Introducing the

The “organizer paper” by Hans Spemann and Hilde Mangold (1924) initiated a new epoch in developmental biology. It marked the climax of Spemann’s life-long research, and the “organizer effect” received special mention by the committee that honoured him with the Nobel Prize for physiology and medicine in 1935. This introduction precedes a translation of that paper by Spemann’s student, Viktor Hamburger. Its purpose is to retrace some of the steps by which Spemann arrived at the organizer concept, based on Spemann’s publications, recollections of his students and collaborators (Baltzer 1942; Mangold 1953; Hamburger 1968, 1984, 1988, 1996; Holtfreter 1988, 1991; Waelsch 1992), historical essays (Oppenheimer 1970a, 1985; Saha 1991; Sander 1985, 1997) and detailed studies of original documents preserved at several places in Europe and the U.S.A. (Faessler 1994, 1996, 1997; Faessler and Sander 1996; Sander 1997). An additional document (of sorts), not strictly historical but nonetheless impressive, is a movie in which Richard M. Eakin renders the aged Spemann giving a fictitious lecture (Eakin 1975). Eakin in 1935/36 spent a year in Spemann’s institute and helped him with the English version of his book (Spemann 1938). Two witnesses of that period who had seen the film felt that Eakin’s performance gives a good impression of Spemann’s gist for lecturing. Hans Spemann (Fig. 1) gained early fame by his work on lens induction in frogs (Sander 1985, Saha 1991). Evocation of the “outer parts of the eye” by an outgrowth of the nerve tube (the optical vesicle) had been postulated early in the 19th century (von Baer 1828; Oppenheimer 1970b), but Spemann’s experiments were the first to raise considerable interest; perhaps not so much by their results than by the long-lasting controversy these triggered. The struggle resulted, in a paradigmatic way, from Spemann’s failure to consider a seemingly far-fetched possibility, namely that functional components of a developmental process might differ between closely related species. This contra-intuitive fact, although well documented meanwhile, tends to be overlooked even in our days of "Evo-Devo" research. The controversy was resolved by Spemann’s painstaking documentation of just such differences between Rana fusca and R.