von Baer's law (biology)

The von Baer's law (or Law of Development, or von Baer's law of embryology) is a concept in biology introduced by Karl Ernst von Baer to explain the details of embryo development.^[1] von Baer formulated the statements 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 aimed at rebutting the recapitulation theory introduced by Johann Friedrich Meckel in 1808. According to Meckel's 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 for different animals, and then developed in a branching pattern into increasingly different looking organisms. Defending his ideas, he was also opposed to the theory of common ancestry and descent with modification as proposed by Charles Darwin in 1859, and particularly the revised recapitulation theory ("ontogeny recapitulates phylogeny") of Ernst Haeckel, a supporter of Darwin's theory in Germany.^[3][4]

The laws

von Baer's law is a series of statements generally summarised into four points. According to the translation of 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. Fundamentally, therefore, the embryo of a higher form never resembles any other form, but only its embryo.

Description

von Baer was the discoverer of blastula (early embryo), development of notochord, and mammalian ovum. It was from his observation of these developmental stages in different vertebrates that he realised the flaw in the recapitulation theory. For example, the yolk sac in found in birds, but not in frogs. According to the recapitulation theory, such structure should invariably be present in frogs because, by implication, frogs are more primitive or in lower level of the evolutionary tree. von Baer concluded that it is only the organs that are recapitulated during embryogenesis, and not the organisms.^[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 will be formed in the order, first from those of phylum, then class, order, family, genus, and finally species.^[6]

Reception

von Baer's law received a mixed appreciation. While its details are criticised, it is also regarded as the foundation of modern embryology.^[1] British zoologist Adam Sedgwick was one of the most important critics. He had studied the developing embryos of a dogfish and chicken. In 1894, he noted major differences, such as the green yolk in the former and yellow yolk in the latter, absence of embryonic rim and blastopore in chick that are present in the fish, and subtle 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