{103}"The formation of the eye," as taught by Balfour, "commences with the appearance of a pair of hollow outgrowths from the anterior cerebral vesicle. These outgrowths, known as the optic vesicles, at first open freely into the cavity of the anterior cerebral vesicle. From this they soon, however, become partially constricted, and form vesicles united to the base of the brain by comparatively narrow, hollow stalks, the rudiments of the optic nerves." "After the establishment of the optic nerves, there takes place (1) the formation of the lens, and (2) the formation of the optic cup from the walls of the primary optic vesicle." He then goes on to explain how the formation of the lens forms the optic cup with its double walls from the primary optic vesicle, and says-- "Of its double walls, the inner, or anterior, is formed from the front portion, the outer, or posterior, from the hind portion of the wall of the primary optic vesicle. The inner, or anterior, which very speedily becomes thicker than the other, is converted into the retina; in the outer, or posterior, which remains thin, pigment is eventually deposited, and it ultimately becomes the tesselated pigment-layer of the choroid." The difficulties in connection with this view of the origin of the eye are exceedingly great, so great as to have caused Balfour to discuss seriously Lankester's suggestion that the eye must have been at one time within the brain, and that the ancestor of the vertebrate was therefore a transparent animal, so that light might get to the eye through the outer covering and the brain-mass; a suggestion, the unsatisfactory nature of which Balfour himself confessed. Is there really evidence of any part of either retina or optic nerve being formed from the epithelial lining of the tube?