This structure develops into the neuralfolds which fuse to form the neural tube. The fusion begins in the cervical region but this process proceeds in a somewhat irregular fashion. By the end of the 4th week the central nervous system is a closed tubular structure. The cranial and caudal neuropores close approximately on the 25th and 27th days respectively. The notochord is replaced by the cells that form the future vertebral bodies.
Neural arches develop from selerotomal material and fuse dorsally to form the spinous processes.
Failure of this fusion causes complete or partial rachischisis. This may follow faulty induction by the underlying notochord. The timing of these embryological processes is significant in terms of NTD which are probably established by the end of the 4th week after conception. In spina bifida occulta the defect cannot be seen externally. In most of these situations there is a cutaneous marker at the L2-L3 level eg, a tuft of hair, skin tags, dimples, lipoma, aplasia cutis congenita.