252 DR WALTER II. GASKELL. 



and which ultimately becomes the tesselated pigment layer of 

 the choroid. Nobody has ever suggested that this pigment layer 

 is nervous matter, or ever was, or ever will be, nervous 

 matter ; it is in precisely the same category as the membranous 

 roof of the brain in Ammoccetes, which never was, and never 

 will be, nervous matter ; yet, according to the old embryology 

 both in the case of the eye and the brain, the pigment layer 

 and the so-called choroid plexuses are a part of the tubular 

 nervous system. 



Turning now to the optic nerve, Balfour describes it as 

 derived from the hollow stalk of the optic vesicle. He says : ^ 

 " At first the optic nerve is equally continuous with both walls 

 of the optic cup, as must of necessity be the case, since the 

 interval which primarily exists between the two walls is 

 continuous with the cavity of the stalk. When the cavity 

 within the optic nerve vanishes, and the fibres of the optic 

 nerve appear, all connection is ruptured between the outer wall 

 of the optic cup and the optic nerve, and the optic nerve 

 simply perforates the outer wall, and becomes continuous with 

 the inner one." 



In this description Balfour, because he derived the optic 

 nerve fibres from the epithelial wall of the optic stalk, of 

 necessity supposed that such fibres originally supplied both the 

 outer and inner walls of the optic cup, and therefore, seeing that 

 when the fibres of the optic nerve appear they do not supply the 

 outer wall, he supposes that their original connection with the 

 outer wall is ruptured, because a discontinuity of the epithelial 

 lining takes place coincidently with the appearance of the optic 

 nerve fibres, and, according to him, the optic nerve simply 

 perforates the outer wall and becomes continuous with the 

 inner one. This last statement is very difficult to understand. 

 I presume he meant that some of the fibres of the optic nerve 

 supplied from the beginning the inner wall of the optic cup, 

 but that others which originally supplied the outer wall were 

 first ruptured, then perforated the outer wall, and finally com- 

 pleted the supply of the inner wall or retina. 



This statement of Balfour's is the necessary consequence of 

 his belief, that the epithelial cells of the optic stalk gave rise to 



' dp. nt., p. 405. 



