Developmental History of Animals, written while he was at the University of Konigsberg, in order to counter Johann Friedrich Meckel's recapitulation theory, which held that embryos pass through stages resembling the adult forms of simpler organisms. Von Baer proposed instead that embryos of different species begin from similar general forms and develop through a branching pattern into increasingly distinct organisms, a view he framed against both Meckel's linear model and, later, against Ernst Haeckel's revised recapitulation theory and Charles Darwin's 1859 theory of common descent with modification. Darwin himself nonetheless found von Baer's embryological observations broadly supportive of evolution, since they showed how similarities between embryos reflect older, less modified ancestral conditions, a relationship von Baer summarized in the principle that the more general characters of a large group appear earlier in the embryo than its more specialized characters.