ORGANOGENESIS
In organogenesis, or organ formation, the cells of the three germ layers carry on the progression of pattern formation that culminates in the formation of definite structures. The ectoderm ultimately gives rise to the outer layer of skin, the nervous system, and sense organs. Tissues that ultimately line the digestive tract, and organs that form as outgrowths of the digestive tract (including the liver, pancreas, and lungs) all develop from the endoderm. Skeletal tissue, muscle, and the circulatory, excretory, and reproductive systems all have their origin in the mesoderm.

The notochord, brain, and spinal cord develop in the early vertebrate embryo. Initially, the notochord, which is mesodermal tissue, grows forward along the length of the embryo as a cylindrical rod of cells. The vertebral column eventually replaces the notochord but remnants will remain in the discs of cartilage between the vertebrae.