Group of cells at the anterior end of the primitive streak, involved in secreting chemical signals that regulate differentiation of the germ layers during gastrulation.
Source:
PAC, 2016, 88, 713. 'Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)' on page 796 (https://doi.org/10.1515/pac-2015-1202)