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 walis 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 



