168 RICHARD AS8HBT0N, M.A. 



length of the optic stalk, and for a short distance into the brain. It 

 is, however, not possible to trace them at this stage across to the 

 opposite side of the brain. There is as yet no chiasma. The two 

 indentations (GH., CH.) are caused by the choroidal fissure. 



From the end of the cleft nearest the brain the fibres of the optic 

 nerve are seen issuing (OP. N~.J, and alongside the nerve, but quite 

 separate from it, is the optic stalk (OP. S.) y the lumen of which is 

 continuous with the cavity of the primary optic vesicle at C. OP. V. 

 In the next few sections fibres of the optic nerve can be traced into 

 connection with those arising from cells within the retina, such as are 

 seen at N. F. 



Fig. 2 is a camera drawing of the optic stalk and nerve of the same 

 tadpole, but of the left instead of the right side. In this figure the 

 optic nerve and stalk, cut across at right angles to their longitudinal 

 axes, are seen lying between the band of dense tissue (TR. CR.) and 

 the brain (BR.). 



The evidence here, again, is to prove the optic nerve to be entirely 

 independent of the optic stalk, except that the two structures lie 

 closely opposed to one another. 



A section transverse to the longitudinal axis of the nerve-stalk, 

 taken at any point between the eye and brain at this stage, would 

 give a similar figure. 



The cells of the wall of the stalk on the side on which the nerve 

 lies are as large and in every way similar to those on the opposite side, 

 whence it is impossible to maintain that the nerve-fibres are developed 

 by the differentiation of the cells of the stalk in situ into nerve-fibres. 



Fig. 3 shows the connection of the optic nerve and stalk with the brain. 



On reaching the brain the stalk and nerve separate, the cavity of 

 the optic stalk becoming continuous with the optic recess in the floor 

 of the third ventricle, while the fibres of the optic nerve can be traced 

 as far as the middle line across the great ventral commissure (M. COM.), 

 but at this stage can be traced no further. In later stages (20 mm.) 

 the fibres may be easily traced to the opposite side of the brain, and 

 later (30 — 40 mm.) up into the optic lobe of the opposite side. 



At the stage I have been describing the optic chiasma cannot be 

 said to exist ; or if some of the fibres have indeed crossed, they form 

 such small bundles that they cannot be recognised. 



