Organizers in animal embryos

Transplantation experiments played a central role in early embryology and demonstrated the long-range organizing activity of embryonic tissues. (a) The Spemann organizer. The dorsal blastopore “lip” of an early amphibian embryo can induce a second embryonic axis and embryo when transplanted to the ventral region of a recipient embryo. (b) In the developing chick vertebral limb bud, the zone of polarizing activity (ZPA) organizes pattern along the anteroposterior axis. Transplantation of the ZPA from a posterior to anterior position induces extra digits with reverse polarity.