By several marking and transplantation methods it is feasible to identify in early embryos the specific areas of groups of cells that ultimately make different organs. Identification of such types of areas helps to construct fate maps of dissimilar parts of embryos at different developmental stages. By using identical methods it has been possible to identify the presumptive areas of forelimbs and hindlimbs and localize exactly in the early embryos of several vertebrates. The prospective forelimb area in the tail bud stage embryo of the salamander, Ambystoma maculatum (Amphibia) is displayed in Figure. It is a circular disc-like area just behind the gill area containing ectoderm underlain by lateral plate mesoderm. The limb is actually made from cells in the central part of this disc and those surrounding this central part provide rise to peribranchial tissue and the shoulder girdle.
Figure: Prospective forelimb bud of Ambystoma a salamander. The central area contain those cells which are destined to form the free limb; the cells surrounding free limb are those which give rise to the peribranchial flank tissue and the shoulder girdle. The cells outside these regions usually are not included in limbs but can regulate to form a limb if more central tissues are removed.