anencephaly Fyler:4332 anencephalia MP:0000915 anomaly that occurs when the anterior neuropore of the neural tube fails to close, resulting in the congenital partial or complete absence of the cranial vault accompanied by absence of overlying tissues, including the brain and cerebral hemispheres, skull and scalp; in affected organisms, the forebrain remains in contact with the amniotic fluid and subsequently degenerates, including the cerebrum (areas responsible for cognition); any remaining brain tissue, such as the brain stem, is often exposed MPheno.ontology MP:0001890 abnormal nervous system development