Karl Ernst von Baer and Gallus domesticus the beginnings of comparative embryology

Nineteenth-century comparative embryology lies at the origin of evolutionary developmental biology or, more briefly, evo-devo. Von Baer's and Haeckel's works are the most popular examples of that period, and these authors' "laws" describing the general development of organismic form are of great interest.

Karl Ernst von Baer, a pupil of Ignatius Dollinger, a professor at Wiirzburg, is known as the founder of embryology as a scientific endeavor. Although representatives of German transcendentalism had provided some insights into the field of embryology, it was only in 1828 that von Baer's Uber die Entwickelungs-geschichte der Thiere: Beobachtung und Reflexion appeared in print and made even contemporaries recognize him as the founder of embryology. In the first volume of his masterpiece, von Baer concentrated on the development of the chicken (Gallus domesticus), but he also bore general laws of development in mind. He worked with dissecting needles and a simple microscope: the "Scholia" describe the deductions he made. The accuracy and minuteness of his fundamental observations is absolutely astonishing. Russell (1982 p 114) was unable to hide his admiration: "His account of the development of the chicken is a model of what a scientific memoir ought to be ''

Von Baer's ideas incorporate the truly important distinction between the type (Wiesemuller et al. 2002) of organization (=the structural plan) and the grade of differentiation (modifications of this plan). The aim of comparative anatomy to reveal group-specific Baupläne now had a prominent new tool by integrating embryological data.

As Charles Darwin was not a professional morphologist, it was others who introduced evolutionary thinking into the realm of comparative embryology. Ernst Haeckel's formulation of the biogenetic law (ontogeny as the short and rapid recapitulation of phylogeny) presents a radicalized phylogenetic approach. Recapitulatory ideas were not new since Müller (Russell 1982) had already supported a similar hypothesis in 1864. Yet Haeckel's verve and passion—very striking in a commemorative speech in 1909 (Das Weltbild von Darwin und Lamarck), on the occasion of the 100th birthday of Charles Darwin, favoring Goethe's monistic world view as being the ultima ratio—made it famous. The interpretations of heterochrony and its implication for paleoanthropology are discussed later on. However, how does today's evolutionary developmental biology reflect the ideas of von Baer and Haeckel?

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