The three-layered embryonic disc (ectoderm, mesoderm, endoderm) forms from the two-layered disc through a process called:

1. Early in the third week, the embryonic disc has only two cell layers, the epiblast and hypoblast. 2. Cells then migrate through a structure called the primitive streak to form new layers. 3. This overall process gastrulation, which produces the three germ layers. 4. Neurulation is a later, more specific process forming the neural tube from ectoderm, so it is ruled out as the name for germ layer formation itself. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 28 "Development and Inheritance", p.1330-1331 §Embryogenesis_