"Stretching radially from the inner to the outer limiting membrane in all regions of the retina are certain peculiar-shaped bodies known as the radial fibres of Müller. Each fibre is the outcome of the changes undergone by what was at first a simple columnar epithelial cell. The changes are, in the main, that the columnar form is elongated into that of a more or less prismatic fibre, the edges of which become variously branched, and that while the nucleus is retained the cell substance becomes converted into neuro-keratin. And, indeed, at the _ora serrata_ the fibres of Müller may be seen suddenly to lose their peculiar features and to pass into the ordinary columnar cells which form the _pars ciliaris retinæ_." [Illustration: FIG. 44.--DIAGRAM REPRESENTING THE SINGLE-LAYERED EPITHELIAL TUBE OF THE VERTEBRATE EYE AFTER REMOVAL OF THE NERVOUS AND RETINAL ELEMENTS. _O.n._, axial core of cells in optic nerve; _p._, pigment epithelium; _p.c.r., pars ciliaris retinæ_; _m.f._, Müllerian fibres; _l._, lens.] It is then absolutely clear that the essential parts of the eye may be considered as composed of two parts-- 1. A tube or diverticulum from the tube of the central nervous system, composed throughout of a single layer of epithelium, which forms the supporting axial cells in the optic nerve, the pigment epithelium and the Müllerian fibres of the retina. Such a tube would be represented by the accompanying Fig. 44, and the left side of Fig. 41. 2. The retina proper with the retinal ganglion and the optic nerve-fibres as already described. In this part supporting elements are found, just as in any other compound retina, of the nature of neuroglia, which are independent of the Müllerian fibres. {109}Of these two parts we have already seen that the second is to all intents and purposes a compound retina of a crustacean eye, and seeing that the single-layered epithelial tube is continuous with the single-layered epithelial tube of the central nervous system--_i.e._ with the cephalic part of the gut of the arthropod ancestor--it follows with certainty that the ancestor of the vertebrates must have possessed two anterior diverticula of the gut, with the wall of which, near the anterior extremity, the compound retina has amalgamated on either side, just as the infra-oesophageal ganglia have amalgamated with the ventral wall of the main gut-tube. In this way, and in this way alone, does the interpretation of the structure of the vertebrate lateral eye harmonize in the most perfect manner with the rest of the conclusions already arrived at. The question therefore arises:--Have we any grounds for believing that the ancient forms of primitive crustaceans and primitive arachnids, which were so abundant in the time when the Cephalaspids appeared, possessed two anterior diverticula of the stomach, such as the consideration of the vertebrate eye strongly indicates must have been the case?