"The Potency of the First Two Cleavage Cells in Echinoderm Development. Experimental Production of Partial and Double Formations" (1891-1892), by Hans Driesch

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 twocelled sea urchin [5] (Echinus microtuberculatus) 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 [6]'s experiments with frog [7] eggs from which Roux had concluded that embryonic cells have predetermined fates?an embryonic cell couldn't form into one thing when separated from other embryonic cells and form into something else when left unseparated. To Roux, embryos were mosaics of cells, all of which were important and necessary for viable [8] embryos to form. Driesch, in contrast, showed that individual cells resulting from cleavage of the fertilized egg [9] were all able to form into viable [8] embryos, and not just into predetermined parts as Roux believed.