Developmental brain defect: Magnetic Resonance Image (MRI) of the head of a baby girl (aged 20 months) suffering from holoprosencephaly, a mal- formation of the brain that results in the presence of a large abnormal ventricle (fluid- filled cavity), the dark area over much of this image. The cerebral hemispheres fail to form, the olfactory system (smell) is often absent, & defects occur in the eyes, optic nerves & pituitary. A range of facial deformities reflects the severity of the brain defect, as does the degree of mental retardation. Chromosome defects such as trisomy 13 & 18 are frequent causes.