90 EICHAED ASSHETON. 



outer wall has apparently begun to be pushed inwards upon 

 the inner wall^ the walls of the optic stalk are not more than 

 one cell in thickness, and never become any thicker. The 

 same statement may be made about the inner wall of the cup 

 itself. From the time that the optic vesicles first form — that 

 is, during the folding up of the neural plate — till after the 

 fibres of the optic nerve have appeared, the optic stalk is 

 hollow from end to end. With the folding off of the optic 

 vesicles the optic stalk diminishes in diameter, and conse- 

 quently the lumen also diminishes. 



When the fibres first appear (in tadpoles of 6| — 7^ mm. in 

 length) along the outer and ventral border of that part of the 

 stalk nearest the optic cup, the lumen is still continuous 

 throughout (figs. 6 and 7), but the greater part of the cavity 

 between the outer and inner walls of the optic cup has 

 become obliterated by the approximation of those two walls 

 (fig. 12). The lumen of the optic stalk is first obliterated in 

 tadpoles of about 10 — 11 mm. at the point at which the optic 

 stalk and nerve pierce the dense tissue now forming at the 

 side of the brain. At this point the optic stalk becomes 

 squeezed, the lumen obliterated. Close to the brain the 

 lumen persists for a long time, and is not entirely obliterated 

 until the tadpole has attained a length of about -iO ram. 



Fig. 1 is a horizontal section of the eye of a tadpole 

 10 mm. long, in which the fibres of the optic nerve can be seen 

 along the whole length of the optic stalk, and for a short dis- 

 tance 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 [CH., 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. iV.), and alongside the nerve, 

 but quite separate from it, is the optic stalk {OP. S.), 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 A^. F. 



