DEVELOPMENT OF THE OPTIC NERVE OF VERTEBRATES. 167 



nerve processes into the brain, though neuroblasts with the nerve 

 processes directed towards the brain may be easily found. 



I have paid special attention to the development of the nerve 

 processes in the frog, and so shall describe the development of the 

 optic nerve in that animal at some length. 



Fate of the Optic Stalk. 



I wish first to draw attention to the figs. 1, 2, 3, 8, 9, 10, 11, of PI. 

 VIII, which are camera drawings of sections which seem to me to prove 

 conclusively that the optic nerve is not developed from the optic 

 stalk ; that is to say, the nerve-fibres of the optic nerve do not arise 

 by a transformation of the cells of the optic stalk into nerve-fibres. 



During the earliest stages of the folding off of the optic vesicles the 

 walls of the stalk are more than one cell thick ; but by the time the 

 vesicles are definitely formed, and the outer wall has apparently 

 begun to be pushed inwards upon the inner wall, the walls of the optic 

 stalk are not more than one cell in thickness, and never become any 

 thicker. The same statement may be made about the inner wall of 

 the cup itself. From the time that the optic vesicles first form — that 

 is, during the folding up of the neural plate — till after the fibres of 

 the optic nerve have appeared, the optic stalk is hollow from end to 

 end. With the folding off of the optic vesicles the optic stalk 

 diminishes in diameter, and consequently the lumen also diminishes. 



When the fibres first appear (in tadpoles of 6£ — 7£ mm. in length) 

 along the outer and ventral border of that part of the stalk nearest 

 the optic cup, the lumen is still continuous throughout (figs. 6 and 7), 

 but the greater part of the cavity between the outer and inner walls 

 of the optic cup has become obliterated by the approximation of those 

 two walls (fig. 12). The lumen of the optic stalk is first obliterated 

 in tadpoles of about 10 — 11 mm. at the point at which the optic stalk 

 and nerve pierce the dense tissue now forming at the side of the 

 brain. At this point the optic stalk becomes squeezed, the lumen 

 obliterated. Close to the brain the lumen persists for a long time, and 

 is not entirely obliterated until the tadpole has attained a length of 

 about 40 mm. 



. Fig. 1 is a hoi'izontal section of the eye of a tadpole 10 mm. long, 

 in which the fibres of the optic nerve can be seen along the whole 



