Spemann organizer

https://doi.org/10.1351/goldbook.11235

Group of cells in the amphibian embryo that is important in orientation of surrounding cells and facilitates development of the central nervous system.

Note: Cells on the ventral side of the Xenopus blastula secrete factors such as bone morphogenetic protein-4 (BMP-4) to signal the overlying ectoderm to become skin. The Spemann organizer blocks the action of BMP-4 by secreting chordin and noggin, allowing the ectoderm to develop into the brain and spinal cord by default.

Source:
PAC, 2016, 88, 713. 'Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)' on page 806 (https://doi.org/10.1515/pac-2015-1202)