gastrulation

https://doi.org/10.1351/goldbook.10750

Stage of embryo development in which the two-layered blastula transforms into the gastrula by developing ectoderm and endoderm, as well as a third layer (mesoderm), through the movement of specific cells.

Source:
PAC, 2016, 88, 713. (Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)) on page 758 [Terms] [Paper]