Lack of closure
In the fetus the spine develops with two parts that come together. The spinal cord is a flat strip that folds up and closes.
If these two parts do not fold up and close, they leave an opening in the spine with the spinal cord hanging loose. This appears as a sack usually at the base of the spine. The nerves to the legs, bowels and bladder
are often involved. The sack needs to be closed.