DEVELOPMENT OF THE OPTIC NERVE OF VERTEBRATES. 95 



" Comme nous venons de le voir ici deja nous rencontrons les 

 origines cellulaires de quelques fibres optiques lesquelles, 

 d'apres nos recherches sur le retine des oiseaux pourraient 

 bien etre celles qui finissent dans la couche des grains 

 internes par des arborisations libres, courtes et fortement vari- 

 queueses. Nous ignorons si, parmi les fibres du nerf optique, 

 il en existent d'autres d'origine centrale/^ 



Hence it seems almost certain from the study of the 

 adult condition, at any rate in birds, that the optic nerve 

 is composed of two kinds: (1) those which have arisen 

 from cells in the retina and have grown centralwards j 

 (2) those which have arisen in the brain and grown out- 

 wards. 



From a study of the development of the eye and optic 

 nerve of Rana temporaria, I am convinced that at any rate 

 a very large proportion of the nerve-fibres of the optic nerve 

 arise as outgrowths of cells in that portion of the optic cup 

 from which the retina will be formed, which processes grow 

 centralwards at first along the ventral, then along the poste- 

 rior border of the optic stalk, and entering the brain imme- 

 diately posterior to the optic recess cross along the ventral 

 surface of the middle commissure to the opposite side, where 

 they turn dorsalwards and slightly backwards to the roof of 

 the mid-brain. 



In tadpoles of about 7 mm. in length, or a little earlier, the 

 first trace of the optic nerve may be seen. 



Many of the cells of the retinal portion of the optic cup 

 may be seen to be pear-shaped, with their end drawn out into 

 processes which are directed towards the centre of the optic 

 cup. 



It is not possible to follow the individual fibres, but all are 

 directed towards the ventral rim of the optic cup, over which, 

 at any rate, a large number pass and run along the ventral 

 border of the optic stalk, some for a greater, others for a lesser 

 distance. 



Fig. 4 is from a sagittal section of a tadpole (of 7^ ram. 

 in length), and therefore a section transverse to the longi- 



