THE EVIDENCE OF THE ORGANS OF VISION 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 repre- 

 sented by the upper diagram in Fig. 

 43 ; at a later stage, when the choroidal 

 slit is closed, by the lower diagram. 



The evident truth of this manner 

 of looking at the origin of the optic 

 nerve is demonstrated by the appear- 

 ance of the optic nerve in Amrao- 

 ccetes and Fetromyzon. 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 Mtillerian 

 fibres. 



The origin of this axial core of cells is perfectly clear, as has been 

 pointed out by W. Miiller. He says— 



" The development of the optic nerve shows peculiarities in 



On 



Fig. 43. — Diagram op the RELA- 

 tion 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. 



