in an embryonic vertebrate, the zone of polarizing activity (zpa) is a block of tissue located on the posterior side of a limb bud. the zpa provides information about the anterior-posterior axis of the limb. cells nearest the zpa form the most posterior of digits (like our little finger); cells farthest from the zpa form the most anterior digits (like our thumb). an embryo of a vertebrate with limb buds. on the posterior side of the top limb bud, there is a block of tissue called zpa. how would a vertebrate forelimb bud develop if it had two zones of polarizing activities, one on the posterior side and one on the anterior side of the bud?