The start of gastrulation in week 3 is marked by the appearance of a midline structure, a depression, on the epiblast called the primitive streak, shown schematically here.

Cells from the epiblast invade this depression, and migrate into the hypoblast, displacing the cells here entirely, and forming a new layer of cells: the definitive endoderm (or secondary endoderm).

Other cells from the epiblast then migrate between the endoderm and epiblast and form a third germ layer - the intraembryonic mesoderm.

This invasion of the epiplast cells into the space between the epiblast and hypoblast is called gastrulation.

The process of gastrulation converts the bilaminar germ disc into a trilaminar germ disc.

The epiblast is now known as the ectoderm.

Thus, the ectoderm, mesoderm and definitive endoderm are all derived from the epiblast.

Cells remaining in the epiblast, when gastrulation is finished, give rise either to surface ectoderm, or to the neurectoderm of the neural plate.

A tube of mesoderm forms and extends cranially from the primitive node to form the notochordal process, which will develop into the notocord.

The notocord , and axial mesoderm enable the formation of the neural plate in the epiblast, in the cranial region, which is the beginning of the central nervous system.

The lateral lips of the neural plate give rise to the neural crest cells. these cells detach and migrate into the embryo to form a variety of important structures.

Mesoderm lateral to the midline, forms the paraxial, intermediate and lateral plate mesoderm. The paraxial mesoderm eventually becomes organised into somites.

The somites give rise to most of the axial skeleton (vertebrae, occipital bone, volountary musculature of neck, body wall and limbs) and to part of the dermis of neck and trunk.