Before saying anything further on the subject of degeneration, it seems desirable once more to direct attention to the myelonic or cerebral eye which the Ascidian tadpole possesses in common with all Vertebrates. All other animals which have eyes develop the retina or sensitive part of the eye from their _outer skin_ (See Figs. 21 and 22, and explanation.) It is easy to understand that an organ which is to be affected by the light should form on the _surface_ of the body where the light falls. It has long been known as a very puzzling and unaccountable peculiarity of Vertebrates, that the retina or sensitive part of the eye grows out in the embryo as a bud or vesicle of the brain, and thus forms _deeply_ below the surface and _away from the light_ (see Fig. 23, and explanation). The Ascidian tadpole helps us to understand this, for it is perfectly transparent and has its eye actually _inside_ its brain. The light passes through the transparent tissues and acts on the pigmented eye, lying deep in the brain. We are thus led to the conclusion—and I believe this inference to be now for the first time put into so many words—that the original Vertebrate must have been a transparent animal, and had an eye or pair of eyes _inside_ its brain, like that of the Ascidian tadpole. As the tissues of this ancestral Vertebrate grew denser and more opaque, the eye-bearing part of the brain was forced by natural selection to grow outwards towards the surface, in order that it might still be in a position to receive the influence of the sun’s rays. Thus the very peculiar mode of development of the Vertebrate eye from two parts, a brain-vesicle (Fig. 23, _A a_, and _B p r_), and a skin-vesicle (Fig. 23, _B e, l_), is accounted for. [Illustration: FIG. 22.—Section through the eye (“surface-eye”) of a Marine Worm (Neophanta). _i_, integument spreading over the front of the eye _c_; _l_, cuticular lens; _h_, cavity occupied by vitreous body; _p_, retinal cells; _b_, pigment; _o_, optic nerve: _o´_, expansion of optic nerve.] [Illustration: FIG. 23.—_A._ Vertical section through the head of a very young fish, showing in the centre the cavity of the brain _c_. On each side is a hollow outgrowth (_a_) which will form the retina of the fish’s eye (“cerebral eye”); _b_, will become the optic nerve connecting the brain and the retina; _d_, integument.—_B._ Later condition of the hollow outgrowth (_a_) of _A_. Its outer wall _r_ is pressed against its deeper wall _p_ by an ingrowth (_l_) from the outer skin (ectoderm) _e_; _r_, gives rise to the retinal cells, whilst only _l_, the cellular lens, is derived from the surface of the skin.]