ON THE ORIGIN OF VERTEBKATES. 253 



the fibres of the optic nerve. If, instead of this, we follow 

 Kolliker and His, who state that the optic nerve fibres are 

 formed outside the epithelial walls of the optic stalk, and that 

 the cells of the latter form supporting structures for the nerve 

 fibres, then the position of the optic nerve becomes perfectly 

 simple and satisfactory without any rupturing of its connection 

 with the outer wall and subsequent perforation, for the optic 

 nerve fibres from tiieir very first appearance pass directly to 

 supply the retina — i.e., the inner wall of the optic cup and noth- 

 ing else. 



They pass, as is well known, without any perforation by way 

 of the choroidal slit to the inner surface of the inner wall 

 (retina) of the optic cup, then when the choroidal slit becomes 

 closed by the expansion of the optic cup, the optic nerve natu- 

 rally becomes situated in the centre of the base of the cup, and 

 spreads over its inner surface as that surface expands. 



A section across the optic cup at an early stage at the junction 

 of the optic stalk and optic cup would be represented by the 

 upper diagram in fig. 8, at a later stage, when the choroidal slit 

 is closed by the lower diagram. 



The evident truth of this manner of looking at the orifjin of 

 the optic nerve is demonstrated by the appearance of the optic 

 nerve in Ammoccetes and Petromyzon. In the latter, although 

 the development is complete, and the eye, and consequently also 

 the optic nerve fibres, are fully functional, there is still present 

 in the axial core of the nerve a row of epithelial cells (Axen- 

 strang) which are altered so as to form supporting structures, in 

 the same way as a row of epithelial cells in the retina is altered 

 to form the system of supporting cells known by the name of 

 the Miillerian fibres. 



The origin of this axial core of cells is perfectly clear, as has 

 been pointed out by W. Milller. He says :^ " The development 

 of the optic nerve shows peculiarities in Petromyzon of such a 

 character as to make this animal one of the most valuable 

 objects for the deciding of the various controversial questions 

 connected with the genesis of its elements. The lumen of the 

 stalk of the primary optic vesicle is obliterated quite early by 

 a proliferation of its lining epithelium. Also the original con- 



' Op. cit., p. xxxvi. 



