DEVELOPMENT OF THE OPTIO NERVE OF VERTEBRATES. 91 



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 {TE. 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 apposed 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 difi'erentiation 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. 



Figs. 5, 6, 7 represent much earlier stages (tadpole, 

 7 mm.), by which time the nerve-fibres have just made their 

 appearance, and figs. 8, 9, 10, 11 represent considerably older 

 stages (tadpole, 23 mm.). 



Figs. 5, 6, 7 are placed with their dorsal surfaces towards 

 the top of the page, and are figures of sections taken close to 

 the eye. 



Figs. 1, 2, 3 are placed with their anterior surfaces towards 



