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, but 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 
Fic. 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 
