ATTEMPT TO DEDUCE VERTEBRATE KEYES FROM THE SKIN. 349 
cells in the pars ciliaris retine of Vertebrate eyes (see below), 
and as they certainly do in the rete mucosa of the Vertebrate 
skin. That they formerly clarified in the cavity of the pineal 
eye also we have some evidence in another of the facts which 
Leydig adduces as a reason for disbelieving the original ocular | 
function of these structures. The remains of the homogeneous 
clear substance which I assume once filled the cavity of the 
eye still here and there persists, as Leydig himself has shown. 
It appears in some cases as bristle-like streaks or threads of 
clear matter streaming outwards from the surface of the retina, 
or even in a thick layer like a cuticle. That this substance 
ever formed a definite system of rods turned towards the 
cavity of the eye, such as has been suggested by Gaskell,! I 
think somewhat doubtful, although it is quite possible that 
here and there some such differentiation may have taken 
place (cf. the “ rods ” of the Cephalopod eye). 
Summing up this brief sketch of the pineal eye, we note 
that structurally (its embryology will be discussed later 
on) it is quite explicable as a simple invagination of a skin 
of the Invertebrate type, and may well have arisen in the 
manner suggested by our theory, while our ancestors still 
possessed such an undifferentiated epidermis. 
An eye arising in this way would necessarily be what is 
known as a direct vision eye,—that is, the nerves would end in 
the retina without any bending back upon themselves. 
The Vertebrate Eye Proper.—In process of time the 
skin lost the simple character it possessed when, according to 
the foregoing, the pineal eye arose. Cells budded off from 
the palisade layers, while others (e. g. pigment-bearing cells) 
migrated through the palisade layer, and these together, the 
1 *Quart. Journ. Micr. Sci.,’ vol. xxxi. At the same time it is obvious 
that the interpretation of the facts here adopted is hardly reconcilable with 
the deduction of Vertebrates from the Arthropods; a chitinous exoskeleton 
does not lend itself to such simple invagination as we have here assumed. 
I do not think the depression in the centre of the retina of the pineal eye of 
Petromyzon planeri at all justifies Gaskell’s comparison of that eye with 
the eye of the Acilius larva. A comparison with other pineal eyes shows that 
that depression is hardly primitive ; it may even be a result of degeneration. 
