480 J. T. GRADON. 



its complex framework, it will be my aim in the present 

 paper to describe its origin and development as a whole. 

 The subject will be dealt with under the following heads : 



PAGE 



I. The Relation of the Optic Stalk to tiie Nerve-fibres . . 480 



II. Cellular Segmentation ..... 481 



III. Obliteration of the Lumen of the Optic Stalk . . 482 



IV. Period of slow growth, followed by one of great activity, conse- 



quent on the formation of the Arachnoid Sheath and the 

 Enclosure of the Subarachnoidal Lymph Space . . 483 



V, Pigmentation of the Trabeculse .... 485 

 VI. Supplementary Remarks .... 486 



VII. formation of the Pial Sheath, and its Relation to the Membrana 



Limitans Externa ..... 487 

 VIII. Some Variations of Epiblastic Trabeculse met with in the Deve- 

 loping Optic Nerve of the Mouse, the Trout, the Dog-fish, 

 and the Chick . . . . .487 



I. The Relation op the Optic Stalk to the Nerve-fibres. 



In discussing Assheton's (1) contention that the first nerve- 

 fibres, though lying along the posterior border of the stalk, 

 are at first entirely outside it and separate from it, Robinson 

 (7), after showing that this conclusion is altogether at vari- 

 ance with the observations of W. Miiller (6) upon the lamprey, 

 of Kolliker (5) upon rabbits, pigs, and calves, of Keibel (4) 

 upon reptiles, and of Froriep (3) upon cartilaginous fishes, 

 makes the following very important statement : — " If the 

 condition which Assheton (1) found in the frog is present in 

 mammals also, then it follows that the sustcntacular frame- 

 work of the optic nerve of man may consist, for the most part, 

 like the framework of an ordinary cerebro-spinal nerve, of 

 mcsoblastic tissue surrounding and embedding the epiblastic 

 nerve-fibres, but if Miiller's and Kolliker's statements are 

 well founded, then the sustentacular tissue of the optic nerve 

 in man and mammals must consist chiefly of epiblastic tissue 

 derived from the primitive epithelial cells of the optic 



