THE EVIDENCE OF THE ORGANS OF VISION 103 
“The formation of the eye,” as taught by Balfour, “ commences 
with the appearance of a pair of hollow outgrowths from the anterior 
cerebral vesicle. These outgrowths, known as the optic vesicles, at 
first open freely into the cavity of the anterior cerebral vesicle. 
From this they soon, however, become partially constricted, and 
form vesicles united to the base of the brain by comparatively 
narrow, hollow stalks, the rudiments of the optic nerves.” 
“ After the establishment of the optic nerves, there takes place 
(1) the formation of the lens, and (2) the formation of the optic cup 
from the walls of the primary optic vesicle.” 
He then goes on to explain how the formation of the lens forms 
the optic cup with its double walls from the primary optic vesicle, 
and says— 
“Of its double walls, the inner, or anterior, is formed from the 
front portion, the outer, or posterior, from the hind portion of the 
wall of the primary optic vesicle. The inner, or anterior, which very 
speedily becomes thicker than the other, is converted into the retina ; 
in the outer, or posterior, which remains thin, pigment is eventually 
deposited, and it ultimately becomes the tesselated pigment-layer of 
the choroid.” 
The difficulties in connection with this view of the origin of the 
eye are exceedingly great, so great as to have caused Balfour to 
discuss seriously Lankester’s suggestion that the eye must have been 
at one time within the brain, and that the ancestor of the vertebrate 
was therefore a transparent animal, so that light might get to the eye 
through the outer covering and the brain-mass ; a suggestion, the 
unsatisfactory nature of which Balfour himself confessed. Is there 
really evidence of any part of either retina or optic nerve being 
formed from the epithelial lining of the tube ? 
This tube is formed as a direct continuation of the tube of the 
central nervous system, and we can therefore apply to it the same 
arguments as have been used in the discussion of the meaning of the 
latter tube. Now, the striking point in the latter case is the fact 
that the lining membrane of the central canal is in so many parts 
absolutely free from nervous matter, and so shows, as in the so-called 
choroid plexuses, its simple, non-nervous epithelial structure. This 
also we find in the optic diverticulum. Where there is no evidence 
of any invasion of the tube by nervous elements, there it retains its 
simple non-nervous character of a tube composed of a single layer of 
