"The Potency of the First Two Cleavage Cells in Echinoderm Development. Experimental Production of Partial and Double Formations" (1891-1892), by Hans Driesch
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Hans Adolf Eduard Driesch was a late-nineteenth and early-twentieth century philosopher and developmental biologist. In the spring of 1891 Driesch performed experiments using two-celled sea urchin embryos, the results of which challenged the then-accepted understanding of embryo development. Driesch showed that the cells of an early embryo, when separated, could each continue to develop into normal larval forms. This finding contrasted with Wilhelm Roux's experiments with frog eggs from which Roux concluded that embryonic cells have predetermined fates - they cannot form into one thing when separated, and a different form when left unseparated. To Roux, embryos were made up of a mosaic of cells, all of which were important and necessary for the viable embryos to form. Driesch, on the other hand, was able to show that individual cells resulting from cleavage of the fertilized egg were all able to form into viable embryos, and not just predetermined parts that Roux believed.