Page:Popular Science Monthly Volume 19.djvu/401

Rh body. The eye of the larva likewise disappears, and all that remains to the adult ascidian is a nerve-mass, called by courtesy the "brain," and which serves to regulate the few acts that mark the placid and rooted existence of the race. Attention has been recently directed in a special manner to the resemblance which exists between the eye of the larval sea-squirt and that of vertebrates—a statement to be taken along with that which conversely declares the unlikeness of the ascidian eye to that of all other invertebrate animals. It is matter of fact that the chief parts of the eye of a vertebrate animal grow inward as developments from the skin, and unite with an outgrowth from the brain. This outgrowth forms the retina, a nervous network of the eye, whereon the images of things seen are duly received for transmission to brain and sensorium. Now, in invertebrate animals the retina is formed from the skin-layer. This latter method of growth, it has been remarked, is a perfectly natural one. It was to be expected that, as the retina is to be affected in the discharge of its duty by light rays, it should form on the surface of the body where the light-rays fall. In the vertebrate, and in the sea-quirt larva, the retina, on the contrary, forms away below the skin-surface, and grows outward from the brain. Why is this so? Professor Ray Lankester maintains that because the ascidian larva is perfectly transparent, the light-rays pass through to its brain-eye, and thus give rise to sensations of sight. Hence, if the original and primitive vertebrate animal or root-stock were like the larval sea-squirt, as we suppose it to have been, its body would be transparent, and its eye or eyes, situated on its brain, would receive light-rays through its clear body. But, as the evolution of the vertebrate race proceeded, the tissues became firmer and denser. By "natural selection"—or, in other words, by the exercise of accommodating power to function the eyed region of the brain would tend to grow more and more toward the body's surface, to receive the rays of light. As development, therefore, proceeded, the mode of growth of the vertebrate eye would be adapted to the exigencies of its new surroundings. Thus, to-day, the vertebrate eye grows from without inward, because light-rays strike naturally on the outer surface of the body. But it likewise grows from within outward as well, because of the ancestral and hereditary tendencies which cause it to repeat in the individual growth the passage to the surface it had to make in the evolution of the race. If one might add a suggestion to such an explanation, it would consist in an endeavor to account for that affinity between brain and outer surface of body which we see to exist. Why the brain should grow outward, as it does in eye, ear, and nose likewise, to connect with the body's surface, and so to form organs of sense, is plain enough. We must bear in mind that the brain itself is formed from the outer layer or epiblast of the larva, and from the same layer which develops into the skin. Brain and skin, to begin with, arise from the same layer. Hence, before even the matter of