Fig. 6. A section through the eye of a 15-mm human embryo showing the elongation of the primary fibers as they encroach upon the lumen. The crescentic lumen of the lens vesicle still contains some epitrichial cells. Note the anterior migration of the nuclei of the primary fibers, producing an anuclear zone in the posterior portion of the lens and a bow pattern at the lens equator. Carnegie embryo 6520. (Courtesy of Victoria Ozanics, Columbia University)