The notochord is a cellular, rod-shaped body that is present in the embryos of all chordates, including humans. It is comprised of cells from the mesoderm, and functions by designing the axis of the embryo.
In humans, the notochord is responsible for developing into the central nervous system, which includes the brain and the spinal cord. It also is responsible for defining the polar axis of the human body and determining which end of the embryo develops into the mouth, and which end develops into the anus and rectum. Notochord development begins in the early days of embryo development, and continues throughout many of the first weeks of pregnancy.