Law of embryonic resemblance

Basic principles of Karl Ernst von Baer's law

Karl Ernst von Baer formulated four key rules describing the general patterns of embryogenesis. They explain the order in which anatomical structures are laid down in the embryo.

1. Sequence of trait appearance. During intrauterine development, traits characteristic of large taxonomic groups (such as phylum or class) always form earlier than highly specialized features of a particular species. As an example, consider the early stages of vertebrate development: all embryos without exception (whether fish, salamander, chicken, or human) initially develop a notochord, a neural tube, and pharyngeal slits.

1. Direction of differentiation. Embryonic development is strictly directed from the most general traits to less general ones. Only at the final stages do the most specialized structures form. For example, the integument in all embryos initially looks identical. Only at later stages does it differentiate into specific skin derivatives: scales, feathers, hair, or claws depending on the species.

1. Divergence (divergence of traits).l The developing embryo of a particular species never passes through stages in its development that are identical to the adult forms of other animals. On the contrary, as it grows, it moves further and further away from them, acquiring its own unique features. A striking example is the fate of the pharyngeal slits. In fish embryos, they ultimately develop into full-fledged permanent gills, while in mammals, these same initial structures transform into the Eustachian tubes.

1. Resemblance exclusively between embryos. An early embryo of a highly organized animal is never similar to an adult of a lower-organized species. It can only be similar to its embryo. Thus, at the beginning of embryogenesis, the maximum number of common features is observed, and then, as growth proceeds, differences begin to increase rapidly.

Biogenetic law and its refinement

Along with Baer's rules, embryology features the biogenetic law, formulated by researchers Ernst Haeckel and Fritz Müller.

Its essence is as follows: ontogeny (the individual development of a particular organism from conception to the end of life) represents a brief and very rapid recapitulation of phylogeny (the lengthy historical development of the entire species in the course of evolution).

Later, this law was substantially supplemented by the work of the Russian biologist A.N. Severtsov. Severtsov introduced a crucial refinement consistent with Baer's rules: during ontogeny, the developing organism repeats the structure of precisely the embryos of its evolutionary ancestors, rather than their adult, fully formed forms.

Significance for evolutionary theory

The patterns described by Karl Ernst von Baer had a huge impact on the development of biology. In particular, Charles Darwin actively used the law of embryonic resemblance to support his evolutionary theory.

According to Darwin's conclusions, the resemblance of embryos undeniably proves two facts:

• The existence of a common origin for all animals.
• The strict unity of the initial stages of evolution among animals belonging to the same phylum.

That is why at early stages of development, fish, salamander, chicken, and human embryos possess a striking resemblance, and only at later stages do the differences between them become significant and obvious.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of trait divergence in embryogenesis
• Comparative characteristics of ontogeny and phylogeny
• Formation of integumentary derivatives in vertebrates
• The fate of branchial arches and clefts in various animal classes
• The role of A.N. Severtsov in the development of evolutionary embryology

Other topics in the section "Patterns of Individual Development"

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