One of the key issues in developmental biology is the question of specification. Specification is the commitment of a group of cells to proceed down a developmental pathway without further extraneous signals. In order identify the development period in which cells have acquired all the necessary information to become committed to a developmental pathway, such as the formation of an organ or tissue, cells are explanted from embryos at various stages and cultured in vitro in a neutral medium. Analysis of the tissues that are formed by the explants allows the investigator to define the developmental period during which specification of a tissue is completed. From these results, one can infer that if cell-to-cell signals are involved in specification of the tissue, they must have occurred earlier than the defined developmental period.
