13. Description of Video
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
The primitive pit is continuous with a groove, known as the primitive
groove, which runs caudally along the midline of the primitive streak.
14. 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
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
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