104 THE ORIGIN OF VERTEBRATES 



epithelial cells — viz. in that part of the tube which, as Balfour says, 

 remains thin, in which pigment is eventually deposited, 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 Ammocoetes, 

 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 ecrually 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 witli 

 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 under- 

 stand. 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 completed the supply to 

 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 the 

 fibres of the optic nerve. If, instead of this, we follow Kolliker and 

 His, who state that the optic nerve-fibres are formed outside the 



