During the first month after conception, the embryo develops a primitive tissue structure known as the neural tube. This structure gradually develops into bones, nerves, and tissue. These eventually form the nervous system, the spinal column, and the spine, a ridge of bone that protects and surrounds the nerves. In spina bifida, the neural tube and spinal column do not develop properly. The spine does not close fully, and the spinal column remains exposed along several vertebrae. A sac forms on the fetus’ back as the membranes and spinal cord push out. The sac may be covered with meninges, or membranes