Baer’s laws on embryonic development


Baer (1828) formulated four rules.

A formulation from Russell (1916):
(I) The general characters of the big group to which an embryo belongs appear in development earlier than the special characters.
(II) The less general structural relations are formed after the more general, and so on, until the most specific appear.
(III) The embryo of any given form, instead of passing through the state [i.e., particular configuration] of other definite forms [i.e., of other species], on the contrary, [gradually] separates itself from them.
(IV) Fundamentally the embryo of a higher animal form never resembles the adult of another animal form [which it might do superficially, as when mammalian embryos go through a stage when they have gill clefts], but only its embryo.

A formulation from Raff & Kaufman (1983: 9):
(I) The more general characters of a large group of animals appear earlier in their embryos than the more special characters.
(II) From the most general forms the less general are developed, and so on, until finally the most special arise.
(III) Every embryo of a given animal form instead of passing through the other forms, becomes separate from them.
(IV) Fundamentally, therefore, the embryo of a higher form never resembles any other form, but only its embryo.

A formulation from Gould (1977: 486):
Development proceeds from the general to the special. The earliest embryonic stages of related organisms are identical; distinguishing features are added later as heterogeneity differentiates from homogeneity. Recapitulation is impossible; young embryos are undifferentiated general forms, not previous adult ancestors.

A formulation from Salthe (1993: 56-57):
These can be summed up as Baer's Law to the effect that, as organisms develop during ontogeny, they begin more alike (minimally insofar as earlier stages are vaguer and so necesarily poorly distinguishable) and gradually appear to become more and more different as they differentiate and individuate. In other words, development moves from the general to the particular; transformation is irreversibely into more highly differentiated states, requiring increasing specification by an observer.


These Baer’s laws should not be mixed up with his law known in geography (Baer-Babinet' Law), on the asymmetry of river coasts.


References

Baer K. E. v. 1828-1837. Uber Entwickelungsgeschichte der Thiere: Beobachtung und Reflektion. 2 vols. Königsberg: Gebrüder Borntraeger, pt. 1.

Gould S. J. 1977. Ontogeny and Phylogeny. Cambridge (Mass.): The Belknap Press, xiv+501.

Raff Rudolf A., Kaufman Thomas C. 1983. Embryos, Genes and Evolution: The Developmental-Genetic Basis of Evolutionary Change. New York: Macmillan Publishing Co.

Russell E. S. 1916. Form and Function: A Contribution to the History of Animal Morphology. London: J.Murray, 383.

Salthe S. N. 1993. Development and Evolution: Complexity and Change in Biology. Cambridge: A Bradford Book, The MIT Press, xiv+357.


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