Baer's Laws
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In
developmental biology Developmental biology is the study of the process by which animals and plants grow and develop. Developmental biology also encompasses the biology of Regeneration (biology), regeneration, asexual reproduction, metamorphosis, and the growth and di ...
, von Baer's laws of embryology (or laws of development) are four rules proposed by
Karl Ernst von Baer Karl Ernst Ritter von Baer Edler von Huthorn (; – ) was a Baltic German scientist and explorer. Baer was a naturalist, biologist, geologist, meteorologist, geographer, and is considered a, or the, founding father of embryology. He was a m ...
to explain the observed pattern of
embryonic development In developmental biology, animal embryonic development, also known as animal embryogenesis, is the developmental stage of an animal embryo. Embryonic development starts with the fertilization of an egg cell (ovum) by a sperm, sperm cell (spermat ...
in different
species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
. von Baer formulated the laws in his book ''On the Developmental History of Animals'' (), published in 1828, while working at the
University of Königsberg The University of Königsberg () was the university of Königsberg in Duchy of Prussia, which was a fief of Poland. It was founded in 1544 as the world's second Protestant Reformation, Protestant academy (after the University of Marburg) by Duke A ...
. He specifically intended to rebut
Johann Friedrich Meckel Johann Friedrich Meckel (17 October 1781 – 31 October 1833), often referred to as Johann Friedrich Meckel, the Younger, was a German anatomist born in Halle. He worked as a professor of anatomy, pathology and zoology at the University of Halle, ...
's 1808
recapitulation theory The theory of recapitulation, also called the biogenetic law or embryological parallelism—often expressed using Ernst Haeckel's phrase "ontogeny recapitulates phylogeny"—is a historical hypothesis that the development of the embryo of an ...
. According to that theory, embryos pass through successive stages that represent the adult forms of less complex
organism An organism is any life, living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have be ...
s in the course of development, and that ultimately reflects (the
great chain of being The great chain of being is a hierarchical structure of all matter and life, thought by medieval Christianity to have been decreed by God. The chain begins with God and descends through angels, Human, humans, Animal, animals and Plant, plants to ...
). 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 Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English Natural history#Before 1900, naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all speci ...
's 1859 theory of
common ancestry Common descent is a concept in evolutionary biology applicable when one species is the ancestor of two or more species later in time. According to modern evolutionary biology, all living beings could be descendants of a unique ancestor commonl ...
and descent with modification, and particularly to
Ernst Haeckel Ernst Heinrich Philipp August Haeckel (; ; 16 February 1834 – 9 August 1919) was a German zoologist, natural history, naturalist, eugenics, eugenicist, Philosophy, philosopher, physician, professor, marine biology, marine biologist and artist ...
's revised recapitulation theory with its slogan "
ontogeny recapitulates phylogeny Ontogeny (also ontogenesis) is the origination and development of an organism (both physical and psychological, e.g., moral development), usually from the time of fertilization of the ovum, egg to adult. The term can also be used to refer to t ...
". 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 Thomas Henry Huxley (4 May 1825 – 29 June 1895) was an English biologist and anthropologist who specialized in comparative anatomy. He has become known as "Darwin's Bulldog" for his advocacy of Charles Darwin's theory of evolution. The stor ...
in his ''Scientific Memoirs'': # The more general characters of a large group appear earlier in the embryo than the more special characters. # From the most general forms the less general are developed, and so on, until finally the most special arises. # Every embryo of a given animal form, instead of passing through the other forms, rather becomes separated from them. # The embryo of a higher form never resembles any other form, but only its embryo.


Description

Von Baer discovered the
blastula Blastulation is the stage in early animal embryonic development that produces the blastula. In mammalian development, the blastula develops into the blastocyst with a differentiated inner cell mass and an outer trophectoderm. The blastula (fr ...
(the early hollow ball stage of an embryo) and the development of the
notochord The notochord is an elastic, rod-like structure found in chordates. In vertebrates the notochord is an embryonic structure that disintegrates, as the vertebrae develop, to become the nucleus pulposus in the intervertebral discs of the verteb ...
(the stiffening rod along the back of all
chordate A chordate ( ) is a bilaterian animal belonging to the phylum Chordata ( ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics ( synapomorphies) that distinguish them from ot ...
s, that forms after the blastula and
gastrula Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of Cell (biology), cells), or in mammals, the blastocyst, is reorganized into a two-layered or three-layered e ...
stages). From his observations of these stages in different vertebrates, he realised that
Johann Friedrich Meckel Johann Friedrich Meckel (17 October 1781 – 31 October 1833), often referred to as Johann Friedrich Meckel, the Younger, was a German anatomist born in Halle. He worked as a professor of anatomy, pathology and zoology at the University of Halle, ...
's
recapitulation theory The theory of recapitulation, also called the biogenetic law or embryological parallelism—often expressed using Ernst Haeckel's phrase "ontogeny recapitulates phylogeny"—is a historical hypothesis that the development of the embryo of an ...
must be wrong. For example, he noticed that the
yolk sac The yolk sac is a membranous wikt:sac, sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac' ...
is found in
bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...
s, but not in
frog A frog is any member of a diverse and largely semiaquatic group of short-bodied, tailless amphibian vertebrates composing the order (biology), order Anura (coming from the Ancient Greek , literally 'without tail'). Frog species with rough ski ...
s. 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): 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 In biology, a phylum (; : phyla) is a level of classification, or taxonomic rank, that is below Kingdom (biology), kingdom and above Class (biology), class. Traditionally, in botany the term division (taxonomy), division has been used instead ...
, 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 Embryology (from Ancient Greek, Greek ἔμβρυον, ''embryon'', "the unborn, embryo"; and -λογία, ''-logy, -logia'') is the branch of animal biology that studies the Prenatal development (biology), prenatal development of gametes (sex ...
.


Charles Darwin

The most important supporter of von Baer's laws was
Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English Natural history#Before 1900, naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all speci ...
. Darwin came across von Baer's laws from the work of
Johannes Peter Müller Johannes Peter Müller (14 July 1801 – 28 April 1858) was a German physiologist, comparative anatomist, ichthyologist, and herpetologist, known not only for his discoveries but also for his ability to synthesize knowledge. The paramesonephri ...
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 ''On the Origin of Species'' (or, more completely, ''On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life'')The book's full original title was ''On the Origin of Species by M ...
'' (first edition, 1859): 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 Jean Louis Rodolphe Agassiz ( ; ) FRS (For) FRSE (May 28, 1807 – December 14, 1873) was a Swiss-born American biologist and geologist who is recognized as a scholar of Earth's natural history. Spending his early life in Switzerland, he recei ...
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 f vertebratesundergo 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 Adam Sedgwick FRS (; 22 March 1785 – 27 January 1873) was a British geologist and Anglican priest, one of the founders of modern geology. He proposed the Cambrian and Devonian period of the geological timescale. Based on work which he did ...
studied the developing embryos of dogfish and
chicken The chicken (''Gallus gallus domesticus'') is a domesticated subspecies of the red junglefowl (''Gallus gallus''), originally native to Southeast Asia. It was first domesticated around 8,000 years ago and is now one of the most common and w ...
, 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: 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 The notochord is an elastic, rod-like structure found in chordates. In vertebrates the notochord is an embryonic structure that disintegrates, as the vertebrae develop, to become the nucleus pulposus in the intervertebral discs of the verteb ...
before heart. This is due to the fact that heart is present in many invertebrates, which never have notochord.


See also

*
Evolutionary developmental biology Evolutionary developmental biology, informally known as evo-devo, is a field of biological research that compares the developmental biology, developmental processes of different organisms to infer how developmental processes evolution, evolved. ...


References

{{Biological rules Biological rules Biology theories Evolutionary biology Animal developmental biology 1828 in science