Failure of cranial neural tube closure results in exencephaly in which the brain is totally exposed or actually extruded through an associated skull defect. Abortion of the foetus is almost inevitable, but if it survives the brain at birth consists predominantly of hind brain which has degenerated to a spongiform mass-anencephaly. Anencephaly is found in about 1:1000 live births. At times it is associated with an extensive defect of spinal neural tube closure (rachischisis).

Cranioschisis consists of a skull defect through which various tissues protrude. The defect is usually in the midline region of the occiput or between the frontal and nasal bones. Herniation of the arachnoid alone produces a meningocoele, of both the malformed brain tissue and arachnoid, a meningo-
encephalocoele.
Malformations which involve the caudal end of the neural tube and the overlying vertebral arches constitute the various forms of spinal bifida .