In 1927 Hans Spemann wrote in his article New work on the organizer: "What has been achieved is but the first step; we still stand in the presence of riddles, but not without hope of solving them. And riddles with the hope of solution-what more can a scientist desire?" The problem referred to, which to date still waits for a solution, arose from an experiment that Hans Spemann and Hilde Mangold performed in 1924 where they transplanted the dorsal lip of a Triturus gastrula to the ventral side of a host gastrula (Spemann and Mangold, 1924). This manipulation resulted in the induction of a secondary body axis in the recipient embryo. Analysis of the secondary axis revealed that the histological composition of the induced structures was "chimaerisch", meaning composed of both graft and host tissue. The grafted material differentiated into notochord, portions of the somites and neur. al tube. Muscle and neural tissue consisted of both host and graft material. This observation lead to the conclusion that the dorsal lip is able to induce and organize mesodermal and neur. al tissue in the host. The ability of the "Urmundregion" (blastopore lip) to act as an organizing center for cell differentiation had been observed ear. lier by Spemann. when he separated the cells of a 2-cell newt embryo using the ligature technique (Spemann. 1919). Only those blastomeres which contained the future "Urmund" devel. oped an axis whereas the other blastomeres became a "BauchstQck" (belly piece) without axis structures. To explain these results, one has to postulate an unequal distribution of maternal components in the fertilized egg and the early embryo. The questions that arose from these experiments were: What are the determinants responsible for the organizer activity? What activity induces the organizer?