A morphogenetic field is a region of an embryo that forms a discrete structure, such as a limb or heart. Morphogenetic fields became known through the experimental work of Ross G. Harrison, one of the most deserving scientists never to have won a Nobel Prize. The regions are described as fields instead of discrete cells because they can recover from the effects of partial destruction. For example, if half of a salamander's forelimb field is destroyed, it will still develop into a reasonable approximation of a complete limb, not a half-limb. If the limb field is transplanted to a novel region, such as the mid-flank of a host embryo, it will develop into an extra limb. These remarkable and still valid discoveries were widely reported in the scientific and popular media during the Golden Age of experimental embryology in the first half of the 20th century, but have been partially eclipsed by the emergence of more modern, reductionistic approaches to developmental problems.
The morphogenetic field offers important lessons about the nature of development and genetic determination. A morphogenetic field has the property of self-organization, forming the best possible whole from available cells. The field is a cellular ecosystem that will not work if the fates of component cells are predetermined entities that lack the requisite plasticity. The cellular community of a field is coordinated by a chemical gradient and, therefore, is not scalable, which is the reason why all embryos are small and about the same size, whether mouse or great blue whale. Self-organization of a morphogenetic field brings the benefit of error correction, a tremendous advantage to a complex, developing system, where a lot can and does go wrong. For example, if a cell in the field is missing, another will be reprogrammed to take its place, or if an errant cell wanders into the field, its developmental program will be overridden by its neighbors, in both cases forming the best possible whole.
The probabilistic, epigenetic processes of morphogenetic fields force a reconsideration of what it means to be "genetically determined" and illustrate why genes are better understood as recipes than blueprints; genes provide instructions for assembly, not a detailed plan for the final product. For psychologists and other social scientists whose developmental studies are based more on philosophical than biological foundations, morphogenetic fields provide a good starting point for learning how development works. Embryos provide excellent instruction about development for those knowing where to look and how to see.