primitive streak

https://doi.org/10.1351/goldbook.11131

Structure forming in the blastula during early embryogenesis that establishes bilateral symmetry and determines the site of gastrulation and germ layer formation.

Source:
PAC, 2016, 88, 713. (Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)) on page 796 [Terms] [Paper]