Introducing the Spemann-Mangold organizer: experiments and insights that generated a key concept in developmental biology.

The "organizer paper", published by Hans Spemann and Hilde Mangold in 1924, initiated a new epoch in developmental biology. Also it marked the climax of Spemann's life-long research which began at the end of the nineteenth century. This introduction retraces some of the steps by which Spemann arrived at the organizer concept: The problem of amphibian lens induction including the so-called lens controversy, the early constriction experiments creating double headed malformations, and the homeo- and heteroplastic transplantations during gastrula stages of the newt. Furthermore this paper will--based on historical documents--repudiate some objections raised to the contribution of Spemann and Hilde Mangold to the discovery and interpretation of the organizer effect.