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Slowly an important idea emerged: the gradient hypothesis. One of the proposers of this idea was Theodor H. Boveri of the University of Würzburg, the founder of the chromosomal theory of inheritance. Boveri suggested that "a something increases or decreases in concentration" from one end of an egg to the other. The hypothesis, in essence, is that cells in a developing field respond to a special substance - a morphogen - the concentration of which gradually increases in a certain direction, forming a gradient. Different concentrations of the morphogen were postulated to cause different responses in cells.