Mechanistically neural tube closure can be described as the bending, elevation and fusion of neural folds.
Neural tube closure is initiated at several points along the neural tube, at different developmental stages. In a mouse embryo, the tips of the neural ridges typically have met and fused at the midline of the hindbrain/cervical boundary first, and neural tube fusion proceeds both rostrally (toward the hindbrain) and caudally (toward the trunk).
Closure initiated from this point is named closure 1. Next, the neural ridges meet and fuse at the forebrain/midbrain boundary, initiating closure 2, which also proceeds bi-directionally from the contact point.
A third fusion initiation point begins at the rostral end of the neural plate. This closure 3 proceeds caudally and meets the rostrally directed wave of closure 2 to seal the anterior neuropore. The caudally directed closure 2 meets the rostrally directed closure 1 to seal the midbrain-hindbrain neuropore.
What is the MOST likely consequence of failure in closure 2?