- Each vertebra develops in three parts - the body and two halves of the neural arch.  These ossify in cartilage and, at birth, the three parts are still separate but they soon fuse.  Failure of such fusion gives rise to the condition of spina bifida.  This may be symptomless (spina bifida occulta) although the site of the lesion may be marked by a tuft of hair.  If the defect is large, the meninges or even the spinal cord may herniate onto the surface (meningocele) and this may produce neurological symptoms.

Reference:
1. Anatomy at a Glance. Omar Faiz and David Moffat.  Blackwell Science. 2002.