172 RICHARD ASSHBTON, M.A. 



grown centralwards ; (2) those which have arisen in the brain and 

 grown outwards. 



From a study of the development of the eye and optic nerve of 

 Rana temporaria, I am convinced that at any rate a very large pro- 

 portion of the nerve-fibres of the optic nerve arise as outgrowths of 

 cells in that portion of the optic cup from which the retina will be 

 formed, which processes grow centralwards at first along the ventral, 

 then along the posterior border of the optic stalk, and entering the 

 brain immediately posterior to the optic recess cross along the ventral 

 surface of the middle commissure to the opposite side, where they 

 turn dorsalwards and slightly backwards to the roof of the mid-brain. 



In tadpoles of about 7 mm. in length, or a little earlier, the first 

 trace of the optic nerve may be seen. 



Many of the cells of the retinal portion of the optic cup may be 

 seen to be pear-shaped, with their end drawn out into processes which 

 are directed towards the centre of the optic cup. 



It is not possible to follow the individual fibres, but all are directed 

 towards the ventral rim of the optic cup, over which, at any rate, a 

 large number pass and run along the ventral border of the optic 

 stalk, some for a greater, others for a lesser distance. 



Fig. 4 is from a sagittal section of a tadpole (of 1\ mm. in length), 

 and therefore a section transverse to the longitudinal axis of the optic 

 stalk, taken at the point where the fibres are crossing the ventral lip 

 of the optic cup. 



It will be seen that the fibres pass through a cleft, or rather, I 

 should say, the walls of the optic cup have grown up round the 

 bundle of nerve-fibres since they passed over the rim, for there is no 

 such apparent cleft before the fibres are developed. Fig. 5 is a section 

 of the optic stalk from the same series as Fig. 4, but taken between 

 the eye and brain, close to the former. Here on the ventral border is 

 seen the bundle of fibres, and the wall of the optic stalk is slightly 

 bulged in along the line of the fibres. The bundle here is rather 

 smaller than in Fig. 5. 



In Fig. 6, which is also from the same series of sections, but taken 

 nearer the brain, the bulging in of the optic stalk is not so marked, 

 and the bundle of fibres considerably diminished in size. Between 

 this section and the brain the fibres become less and less distinct, and 



