492 J. T. GRADON. 



The resting nuclei of the stalk, except iu Fig. 16, have been outlined only, 

 and all the nuclei of cells entering into the formation of the pial sheath have 

 been shaded. 



Tiie top of the page represents " dorsal." 



Fig. 8. — 32 mm. The nerve fibres and lymph channels are shown in part 

 of the drawing only. X 500. 



Fig. 9. — Oblique transverse section, from the same series as Fig. 6, taken 

 close to the brain, x 800. 



Fig. 10. — From a transverse section of the optic nerve of an adult frog. 

 Taken 160 /* from the eye. Only a sector has been filled in with the trans- 

 verse trabeculee. X 500. 



Fig. 11. — Horizontal section from an 8'5 mm, tadpole; showing fibrils of 

 the transverse, oblique, and longitudinal trabeculse. x 800. 



PLATE 27. 

 Fig. 12. — Longitudinal section, from a 15 mm. tadpole. Taken near the 

 brain, x 800. 



Fig. 13. — Peripheral longitudinal section, from a 29 mm. tadpole. Taken 

 midway between the eye and the brain. X 800. 



Fig. 14. — Longitudinal section, from a 32 mm. tadpole, x 800. 



Fig. 15. — Longitudinal section, from a 14 day embryo mouse, x 500, 



Fig. 16. — Longitudinal section, from a developing trout. Body length not 

 known ; length of stalk "5 mm. X 500. 



Fig. 17. — Transverse section from a 33 mm. dog-fish. The two large 



spaces between the pial sheath and the membrana limitans externa, here 



represented by the epiblastic trabecule, are due to sectiouising, and they 



sliow that the pial sheath forms a separate layer around the optic nerve, 



X 500. 



Fig. 18. — From a transverse section of tlie optic nerve of an 8-day chick. 

 X 800, 



