von Baer's laws (embryology)

[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.

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).

[5] They are a series of statements generally summarised into four points, as translated by Thomas Henry Huxley in his Scientific Memoirs:[6] Von Baer discovered the blastula (the early hollow ball stage of an embryo) and the development of the notochord (the stiffening rod along the back of all chordates, that forms after the blastula and gastrula stages).

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.

[8]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.

Darwin came across von Baer's laws from the work of Johannes Peter Müller in 1842, and realised that it was a support for his own theory of descent with modification.

[14] He further explained in the later editions of Origin of Species (from third to sixth editions), and wrote: It might be thought that the amount of change which the various parts and organs [of vertebrates] undergo in their development from the embryo to maturity would suffice as a standard of comparison; but there are cases, as with certain parasitic crustaceans, in which several parts of the structure become less perfect, so that the mature animal cannot be called higher than its larva.

In one line of argument, it is said that although every detail of von Baer's law may not work, the basic assumption that early developmental stages of animals are highly conserved is a biological fact.