Development During the third week, a process called gastrulation creates a mesodermal layer between the endoderm and the ectoderm. This process begins with the formation of a primitive streak on the surface of the epiblast. The cells of the layers move between the epiblast and the hypoblast, and begin to spread laterally and cranially. The cells of the epiblast move toward the primitive streak and slip beneath it, in a process called "invagination". Some of the migrating cells displace the hypoblast and create the endoderm, and other cells migrate between the endoderm and the epiblast to create the mesoderm. The remaining cells form the ectoderm. After that, the epiblast and the hypoblast establish contact with the extraembryonic mesoderm until they cover the yolk sac and amnion. They move onto either side of the prechordal plate. The prechordal cells migrate to the midline to form the notochordal plate. The chordamesoderm is the central region of trunk mesoderm. This forms the notochord, which induces the formation of the neural tube, and establishes the anterior-posterior body axis. The notochord extends beneath the neural tube from the head to the tail. The mesoderm moves to the midline until it covers the notochord. When the mesoderm cells proliferate, they form the paraxial mesoderm. In each side, the mesoderm remains thin, and is known as the lateral plate. The intermediate mesoderm lies between the paraxial mesoderm and the lateral plate. Between days 13 and 15, the proliferation of extraembryonic mesoderm, primitive streak, and embryonic mesoderm take place. The notochord process occurs between days 15 and 17. Eventually, the development of the notochord canal and the axial canal takes place between days 17 and 19, when the first three somites are formed.
Development During the third week a process called gastrulation creates a mesodermal layer between the endoderm and the ectoderm This process begins with the formation of a primitive streak on the surface of the epiblast The cells of the layers move between the epiblast and the hypoblast and begin to spread laterally and cranially The cells of the epiblast move toward the primitive streak and slip beneath it in a process called "invagination" Some of the migrating cells displace the hypoblast and create the endoderm and other cells migrate between the endoderm and the epiblast to create the mesoderm The remaining cells form the ectoderm After that the epiblast and the hypoblast establish contact with the extraembryonic mesoderm until they cover the yolk sac and amnion They move onto either side of the prechordal plate The prechordal cells migrate to the midline to form the notochordal plate The chordamesoderm is the central region of trunk mesoderm This forms the notochord which induces the formation of the neural tube and establishes the anterior-posterior body axis The notochord extends beneath the neural tube from the head to the tail The mesoderm moves to the midline until it covers the notochord When the mesoderm cells proliferate they form the paraxial mesoderm In each side the mesoderm remains thin and is known as the lateral plate The intermediate mesoderm lies between the paraxial mesoderm and the lateral plate Between days 13 and 15 the proliferation of extraembryonic mesoderm primitive streak and embryonic mesoderm take place The notochord process occurs between days 15 and 17 Eventually the development of the notochord canal and the axial canal takes place between days 17 and 19 when the first three somites are formed