of two blastomeres to kill it. He then observed how the remaining blastomere developed, and found that it became a half embryo. In 1892 Hans Driesch performed similar experiments on sea urchin embryos, but instead of killing one of the two blastomeres he put many embryos in a tube and shook it to separate the cells. He reported that, contrary to Roux's findings, he ended up with completely formed but smaller embryos. The reason for this discrepancy has been widely attributed to Driesch separating the two blastomeres completely rather than just killing one as Roux had done. Others, including Thomas Hunt Morgan and Oscar Hertwig, attempted to separate the two cells, for the matter was of great importance, particularly to the arguments between proponents of epigenesis and preformation, but satisfactory results could not be achieved. As a master of micro-surgical technique, beginning with his continuing work on the amphibian eye, Spemann's papers in the early years of the 20th century on this vexed question were to be a great contribution to the development of experimental morphogenesis, causing him to be hailed in some quarters as the true founder of micro-surgery. He succeeded in dividing the cells with a noose of baby hair. Spemann found that one half could indeed form a whole embryo, but observed that the plane of division was crucial. This dispatched the theory of preformation and gave some support to the concept of a morphogenetic field, a concept of which Spemann learned from Paul Alfred Weiss. Induction and organizers Spemann was appointed Professor of Zoology and Comparative Anatomy at Rostock in 1908 and, in 1914, Associate Director of the Kaiser Wilhelm Institute of Biology at Dahlem, Berlin. Here he undertook the experiments that would make him famous. Drawing upon the recent work of Warren H. Lewis and Ethel Browne Harvey, he turned his skills to the gastrula, grafting a "field" of cells (the Primitive knot) from one embryo onto another. The experiments, aided by Hilde Proescholdt (later Mangold), a Ph.D. candidate in Spemann's laboratory in Freiburg, took place over several years and were published in full only in 1924. They described an area in the embryo, the portions of which, upon transplantation into a second embryo, organized or "induced" secondary embryonic primordia regardless of location. Spemann called these areas "organiser centres" or "organisers". Later he showed that different parts of the organiser centre produce different parts of