Spina bifida. Artwork showing the three main types of the birth defect spina bifida. The condition arises from the incomplete closure of the neural tube during embryonic development, which prevents the bones of the spine fusing properly. If the gap in the bones is large enough, part of the spinal cord and its surrounding membranes (meninges) can protrude through the bones. This is known as a myelomeningocele (top right). It is the most serious form of spina bifida and can cause paralysis below the defect. If the gap is smaller only the meninges will protrude, known as meningocele (bottom right). With very small gaps there will not be a protrusion, but the skin over the gap may be hairy, dimpled, or darkened, known as spina bifida occulta. These last two types rarely cause symptoms.