The Spemann organizer transplantation experiment in Xenopus. A small region of the dorsal blastopore lip transplanted into the ventral side of a gastrula host (lower right, albino transplanted tissue) results in a perfectly patterned second body axis. The Spemann organizer region has been subjected to saturating molecular screens that identified many novel secreted growth factor antagonists such as Chordin, Noggin, Frzb, Cerberus, and Dickkopf. (Reproduced with permission from De Robertis JB, 2006, Nat Rev Mol Cell Biol 7: 296–302, © Springer Nature).