96 RICHARD ASSHETON. 



tudinal 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 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 termina- 

 tions 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 tadpoles 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 13 — 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 



