von Baer's laws (embryology)

von Baer's laws of embryology (or laws of development) is a set of four rules discovered by Karl Ernst von Baer to explain the observed pattern of embryonic development in different species.^[1]

von Baer formulated the laws in the book Über Entwickelungsgeschichte der Thiere ("On the Developmental History of Animals"), 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).^[2] 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".^[3][4]

The laws

von Baer's law is a series of statements generally summarised into four points. As translated by Thomas Henry Huxley in his Scientific Memoirs:^[5]

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 that he realised that 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 structure 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.^[6] 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.^[7]

In terms of taxonomic hierarchy, 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.^[6]

Reception

von Baer's laws of embryology received a mixed appreciation. While they were criticised in detail, they formed the foundation of modern embryology.^[1] 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.^[8]

The most important supporter of von Baer's law was Charles Darwin, who wrote in his Origin of Species:

[The] adult [animal] 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.^[9]

Darwin took up the concept to support his theory of common descent. But von Baer was one of the most vociferous anti-Darwinists, who devoted much of his scholarly efforts on criticising Darwinism. His criticism culminated with his last work Über Darwins Lehre ("On the Doctrine of Darwin"), published in the year of his death in 1876.^[10]

References