 Wolpert’s answer was that each cell in a developing system has a unique value reflecting its position in that system, encoded by the spatial gradient of a diffusible molecule across the system. Specification of a cell’s position in the system is relative to one or more points in the system: the source(s) of the diffusible molecule or cell property. Cells that share the same set of points relative to which their position is specified make up a field. Wolpert used this model to explain a range of experimental results and observations of development in various species (as well as the toy “French flag” example). In 1969, Wolpert saw the next task as discovering and articulating rules (and underlying mechanisms) for specification of positional information and polarity, the nature of points and boundaries, and “new meaning to classical concepts such as induction, dominance and field” (1969: 1).