Time, place and positional value in the chick limb-bud.
Quantitative experimental evidence is presented for the progress zone theory of limb development. The theory, here formulated mathematically, states that the parts of the limb are specified in proximo-distal succession by an autonomous timing mechanism operating in a 'progress zone' of undifferentiated growing mesenchyme under the influence of the apical ectodermal ridge. By the exchange of distal tips between young and old wing-buds, it is shown that there are no long-range morphogenetic signals from proximal to distal tissue. The width of the progress zone is calculated, and it is found autoradiographically that practically all its cells are dividing.
D000818:Animals; D002454:Cell Differentiation; D002455:Cell Division; D002642:Chick Embryo / Q000254:growth & development*; D008954:Models, Biological; D009024:Morphogenesis; D013997:Time Factors; D014184:Transplantation, Homologous; D014921:Wings, Animal / Q000196:embryology / Q000637:transplantation