How does gastrulation produce the three embryonic germ layers?

Gastrulation is the critical developmental process during which the blastula is reorganized into three primary germ layers: the ectoderm, mesoderm, and endoderm.12

In higher vertebrates, including humans, this process begins with the formation of two layers: the hypoblast and the epiblast. The epiblast serves as the source for the three definitive layers. During gastrulation, cells of the epiblast migrate toward a specialized midline structure known as the primitive streak.34

The formation proceeds as follows:
* Endoderm: Cells migrate through the primitive streak and displace the hypoblast to form the innermost layer.34
* Mesoderm: Subsequent cells migrating through the streak position themselves in the intermediate space between the epiblast and the newly formed endoderm.3
* Ectoderm: The remaining cells of the epiblast that do not migrate inward become the definitive outermost layer.3

Once these layers are established, they serve as the foundational assembly grounds from which all future tissues and organ systems emerge during organogenesis.13