Waddington's epigenetics or the pictorial meetings of development and genetics.

In 1956, in his Principles of Embryology, Conrad Hal Waddington explained that the word "epigenetics" should be used to translate and update Wilhelm Roux' German notion of "Entwicklungsmechanik" (1890) to qualify the studies focusing on the mechanisms of development. When Waddington mentioned it in 1956, the notion of epigenetics was not yet popular, as it would become from the 1980s. However, Waddington referred first to the notion in the late 1930s. While his late allusion clearly reveals that Waddington readily associated the notion of epigenetics with the developmental process, in the contemporary uses of the notion this developmental connotation seems to have disappeared. The advent and success of molecular biology have probably contributed to focusing biologists' attention on the "genetic" or the "non-genetic" over the "developmental". In the present paper, I first examine the links that exist, in Waddington's work, between the classical notion of epigenesis in embryology and those of epigenetics that Waddington proposed to connect, and even synthesize, data both from embryology and genetics. Second, I show that Waddington's own view of epigenetics has changed over time and I analyze how these changes appear through his many representations (both schematic or metaphorical images) of the relationships between genetic signals and developmental processes.