Anencephaly is absence of the fetal cranial vault. Exposure of cerebral tissue to amniotic fluid precludes brain development. Anencephaly is a form of neural tube defect that is typically an isolated birth defect that is not related to chromosomal abnormalities. Embryologically, there is failure of the anterior neuropore to close, resulting in failure of the calvaria to form (called acrania). The lack of a protective vault leaves any developing brain vulnerable to destruction. Technically, the term 'meroanencephaly' is more correct, since there is typically some residual neural tissue at the base of the defect, but this term is not widely used (likewise the term 'Caledonian Canal' is not recognized by most persons, while 'Loch Ness' is).