ON THE ORIGIN OF VEETEBRATES. 255 



this tube or stalk the fibres of the optic nerve make their ap- 

 pearance arising from the ganglion cell layer of the retina, and, 

 passing over the surface of the epithelial tube at the choroidal 

 fissure, proceed to the brain by way of the optic chiasma. Owing 

 to the large number of fibres, their crossing at the junction of 

 the stalk with the bulb, and the narrowness at this neck, the 

 obliteration of the lumen of the tube which takes place in the 

 stalk is carried to a still further extent at this narrow part, with 

 the result that all continuity of the cell layers of the original 

 tube of the optic stalk with those of both the inner and outer 

 walls of the bulb is interrupted, and all that remains in this 

 spot of the original continuous line of cells which connected the 

 tube of the stalk with that of the bulb are possibly some among 

 the groups of cells which are found scattered among the fibres 

 of the optic nerve at their entrance into the retina. Such 

 separation of the originally continuous elements of the epithelial 

 wall of the optic stalk, which is apparent only at this neck of 

 the nerve in Petromyzon, takes place along the whole of the 

 optic nerve in the higher vertebrates, so that no continuous 

 axial core of cells exists, but only scattered supporting cells. 



If further proof be wanted, it is given by the evidence of 

 physiology, which shows that the fibres of the optic nerve are no 

 different to 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 ganglionic 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 nervous system as a whole, 

 have amalgamated so closely with the elements of this tube as to 

 utilise them for supporting structures, with the exception of its 

 dorsal wall, which, like the corresponding part of the brain tube, 

 still retains its original character, and by the deposition of 



VOL. XXXV. (N.S. VOL. XV.) — JAN. 1901. R 



