During early development of the mammalian eye, invagination and differential growth result in the formation of a cleft, the optic fissure, through which the hyaloid artery reaches the interior of the optic cup. Closure of this fissure was studied by electron microscopy in hamster embryos from days 10 through 12 of gestation. Closure occurred only when and where the basal lamina, which invests the entire wall of the optic cup, had disappeared. No morphological evidence was found that indicated a mechanism for the breakdown of the basal lamina lining the fissure, the fusion of surface cells of opposing sides and restoration of the basal lamina along inner and outer margins of the wall after closure. While in previous light microscopic investigations of the developing human eye eversion of the inner layer into the fissure has been reported, an inversion of the outer layer was found in this study. During inversion cells of the outer layer temporarily changed their orientation. While most of these cells remained within the layer when it returned to its normal position, some cells became separated and degenerated. Inversion of the outer layer, breakdown of the basal lamina and degeneration of superfluous cells appear to be necessary events for a normal closure to occur.