What are the known characteristics of the ZPA?
1. If ZPA is transplanted (at the correct time) to an
ectopic
site it will respecify tissue.
2. It acts to induce the correct A/P pattern via
a concentration gradient.
3. Acts over a very narrow time frame.
4. The activity of the ZPA is universal among
 quadrapeds and is the same for the fore limb and hind
limb.