486 J. T. GEADON. 



tinual increase of tlie number of nuclei and fibrils of the 

 trabecul^e^ without any apparent increase in the thickness of 

 the latter. 



It clearly follows^ from what has been said, that the cells 

 of the optic stalk are spongioblasts, and that they, therefore, 

 take no part in the production of optic nerve-fibres, which 

 arise, according to the researches of Ramon y Cajal (2) and 

 other well-known investigators, from neuroblasts, chiefly in 

 the retina. 



VI, SUPPLEMENTAKY REMARKS. 



Even when there is not sufficient protoplasm surrounding 

 the resting nuclei of the stalk to be represented in the 

 drawings, it will be understood that there is still an ex- 

 tremely delicate layer of it covering them, and that the fibrils 

 of the trabecula3 form the continuations of it. This thin sheet 

 of protoplasm may, however, be distinctly recognised around 

 the nuclei that are undergoing division (vide especially 

 fig. 9). 



The fantastic outlines of the nuclear walls are accounted 

 for by the fact that each fibril of the trabecule is being 

 stretched by continual ingrowth of nerve-fibres, and is, 

 therefore, pulling its nucleus towards the point of its attach- 

 ment. 



In this connection it will be interesting to compare the 

 nuclear outlines of the densely-packed optic nerve of the 

 frog (fig, 8), with those of the much less densely packed 

 optic nerve of the dog-fish (fig. 17), 



The nerve-fibres contained in the optic nerve of the latter 

 are so comparatively few in number, and the lymph channels 

 so wide and numerous, that, when favourable transverse 

 sections of it are viewed with a very low power, the nuclei 

 themselves appear to form a well-defined framework, through 

 being pulled into mere threads between the nerve-fibres. 



In the frog, on the other hand, the lymph channels, though 



