DEVELOPMENT OF THE OPTIC NERVE OF VERTEBRATES. 89 



specimeus and with thin sections, to find the early commence- 

 ment of nerve-fibre tracts, arising as they do from definite 

 groups of neuroblasts. I do not doubt that the conclusions 

 of His are correct, namely, that the fibres of the dorsal roots 

 of the spinal nerves and the sensory fibres of the cranial 

 nerves arise as processes from neuroblasts of the spinal gan- 

 glia in the one case, of the cranial ganglia in the other case, 

 and grow inwards to the central nervous system. 



Similarly also the sensory fibres of the sense-organs may be 

 expected to grow inwards from the sensory epithelium of the 

 sense-organ to the central nervous system. 



In no sense-organ can the outgrowths of the fibres to the 

 central nervous system be more easily traced than in the case 

 of the fibres arising in connection with the optic organ ; as 

 the distance between the place of origin of the fibres (in the 

 retina) and the final destination of the fibres (the brain) is 

 relatively greater than in such cases as the distance between 

 the olfactory epithelium and brain, or between ganglia of 

 cranial and spinal nerves and the neural tube, in which cases 

 it is exceedingly diflScult to trace the centralwards growth 

 of the 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 develop- 

 ment 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. XI, 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 oflF 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 



