"The epithelium of the optic stalk develops entirely into supporting cells, which in Petromyzon fill up the original lumen and so form an axial core (Axenstrang) to the nerve-fibres which are formed entirely outside them; the projections of these supporting cells are directed towards the periphery, and so separate the bundles of the optic nerve-fibres. The mesodermal coat of the optic stalk takes no part in this separation; it simply forms the connective tissue sheath of the optic nerve. The development of the optic nerve in the higher vertebrates also obeys the same law, as I am bound to conclude from my own observations."
The evidence, then, of Ammocœtes is very conclusive. Originally a tube composed of a single layer of epithelial cells became expanded at the anterior end to form a bulb. On the outside of this tube or stalk the fibres of the optic nerve make their appearance, arising from the ganglion-cell layer of the retina, and, passing over the surface of the epithelial tube at the choroidal fissure, proceed to the brain by way of the optic chiasma. Owing to the large number of fibres, their crossing at the junction of the stalk with the bulb, and the narrowness at this neck, the obliteration of the lumen of the tube which takes place in the stalk is carried out to a still greater extent at this narrow part. The result of this is that all continuity of the cell-layers of the original tube of the optic stalk with those of both the inner and outer walls of the bulb is interrupted, and all that remains in this spot of the original continuous line of cells which connected the tube of the stalk with that of the bulb are possibly some of the groups of cells which are found scattered among the fibres of the optic nerve at their entrance into the retina. Such separation of the originally continuous elements of the epithelial wall of the optic stalk, which is apparent only at this neck of the nerve in Petromyzon, takes place along the whole of the optic nerve in the higher vertebrates, so that no continuous axial core of cells exist, but only scattered supporting cells.
If further proof in support of this view be wanted, it is given by the evidence of physiology, which shows that the fibres of the optic nerve are not different from other nerve-fibres of the central nervous system, but that they degenerate when separated from their nerve-cell, and that the nerve-cell of which the optic nerve-fibre is a process is the large ganglion-cell of the ganglionic layer of the retina. The origin of the ganglionic layer of the retina cannot therefore be separated from that of the optic nerve-fibres. If the one is outside the epithelial tube, so is the other, and what holds true of the ganglionic layer must hold good of the rest of the retinal ganglion and, from all that has been said, of the retina itself. We therefore come to the conclusion that the evidence is distinctly in favour of the view, that the retina and optic nerve in the true sense are structures which originally were outside a non-nervous tube, but, just like the central nervous system as a whole, have amalgamated so closely with the elements of this tube as to utilize them for supporting structures. One part of this non-nervous tube, its dorsal wall, like the corresponding part of the brain-tube, still retains its original character, and by the deposition of pigment has been pressed into the service of the eye to form the pigmented epithelial layer.
We can, however, go further than this, for we know definitely in the case of the retina what the fate of the epithelial cells lining this tube has been. They have become the system of supporting structures known as Müllerian fibres.
The epithelial layer of the primary optic vesicle can be traced into direct continuity with the lining epithelium of the brain cavity, as a single layer of epithelial cells in the core of the optic nerve, forming the optic stalk, which, in consequence of close contact, becomes the well-known axial layer of supporting cells. This epithelial layer of the optic stalk then expands to form the optic bulb, the outer or dorsal wall of which still remains as a single layer of epithelium and becomes the layer of pigment epithelium. This layer of epithelium becomes doubled on itself by the approximation of the inner or ventral wall of the optic cup to the outer or dorsal wall in consequence of the presence of the lens, and still remaining a single layer, forms the pars ciliaris retinæ; then suddenly, at the ora serrata, the single epithelial layer vanishes, and the layers of the retina take its place. It has long been known, however, that even throughout the retina this single epithelial layer still continues, being known as the fibres of Müller. This is how the fact is described in the last edition of Foster's "Text-book of Physiology," p. 1308—
"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."
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—