DEVELOPMENT OP THE OPTIC NERVE OF VERTEBRATES. 169 



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 the top 

 of the page, and are figures of sections taken close to the eye, midway 

 between the eye and brain, and close to the brain, respectively. 



Figs. 8, 9 are placed with their dorsal surfaces towards the top of 

 the page, and are figures of sections taken close to the brain. 



From a study of these figures it will be seen that the optic nerve 

 lies at its retinal end ventral to the stalk ; but as it nears its cerebral 

 end it lies along the posterior border of the stalk, and even in later 

 stages (fig. 8) lies almost dorsal to the stalk. 



The history of the relation of the stalk to the nerve in later stages 

 is as follows : — In tadpoles of 11 — 12 mm. the trabecula cranii cartilage 

 growing up under the optic stalk and nerve causes the lumen of the 

 stalk to become obliterated at that point, from which point the 

 obliteration of the whole lumen gradually proceeds. The last part to 

 remain open is the part nearest to the brain, which in tadpoles of 

 23 mm. is still open (fig. 8). 



As the nerve-fibres increase in number they seem to tend to grow in 

 between the cells of the walls of the stalk (fig. 9), and eventually 

 (fig. 10) the walls of the stalk become completely broken up, and the 

 cells remain separated from one another, and lie among the fibres of 

 the optic nerve as in fig. 10. Fig. 11 is a longitudinal vertical section 

 of the optic nerve of a 23 mm. tadpole. Very possibly this breaking 

 of the optic stalk is caused not so much, if at all, by the nerve-fibres 

 growing in amongst the cells, but primarily, by reason of increase in 

 distance between the eye and brain, the stalk becomes stretched and 

 broken up. 



It might be argued that the nerve-fibres grow out of processes from 

 the cells of the stalk, but of this there is no trace at any time. The 

 cells of the stalk never show any processes, such as are easily and 

 distinctly seen in the neuroblasts of the rest of the nervous system, 

 and excepting a considerable diminution in size (which is common to 

 almost all cells of all parts of the animal), the cells of the optic stalk 



