Préformation et épigenèse en développement : une analyse épistémologique de l'Entwicklungsgeschichte après Darwin et de l'Entwicklungsmechanik
The main goal of this dissertation is to demonstrate the existence of a logic of embryological discovery which contributed to the genesis of Entwicklungsmechanik, and which renewed the opposition between preformation and epigenesis. On the one hand, the synthesis between a typology of development and a mechanistic heuristic led to a causal reductionist account of phenomena. Embryological experimentation, on the other hand revealed complex processes of epigenetic regulation which forced biologists to transform their models of explanation. This dissertation begins with an analysis of the role of Darwinism in the development of a mechanistic heuristic applied to morphology. Ernst Haeckel's Entwicklungsgeschichte proposed a synthesis between Karl von Baer's embryological program and Darwinism, which led to a physiological-reductionist view of typological epigenesis on the basis of the recapitulation law. A physiology of development thus frees itself from Haeckelian methodology and its phylogenic framework. While relying on Wilhelm His' physics of development (1874), Wilhelm Roux carried out a physiological synthesis of Darwinism in Der Kampf der Theile im Organismus (The Struggle of Parts in the Organism 1881). The third part of this dissertation examines the Entwicklungsmechanik and its relationship to August Weismann's Neo-Darwinism. The essential steps of the nuclear-idioplasmic theory of heredity are then described. The discoveries of mitosis and fertilization (1873-1884), combined with Carl von Nägeli's theory (1884), served as preconditions to Weismann's Neo-Preformationism. By 1885 Weismann postulates the existence of an internal predetermination of development, which would be carried out by unequal division of inherited qualities located in the nucleus chromatic architecture. This theory is first supported by experiments on blastomere destruction (Chabry 1887, Roux 1888), but it ends up being challenged by the discovery of Theilbildung (Hans Driesch 1892). The coordinated actions of differentiating cells which led to the reorganization of their morphogenetic fates (Umdifferenzierung) could not be accounted for by a mechanical model of development. Two outcomes are then examined: (1) the development of the first model of embryonic induction (Driesch 1894) coupled with a Neo-vitalistic epigenesis; (2) Oscar Hertwig's organicism, which combined principles of cell theory with a rejection of the use of mechanistic causality in biology. An epistemological analysis of experimental embryology's founding principles thus shows how a logic of discovery has structured the evolution of contemporary developmental biology by taking into account preformationist ideas and mechanistic models.
Entwicklungsmechanik Haeckel Roux-Weismann History of science Epigenesis Epistemology Embryology Mechanism Biology Logic of discovery Vitalism Teleology