92 RICHARD ASSHETON. 



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 grow- 

 ing 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 pro- 

 cesses 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 of a 40 mm. 



