The Song of the Cell: An Exploration of Medicine and the New Human

by Siddhartha Mukherjee

The embryo is now ready for the final sequence of activities. Within the mesoderm, a series of cells assemble along a thin axis to form a rodlike structure called the notochord, which spans from the front of the embryo to its back. The notochord will become the GPS of the developing embryo, determining the position and axis of the internal organs as well as secreting proteins called inducers. In response, just above the notochord, a section of the ectoderm—the outer layer—invaginates, folding inward and forming a tube. This tube will become the precursor of the nervous system, made up of the brain, spinal cord, and nerves. In one of embryology’s many ironies, having set up the framework of the embryo, the human notochord will lose its prominence and function between embryonic development and adulthood. Its only cellular remnant in the adult human body is the pulp that remains stuck between the skeletal bones. In the end, the master maker of the embryo is trapped inside the bony prison of the very creature it has created.