[Hans Driesch (1867-1941) and the end of any preformationist vision of development].

While the work of early 19th century embryologists marked the end of preformation theory in its initial form, the first experimental attempts at the turn of the century led to the development of the concept of the mosaic egg - a more elaborate preformationist view of development - in which the embryo is made up of a mosaic of territories with a determined future and subjected to autonomous development. By separating sea urchin blastomeres at 2/4 cell stages, a young researcher, Hans Driesch shows that each of the blastomeres develops to form a complete and harmonious larva and that the young embryo is therefore capable of regulation. Thus this invalidated the concept of mosaic development, which predicted the independent development of two hemi-embryos and it opened up new fields of investigation for modern embryology.