ON THE ORIGIN OF VERTEBRATES. 251 



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 con- 

 verted 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 coverincr 

 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 epithelium 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 tul)e. 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 

 epithelial cells — viz., in that part of the tube which, as Balfour 

 says, remains thin, in which pigment is eventually deposited, 



