germ layer

https://doi.org/10.1351/goldbook.10763

Any of three distinct layers of cells (endoderm, ectoderm, and mesoderm) that become recognizable as the embryo develops.

Source:
PAC, 2016, 88, 713. (Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)) on page 760 [Terms] [Paper]