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By the end of the second week of development, the bilaminar embryonic disc, consisting of the hypoblast and epiblast, has formed.
Throughout the third week of development, this bilaminar disc differentiates to establish three primary germ layers, in a process known as gastrulation.
Approximately 15 days after fertilization, a thickened structure forms along the midline in the epiblast, near the caudal end of the bilaminar embryonic disc. This is called the primitive streak.
At this stage, the formation of the primitive streak defines the major body axes of the embryo, including the cranial end (towards the head) and caudal end (towards the tail) as well as the left and right sides of the embryo.
At the cranial end of the embryonic disc, the primitive streak expands to create a primitive node, which contains a circular depression, known as a primitive pit. The primitive pit is continuous with a groove, known as the primitive groove, which runs caudally along the midline of the primitive streak.
Once the primitive groove has formed, cells of the epiblast migrate inwards towards the streak, detach from the epiblast, and slip beneath into the interior of the embryonic disc. This process is known as invagination.
The first cells to invaginate through the primitive groove invade the hypoblast, and displace its cells. The hypoblast cells are eventually completely replaced by a new cell layer, which is referred to as the definitive endoderm.
By day 16, the majority of the hypoblast has been replaced. Some of the invaginated epiblast cells remain in the space between the epiblast and newly formed definitive endoderm.
These cells form a germ layer known as the mesoderm, which subsequently forms the notochord, the basis for the axial skeleton.
Once the formation of the definitive endoderm and mesoderm is complete, epiblast cells no longer migrate towards the primitive streak.
At this point, the remaining cells of the epiblast are referred to as the ectoderm, and form the third germ layer.
The ectoderm forms from the cranial to the caudal end of the embryo, such that by the end of the third week, the three primary germ layers complete the embryonic disc.
The gastrulation process is finally complete.