Myelomeningocele is a sacculation of the meninx and spinal cord through a vertebral defect that develops because of a closure defect of the posterior neuropore
, which is the latest phase of neurulation in the third and fourth weeks of gestation.
Spina bifida with rachischisis occurs when the posterior neuropore
of the neural tube fails to close during week 4 of development.
A short distance posterior to the cross-sectional level in Figure 4G, the stomochord and perihaemal coeloms disappear, and the collar cord emerges at the posterior neuropore
to continue as the dorsal nerve cord of the trunk.