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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.

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). 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". Darwin was however broadly supportive of von Baer's view of the relationship between embryology and evolution.

The laws

Von Baer described his laws in his book Über Entwickelungsgeschichte der Thiere. Beobachtung und Reflexion published in 1828. They 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 (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). 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 (as translated):

The embryo successively adds the organs that characterize the animal classes in the ascending scale. When the human embryo, for instance, is but a simple vesicle, it is an infusorian; when it has gained a liver, it is a mussel; with the appearance of the osseous system, it enters the class of fishes; and so forth, until it becomes a mammal and then a human being.

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.

Charles Darwin

The most important supporter of von Baer's laws was Charles Darwin. 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. Darwin was a critique of the recapitulation theory and agreed with von Baer that an adult animal is not reflected by an embryo of another animal, and only embryos of different animals appear similar. He wrote in his Origin of Species (first edition, 1859):

adult differs from its embryo, owing to variations supervening at a not early age, and being inherited at a corresponding age. This process, whilst it leaves the embryo almost unaltered, continually adds, in the course of successive generations, more and more difference to the adult. Thus the embryo comes to be left as a sort of picture, preserved by nature, of the ancient and less modified condition of each animal. This view may be true, and yet it may never be capable of full proof.

Darwin also said:

It has already been casually remarked that certain organs in the individual, which when mature become widely different and serve for different purposes, are in the embryo exactly alike. The embryos, also, of distinct animals within the same class are often strikingly similar: a better proof of this cannot be given, than a circumstance mentioned by Agassiz, namely, that having forgotten to ticket the embryo of some vertebrate animal, he cannot now tell whether it be that of a mammal, bird, or reptile.

Darwin's attribution to Louis Agassiz was a mistake, and was corrected in the third edition as von Baer. 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 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. Von Baer's standard seems the most widely applicable and the best, namely, the amount of differentiation of the different parts (in the adult state, as I should be inclined to add) and their specialisation for different functions.

Even so, von Baer was a vociferous anti-Darwinist, although he believed in the common ancestry of species. Devoting much of his scholarly effort to criticising natural selection, his criticism culminated with his last work Über Darwins Lehre ("On Darwin's Doctrine"), published in the year of his death in 1876.

Later biologists

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:

There is no stage of development in which the unaided eye would fail to distinguish between them with ease... A blind man could distinguish between them.

Modern biologists still debate the validity of the laws. 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. But an opposition says that there are conserved genetic conditions in embryos, but not the genetic events that govern the development. One example on the problem of von Baer's law is the formation of notochord before heart. This is due to the fact that heart is present in many invertebrates, which never have notochord.

See also

• Evolutionary developmental biology

References

1. ^ Abzhanov, Arhat (2013). "von Baer's law for the ages: lost and found principles of developmental evolution". Trends in Genetics. 29 (12): 712–722. doi:10.1016/j.tig.2013.09.004. PMID 24120296. S2CID 9158143.
2. Opitz, John M.; Schultka, Rüdiger; Göbbel, Luminita (2006). "Meckel on developmental pathology". American Journal of Medical Genetics Part A. 140A (2): 115–128. doi:10.1002/ajmg.a.31043. PMID 16353245. S2CID 30513424.
3. Garstang, Walter (1922). "The Theory of Recapitulation: A Critical Re-statement of the Biogenetic Law". Journal of the Linnean Society of London, Zoology. 35 (232): 81–101. doi:10.1111/j.1096-3642.1922.tb00464.x.
4. Løvtrup, Søren (1978). "On von Baerian and Haeckelian Recapitulation". Systematic Zoology. 27 (3): 348–352. doi:10.2307/2412887. JSTOR 2412887.
5. Baer, Karl Ernst von (1828). Über Entwickelungsgeschichte der Thiere. Beobachtung und Reflexion. Königsberg: Bei den Gebrüdern Bornträger. doi:10.5962/bhl.title.6303.
6. Huxley, Thomas Henry (1853). Henfrey, Arthur (ed.). Scientific memoirs, selected from the transactions of foreign academies of science, and from foreign journals. Natural history. Taylor and Francis. doi:10.5962/bhl.title.28029.
7. ^ Matthen, Mohan; Stephens, Christopher (2007). Philosophy of Biology. Amsterdam: Elsevier. pp. 444–445. ISBN 978-0-080-47124-2.
8. Ospovat, D. (1976). "The influence of Karl Ernst von Baer's embryology, 1828–1859: A reappraisal in light of Richard Owen's and William B. Carpenter's "Palaeontological application of 'von Baer's law'"". Journal of the History of Biology. 9 (1): 1–28. doi:10.1007/BF00129171. PMID 11615631. S2CID 40800347.
9. Gilbert, Scott F. (2000). Developmental Biology. Sinauer Associates. pp. Online. ISBN 978-0878932436.
10. Barnes, M. Elizabeth (2014). "Karl Ernst von Baer's Laws of Embryology". The Embryo Project Encyclopedia. Retrieved 2020-10-04.
11. Darwin, Charles (1859). On the Origin of Species. London: John Murray. p. 338.
12. Darwin, Charles (1859). On the Origin of Species. London: John Murray. pp. 438–439.
13. Rogers, James Allen (1973). "The Reception of Darwin's Origin of Species by Russian Scientists". Isis. 64 (4): 484–503. doi:10.1086/351177. JSTOR 229645. PMID 4593473. S2CID 36129302.
14. Darwin, Charles (1861). The Origin of Species (3 ed.). London: John Murray. pp. 470–471.
15. Darwin, Charles (1861). On the Origin of Species. John Murray. p. 133.
16. Darwin, Charles (1872). The Origin of Species (6 ed.). London: John Murray. p. 97.
17. Dagg, Joachim L.; Derry, J. F. (2020). "Charles Darwin did not mislead Joseph Hooker in their 1881 Correspondence about Leopold von Buch and Karl Ernst von Baer". Annals of Science. 77 (3): 349–365. doi:10.1080/00033790.2020.1790659. PMID 32755351. S2CID 221016353.
18. Vucinich, Alexander (1988). Darwin in Russian Thought. Berkeley: University of California Press. p. 93. ISBN 978-0-52-006283-2.
19. Sedgwick, Adam (1894). "Memoirs: On the Law of Development commonly known as von Baer's Law; and on the Significance of Ancestral Rudiments in Embryonic Development". Journal of Cell Science. s2-36 (141): 35–52. doi:10.1242/jcs.s2-36.141.35.
20. Garfield, David A.; Wray, Gregory A. (2009). "Comparative embryology without a microscope: using genomic approaches to understand the evolution of development". Journal of Biology. 8 (7): 65. doi:10.1186/jbiol161. PMC 2736668. PMID 19664180.
21. Kalinka, Alex T.; Tomancak, Pavel (2012). "The evolution of early animal embryos: conservation or divergence?". Trends in Ecology & Evolution. 27 (7): 385–393. Bibcode:2012TEcoE..27..385K. doi:10.1016/j.tree.2012.03.007. ISSN 1872-8383. PMID 22520868.
22. Løvtrup, Søren (1987). "Phylogenesis, ontogenesis and evolution". Italian Journal of Zoology. 54 (3): 199–208. doi:10.1080/11250008709355584.

v t e Biological rules
Rules	Allen's rule Shorter appendages in colder climates Bateson's rule Extra limbs mirror their neighbours Bergmann's rule Larger bodies in colder climates Cope's rule Bodies get larger over time Deep-sea gigantism Larger bodies in deep-sea animals Dollo's law Loss of complex traits is irreversible Eichler's rule Parasites co-vary with their hosts Emery's rule Insect social parasites are often in same genus as their hosts Fahrenholz's rule Host and parasite phylogenies become congruent Foster's rule (Insular gigantism, Insular dwarfism) Small species get larger, large species smaller, after colonizing islands Gause's law Complete competitors cannot coexist Gloger's rule Lighter coloration in colder, drier climates Haldane's rule Hybrid sexes that are absent, rare, or sterile, are heterogamic Harrison's rule Parasites co-vary in size with their hosts Hamilton's rule Genes increase in frequency when relatedness of recipient to actor times benefit to recipient exceeds reproductive cost to actor Kleiber's law An animals metabolic rate decreases with its size Hennig's progression rule In cladistics, the most primitive species are found in earliest, central, part of group's area Jarman–Bell principle The correlation between the size of an animal and its diet quality; larger animals can consume lower quality diet Jordan's rule Inverse relationship between water temperature and no. of fin rays, vertebrae Lack's principle Birds lay only as many eggs as they can provide food for Rapoport's rule Latitudinal range increases with latitude Rensch's rule Sexual size dimorphism increases with size when males are larger, decreases with size when females are larger Rosa's rule Groups evolve from character variation in primitive species to a fixed character state in advanced ones Schmalhausen's law A population at limit of tolerance in one aspect is vulnerable to small differences in any other aspect Thayer's law The top of an animals coloration is darker than the bottom Thorson's rule No. of eggs of benthic marine invertebrates decreases with latitude Van Valen's law Probability of extinction of a group is constant over time von Baer's laws Embryos start from a common form and develop into increasingly specialised forms Williston's law Parts in an organism become reduced in number and specialized in function
Related	Countergradient variation Where genetics opposes environment as a factor Gigantothermy Large ectothermic animals more easily maintain constant body temperature

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