104 THE ORIGIN OF VERTEBRATES 
epithelial cells—viz. in that part of the tube whigh, 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 neryous matter, 
or ever was, or ever will be, nervous matter; it is in ptecisely the 
same category as the membranous roof of the brain in Ammoceetes, 
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. te 
Turning now to the optic nerve, Balfour describes Sys derived 
from the hollow stalk of the optic vesicle. He says— ~ 7 
“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 under- 
stand. I presume he meant that some of the fibres of the optic nerve 
supplied from the beginuing 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 
