THE EVIDENCE OF THE ORGANS OF VISION 107 
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 gan- 
glionic 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 corre- 
sponding 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 Millerian 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, form- 
ing 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 ciliarts retine; then suddenly, at the ora 
