ATTEMPY! TO DEDUCE VERTEBRATE EYES FROM THE SKIN, 347 
short within the cutis beneath it, the cells which contain the 
red pigmented granules pass freely on, and are seen everywhere 
forcing their way into the epidermis, and, I believe, helping to 
build up the horny layer. 
On turning to the pineal eye, what do we find? A close 
network of cells filled with similar white matter enveloping 
the eye externally. The white matter here also fails to climb 
up into the retinal cells, just as in the skin it fails to climb up 
into the epidermis, while on the other hand passing up between 
these cells are others laden with red pigment, which makes its 
way into or between the retinal cells, just as the pigment from 
the cutis passes between and into the palisade cells of the 
epidermis.! 
In addition to the pineal eyes of Hatteria and Uromastix, 
I have examined this organ in Petromyzon planeri, ammo: 
coetes of which I obtained near Jena and fixed in corrosive 
sublimate. As is now well known, there is no black pigment 
in these eyes; they appear to me to have become a receptacle 
for concretions similar to those already noted in the cutis of 
Uromastix.? The condition of the eye itself appears to indi- 
cate extreme degeneracy. Having sunk below the skin it 
1 Comparison of the pineal eye and skin of Uromastix reveals another 
fact of some importance for the theory of light sensation which I have 
ventured to put forward. The pigment granules are in this case much 
larger and more clearly defined in the eye than in the skin. It is a noticeable 
fact that in eyes in general the pigment grains are much more sharply and 
distinctly granular, and also apparently not seldom rather larger than are the 
pigment granules in the rest of the body. ‘This fact is certainly in favour of 
my suggestion that their passage up and down between the rods of the 
retina causes a mechanical stimulation. 
2 [ therefore doubt the wisdom of Gaskell’s description of this matter as 
“white pigment.” It is worth noting also that the white concretions, 
which have no definite shape in the cutis, are pointed and fusiform in the cells 
round the eye in Uromastix. ‘The guanin granules have the same or a 
very similar shape in the argentea of fishes. I have seen similar white 
long-oval plates in the eyes of spiders. These facts suggest the possibility 
of associating these last-named granules with the guanin “crystals” found 
in such quantities in the bodies of these as of other Arachnids. (On their 
origin cf. ‘Journ. Roy. Micr. Soc.,’ 1893, p. 427.) 
