DEVELOPMENT OP THE OPTIC NERVE OP VERTEBRATES. 165 



call it the optic or retinal cup; the solid stalk becomes the optic 

 nerve." And again, on page 1142, " At the time when the epithelial 

 cells of the stalk of the retinal cup are developed into the fibres of the 

 optic nerve these become connected with the elements of the inner or 

 retinal wall of the cup ; they pierce the outer wall of pigment epi- 

 thelium, making no connections with the cells of that outer wall." 



On p. 131 of the latter work Professor Marshall writes : "The optic 

 vesicles have already been described as arising at a very early period 

 as lateral outgrowths from the fore-brain ; these soon become con- 

 stricted at their necks, so as to be connected with the brain by 

 narrow stalks, which ultimately become the optic nerves." 



In view of this difference of opinion, which still exists concerning 

 the origin of the fibres of the optic nerve, I think an account of a 

 research made some months ago upon this subject in the frog and 

 chick may be of some interest, especially as certain structures con- 

 nected with the development of the eye and other parts of the nervous 

 system may be more easily understood by appreciation of the fact that 

 the optic nerve and optic stalk are two entirely separate structures. 



Relation of the Optic Stalk to the Optic Nerve. 



The two views held at present are — 1. The optic nerve is formed by 

 the differentiation of the cells of the optic stalk into nerve-fibres, 

 which subsequently lose connection with the inner wall of the optic 

 cup, and, piercing the outer wall, make connection with the outer face 

 thereof. 



2. The optic nerve is formed by the growth of nerve fibres either 

 from the retina (outer wall of the optic cup) or from the brain, along 

 the optic stalk, but outside it and unconnected with it. 



The first of these views has been held by Balfour (1), Haddon (8), 

 Foster (6), Marshall (19), &c. ; while the second view has been held 

 by His (10, 12), Mviller (20), Kolliker (14), Hertwig (9), Orr (21), and 

 more recently supported by Keibel (13), Froriep (7), and Cajal (3). 



In the new edition of ' Quain's Anatomy ' Professor Schafer seems 

 to be uncertain which view to take. He writes on p. 79, " The optic 

 nerves take origin as hollow outgrowths of the brain, which afterwards 

 become solid, while nerve-fibres become developed in their walls. Their 

 mode of origin will be further treated of in connection with the 



