Cyclops: A major event in early embryogenesis is cleavage of the cerebral vesicle to form two telencephalic vesicles. Defective cleavage leads to the condition holoprosencephaly (arrhinencephaly). In its extreme form holoprosencephaly is accompanied by failure of development of eyes and midline facial structures. As in this example, there may be only a single eye, and absence of the nose (cyclops). Cleft palate is often an associated defect.