The phenomenon of induction is one of the most important concepts in embryonic development. Embryonic induction may be defined as the process by which one group of cells and/or cell products causes a second group of cells to differentiate into cells which differ from parent cells. For example, the roof of archenteron causes the overlying ectoderm to differentiate into neural tissue. Modification of genetic and structural characteristics occurs at each induction. Many successive inductions are involved in the development of embryonic structures. The embryo is a dynamic system in which the topographic relations of cells and cell groups are in constant flux. The spatial and temporal order of their relations are critical to the normal development and to the nature of inductive interactions at any given point and time. For example, the original archenteron roof does not remain in contact with the same ectoderm throughout gastrulation; the above structures progressively change their character. The reactive ability of the presumptive ectoderm of the gastrula diminishes with time, and this reactivity is influenced by exposure to previous inductive influences. An initially reacting tissue may even acquire inducing capacity. For example, the neural plate, once induced, is itself capable of inducing early gastrular ectoderm to form neural plate. The optic vesicles also perform an inductive role following their own induction. The phenomenon of successive induction emphasizes the multiplicity and diverse quality of embryonic inductive reactions. Many inducing systems operate during the course of development; one of these, referred to as Spemann’s organizer, is responsible for the formation of the nervous system.
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