Fig. 46-2. Summary of the early development of the eye. A, an embryo of 4 weeks showing the optic vesicle. B, a section through the head including the diencephalon (Di) and both optic vesicles. C, the optic vesicle presents retinal and lens discs. D, the retinal and lens discs have become indented to form the optic cup and the lens pit respectively. E, the lens pit has become closed from the surface to form the lens vesicle. F, the optic stalk (future optic nerve) and cup, showing the retinal fissure and the lens. G, section of the eye at the end of the embryonic period (8 postfertilization weeks). The pigmented and inverted strata of the retina are evident. The rim of the optic cup will give rise to the ciliary and iridial parts of the retina. H, the postnatal retina comprises the pigmented stratum (layer 1) and the nervous stratum (layers 2-10 of the retina). The broad, white arrow shows the direction of the light, which reaches as far as the pigmented stratum. The neural pathway, which proceeds in the reverse order, includes (1) photo receptors [cones and rods], (2) bipolar cells, and (3) ganglion cells, which give rise to the fibers of the optic nerve. Modified from R. O'Rahilly and F. Muller, Human Embryology and Teratology, Wiley-Liss, New York, second edition, 1996. Click for high resolution image.