284 



THE DEVELOPMENT OF THE CHICK 



nerve; moreover, at an early stage of their differentiation it is 

 possible to trace their connection with retinal neuroblasts. 



The first fibers of the optic nerve are formed, as already 

 stated, from the fundus part of the retina; the fibers, therefore, 

 pass directly to the floor of the optic stalk; but on the fifth day 

 the formation of fibers begins from more distal portions of the 

 retina and these do not grow towards the insertion of the optic 

 stalk, l3ut towards the choroid fissure; arrived there, they bend 

 centrally and run in a bundle on each side along the floor of the 

 bulbus oculi to the optic stalk, where they join with the fibers first 

 formed. The later formed fibers pass to still more distal portions 



■ Mes/I 



P., 







t-«*''f; 



«^:Mfe^2^ 





Fig. 164. — Section in the plane of d of Fig. 162, to 

 show the histological structure. (After Bernd.) 

 Abbreviations as before. 



of the choroid fissure, and, as the pecten forms in the manner 

 already described, the fibers of the optic nerve all unite beneath 

 it on their way to the original optic stalk. Thus, the optic nerve 

 obtains an insertion coincident in length with the base of the 

 pecten, and its fibers, radiating off into the retina on each side 

 of the pecten, separate the latter completely from the choroid 

 coat of the eyeball. 



The optic stalk is at first a tubular communication between 

 the optic vesicle and the fore-brain, and its walls are an epithelial 

 layer of the same thickness throughout. The fibers of the optic 



