Von Baer's laws (embryology)


Von Baer's laws of embryology are four rules proposed 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, 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. 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".

The laws

Von Baer's laws are a series of statements generally summarised into four points. As translated by Thomas Henry Huxley in his Scientific Memoirs:
  1. The more general characters of a large group appear earlier in the embryo than the more special characters.
  2. From the most general forms the less general are developed, and so on, until finally the most special arises.
  3. Every embryo of a given animal form, instead of passing through the other forms, rather becomes separated from them.
  4. The embryo of a higher form never resembles any other form, but only its embryo.

    Description

Von Baer discovered the blastula and the development of the notochord. From his observations of these stages in different vertebrates, he realised that Johann Friedrich 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 structures 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 :
In terms of taxonomic hierarchy, according to von Baer, characters in the embryo are formed in top-to-bottom sequence, first from those of the largest and oldest taxon, the phylum, then in turn class, order, family, genus, and finally species.

Reception

The laws received a mixed appreciation. While they were criticised in detail, they formed the foundation of modern embryology. The British zoologist Adam Sedgwick studied the developing embryos of dogfish and chicken, and in 1894 noted a series of differences, such as the green yolk in the dogfish and yellow yolk in the chicken, absence of embryonic rim in chick embryos, absence of blastopore in dogfish, and differences in the gill slits and gill clefts. He concluded:
The most important supporter of von Baer's laws was Charles Darwin, who wrote in his Origin of Species:
Darwin took up the concept of common descent which formed part of his theory of evolution. But von Baer was a vociferous anti-Darwinist, devoting much of his scholarly effort to criticising Darwinism. His criticism culminated with his last work Über Darwins Lehre, published in the year of his death in 1876.