The gene for microphthalmia in the homozygous cinnamon mouse produces colobomatous microphthalmia due to failure of closure of the optic fissure. Optic fissure formation and closure were examined in control cinnamon and homozygous microphthalmic foetuses from the 10th to the 13th gestational day using light and electron microscopy. This study showed that basal lamina degeneration and cell death occurred in the area of fusion in the control eyes. Examination of the optic fissure in homozygous microphthalmic foetuses showed cell death in the fissure margins, but there was failure of optic fissure closure associated with persistence of the basal lamina. It is therefore suggested that absence or abnormality of programmed disintegration of the basal lamina prevents fusion in the mutant leading to the development of colobomatous microphthalmia.