ABSTRACT
The retinocephalic folds (retinocephalon) are rostral expansions of the prosencephalic wall (stage 6-2) related to the rostral end of the notochord. The retinocephalic folds change into optic vesicles before the anterior neuropore closes. The lateral walls of the optic vesicles contact the surface ectodermal epithelium. The contacted ectodermal areas change into the lens placodes. Consequently, the optic vesicles transform into optic cups and, simultaneously, the lens placodes transform into the lens grooves and become detached from the surface as lens vesicles. The lens vesicles are located inside the optic cups. The optic cup is temporarily inferiorly incomplete with a deep notch, which represents the hyaloid fissure. The hyaloid artery runs to the lens vesicle within the hyaloid fissure. As the optic cup expands, the margins of the hyaloid fissure fuse over the artery and the fissure disappears. The cells of the posterior epithelium of the lens vesicle elongate, fill the cavity of the lens vesicle and become primary lens fibers. The secondary lens fibers form during the fetal period and are apposed to the primary fibers.
