The special regions of embryonic tissue that induces the development of structures in new places are called organizers. These organizers have remarkable ability to maintain the development of surrounding tissues. The produced substances are called morphogens.
Morphogens are signaling molecules which use the process of diffusion. They induce various responses in surrounding tissue in a concentration-dependent manner. It is impossible to separate the morphogens by biochemical techniques because it may be a protein or a glycoconjugate or a hormone. Morphogens are also present in very minute quantities.
Good examples for organizers are parts of amphibian embryo and chick limb bud. When a part of a developing amphibian embryo (The Spemann organizer) is transplanted to another site in a recipient embryo, it induces the surrounding tissues to develop into second body axis.
In the same way, when the posterior part of developing chick limb bud (Zone of polarizing activity- ZPA) is transplanted to the anterior part, it induces the formation of extra digits with reversed polarity.