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 Ammo- 
ccetes and Petromyzon. In the latter, Fie. 43—D1acram or tux Reta- 
although the development is complete, 08 OF THE Opric NeRvE To 
and the eye, and consequently also the oe ia 
i i The upper diagram represents a 
optic nerve-fibres, are fully functional, stage before the formation of the 
there is still present in the axial core choroidal slit, the lower one the 
of the nerve a row of epithelial cells “age of closure of the choroidal 
: slit. R., retina; O.mn., optic 
(Axenstrang) which are altered so as perve; p., pigment epithelium. 
to form supperting 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 Miillerian 
fibres. 
The origin of this axial core of cells is perfectly clear, as has been 
pointed out by W. Miller. He says— 
“The development of the optic nerve shows peculiarities in 
