. Comparative anatomy. Anatomy, Comparative. THE SENSE ORGANS 589 Where the optic vesicle, as it expands, comes into contact with the ectoderm, a local thickening of the ectoderm forms as the anlage of the lens. This placode thickens, invaginates, and sinks below the surface to form a small hollow vesicle, which eventually loses connexion with the skin. The lens vesicle is finally converted into a solid lens by the thicken- ing of its medial wall, the epithelial cells of which become elongated into fibers and arranged in layers like the coats of an Fig. 486.—Sections of successive stag
. Comparative anatomy. Anatomy, Comparative. THE SENSE ORGANS 589 Where the optic vesicle, as it expands, comes into contact with the ectoderm, a local thickening of the ectoderm forms as the anlage of the lens. This placode thickens, invaginates, and sinks below the surface to form a small hollow vesicle, which eventually loses connexion with the skin. The lens vesicle is finally converted into a solid lens by the thicken- ing of its medial wall, the epithelial cells of which become elongated into fibers and arranged in layers like the coats of an Fig. 486.—Sections of successive stages in the development of the lens of the eye from the first thickening of the ectoderm (ec) to the complete separation of the lens, /. (From Kingsley's "Comparative Anatomy of ;) While the lens is undergoing these changes, it becomes enclosed by the optic cup, formed by invagination of the optic vesicle. Possibly the ingrowth and enlargement of the lens is a factor in this invagination. The optic vesicle is thus converted into a two-layered optic cup, attached to the brain by the optic stalk. The cup is, however, incomplete, for a fissure CONJUNCTIVA- CORNEA- AQUEOUS HUMOR- IRIS. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Neal, Herbert V. (Herbert Vincent), 1869-1940; Rand, Herbert W. (Herbert Wilbur), 1872-1960. Philadelphia : P. Blakiston's Son
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