Failure of the neural tube to close, resulting in the absence of part of the vertebral arch at the midline of the spinal column and exposure of the spinal cord and its covering membranes.
Source: PAC, 2016, 88, 713. 'Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)' on page 807 (https://doi.org/10.1515/pac-2015-1202)