The Process of Gastrulation

by the end of the second week of development the bilaminar embryonic dis consisting of the hypoblast and epiblast has formed throughout the third week of development this bil laminer dis differentiates to establish three primary germ layers in a process known as gastrulation around 15 days after fertilization a thickened structure forms along the midline in the epiblast

near the coddle 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 coddle ends towards the 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 this depression continues along the midline of the epiblast towards the coddle end of the streak forming a primitive Groove once formed cells of the epiblast migrate inwards towards the streak

detach from the epiblast and slip beneath it into the interior of the embryo 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 proximal cell layer which is referred to as the

definitive endoderm by day 16 the majority of the hypoblast has been replaced the remaining cells of the epiblast are now referred to as the ectoderm and forms the most exterior distal layer some of the invaginated epiblast cells remain in the space between the ectoderm and newly formed defin endoderm these cells form a germ layer