Some of the cells within the primitive streak invaginate, giving rise to a third germ layer, the mesoderm. As differentiation continues throughout development, the three germ layers give rise to all other tissues and organs of the body. Mesoderm cells along the main axis give rise to a notochord, which is eventually replaced by the vertebral column. Neural folds fuse to form a neural tube, which gives rise to the spinal cord and brain. Induction probably requires the presence of certain chemicals that turn genes on.
Somites arise from the mesoderm.
These become the muscles of the body and the vertebrae. A body cavity called the coelom forms and is lined by mesoderm. The coelom becomes the thoracic and abdominal cavities. Extraembryonic membranes extend out beyond the embryo. The amnion envelops the fetus in protective fluid.