von Baer's laws (embryology)

In

Von Baer described his laws in his book Über Entwickelungsgeschichte der Thiere. Beobachtung und Reflexion published in 1828.[5] They are a series of statements generally summarised into four points, as translated by Thomas Henry Huxley in his Scientific Memoirs:^[6]

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

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.[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.^[7]

Reception

The laws received a mixed appreciation. While they were criticised in detail, they formed the foundation of modern embryology.^[1]

Charles Darwin

The most important supporter of von Baer's laws was

[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.^[11]

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.^[12]

Darwin's attribution to Louis Agassiz was a mistake,^[13] and was corrected in the third edition as von Baer.^[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. 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.^[15][16]

Even so, von Baer was a vociferous anti-Darwinist, although he believed in the common ancestry of species.^[17] 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.^[18]

Later biologists

The British zoologist

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.^[19]

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.^[20] But an opposition says that there are conserved genetic conditions in embryos, but not the genetic events that govern the development.^[21] 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.^[22]

See also

• Evolutionary developmental biology

References