DEVELOPMENT OF THE OPTIO NERVE OF VERTEBRATES. 173 



in sections of the optic stalk close to the brain no trace of nerve-fibres 

 can be seen (Fig. 7). Such is the case in tadpoles of 7 — 8 mm. in 

 length. In tadpoles of 8 — 9 mm. the nerve-fibres can be seen along 

 the whole of the stalk. 



In later stages fibres may be traced to the upper regions of the 

 mid brain, though of the actual terminations I have nothing to say. 

 Possibly the appearance of concentric bands of white matter of a 

 molecular appearance in the roof of the optic lobes is concomitant 

 with the branching of the terminations of the fibres that have reached 

 that part of the brain ; but, as far as my own researches go, that is 

 merely conjecture. These molecular layers are first visible in tad 

 poles of 17 — 20 mm. in length. I consider, however, that the fibres 

 have reached the upper part of the optic lobes in younger tadpoles — 

 those of 12 — 15 mm. 



The optic fibres begin to grow as soon as those from any other part 

 of the nervous system, and grow very rapidly. As the eye itself 

 increases in size the bundle of fibres lying along the optic stalk 

 increases also. 



The fibres whose addition causes the increase in size of the bundle 

 arise as outgrowths from neuroblasts (Fig. 13, iV., Plate IX.) near 

 the rim of the optic cup, for along this rim a constant proliferation of 

 cells takes place as long as the eye increases in size. 



Fig. 13 shows the main features in the structure of the retina of 

 a tadpole of 13 mm., and is as far as possible a camera drawing. 



Towards the centre of the cup the development of the retina 

 is furthest advanced. 



The two walls of the optic vesicle are closely approximated, and the 

 originally inner wall is reduced to a single layer of pigmented cells (P.). 



These pigmented cells are directly continuous with the walls of 

 the optic stalk, and thereby with the epithelial lining of the neural 

 tube. These cells represent the epidermic layer of the epiblast, so 

 that from this part of the optic cup it will be seen that the nervous 

 layer is entirely absent. 



In the originally outer wall of the optic vesicle there are many cells 

 derived from the nervous layer, and the epidermic cells are greatly 

 elongated and to a certain extent branched, and form the supporting- 

 elements only of the retina. 



