epithelial-mesenchymal transition in order to lose cell-cell adhesion (E-cadherin), delaminate from the epiblast layer and migrate over the dorsal surface of the epiblast then down through the primitive streak. The first wave of epiblast cells to invaginate through the primitive streak invades and displaces the hypoblast to become the embryonic endoderm. The mesoderm layer is established next as migrating epiblast cells move through the primitive streak then spread out within the space between the endoderm and remaining epiblast, which once the mesoderm layer has formed ultimately becomes the definitive ectoderm. The process of gastrulation results in a trilaminar germ disc, consisting of the ectoderm, mesoderm and endoderm layers. Epiblast diversity Epiblasts exhibit diverse structure across species as a result of early embryo morphogenesis. The human epiblast assumes a disc shape, conforming to the embryonic disc morphology; whereas, the mouse epiblast develops in a cup shape within the cylindrical embryo. During implantation of the blastocyst, both the human and mouse epiblasts form a rosette shape in a process called polarization. Polarization results from the interaction between the mammalian blastocyst and β1-integrin from the extracellular matrix, produced from the extra-embryonic tissues. At this stage, both human and mouse epiblasts consist of a pseudostratified columnar epithelium. Shortly after, the human epiblast will assume a disc shape while the amniotic cavity forms. The epiblast cells adjacent to the trophoblast are specified to become amnion cells. The mouse epiblast transitions from a rosette structure to a cup. A pro-amniotic cavity forms, surrounded by the epiblast cup fused to extraembryonic ectoderm. Mouse epiblast cells are not specified to amnion cell fate. Birds Gastrulation occurs in the epiblast of avian embryos. A local thickening of the epiblast, known as Koller's sickle, is key in inducing the primitive streak, the structure through which gastrulation occurs. Studies on chick embryos have shown that mediolateral cell intercalation occurs before gastrulation. The intercalation event is guided by fibroblast growth factors from the hypoblast. It is suggested that the evolution of the amniote primitive streak from the blastopore was due to the acquisition of the mediolateral intercalation event, which positions the primitive streak and acts independently of mesendoderm formation. Reptiles Ancestors of Amniotes (mammals, birds, reptiles) underwent gastrulation primarily by an infolding of the epiblast layer (involution). Mammals and birds have evolved to rely on ingression during gastrulation where epiblast cells converge at the midline and ingress at the primitive streak. Reptile