This statement of Balfour's is the necessary consequence of his belief, that the epithelial cells of the optic stalk gave rise to the fibres of the optic nerve. If, instead of this, we follow Kölliker and His, who state that the optic nerve-fibres are formed outside the {105}epithelial walls of the optic stalk, and that the cells of the latter form supporting structures for the nerve-fibres, then the position of the optic nerve becomes perfectly simple and satisfactory without any rupturing of its connection with the outer wall and subsequent perforation, for the optic nerve-fibres from their very first appearance pass directly to supply the retina--_i.e._ the inner wall of the optic cup and nothing else. They pass, as is well known, without any perforation by way of the choroidal slit to the inner surface of the inner wall (retina) of the optic cup; then, when the choroidal slit becomes closed by the expansion of the optic cup, the optic nerve naturally becomes situated in the centre of the base of the cup and spreads over its inner surface as that surface expands. A section across the optic cup at an early stage at the junction of the optic stalk and optic cup would be represented by the upper diagram in Fig. 43; at a later stage, when the choroidal slit is closed, by the lower diagram. [Illustration: FIG. 43.--DIAGRAM OF THE RELATION OF THE OPTIC NERVE TO THE OPTIC CUP. The upper diagram represents a stage before the formation of the choroidal slit, the lower one the stage of closure of the choroidal slit. _R._, retina; _O.n._, optic nerve; _p._, pigment epithelium.] The evident truth of this manner of looking at the origin of the optic nerve is demonstrated by the appearance of the optic nerve in Ammocoetes and Petromyzon. In the latter, although the development is complete, and the eye, and consequently also the optic nerve-fibres, are fully functional, there is still present in the axial core of the nerve a row of epithelial cells (Axenstrang) which are altered so as to form supporting structures, in the same way as a row of epithelial cells in the retina is altered to form the system of supporting cells known by the name of the Müllerian fibres. The origin of this axial core of cells is perfectly clear, as has been pointed out by W. Müller. He says-- "The development of the optic nerve shows peculiarities in {106}Petromyzon of such a character as to make this animal one of the most valuable objects for deciding the various controversial questions connected with the genesis of its elements. The lumen of the stalk of the primary optic vesicle is obliterated quite early by a proliferation of its lining epithelium. Also the original continuity of this epithelium with that of the pigment-layer is at an early period interrupted at the point of attachment of the optic stalk. This interruption occurs at the time when the fibres of the optic nerve first become visible." Further on he says--