ATTEMPT TO DEDUCE VERTEBRATE EYES FROM THE SKIN. 301 
eye (or eyes) would again, according to our theory, have 
developed out of the skin; but, inasmuch as the character of 
the skin had changed, the type of eye would differ from that 
of the pineal eye. The same physiological principle would, 
however, come into play; the cells of the palisade layer 
would be irritated by the variation in the pressure of the 
pigmented matter travelling between them under the action 
of light, and these cells could become sensory cells by asso- 
ciation with the nerve-endings now found among the cells of 
the rete mucosa. The new eye might thus from the first be, as 
indicated in Diagram II, an inverse eye, i.e. the nerve-fibres 
(n), passing outwards towards the surface, would have to bend 
back on themselves in order to become associated with the 
palisade-retinal cells. 
The gradually thickening nerve-strands of this hypothetical 
epidermal eye may be assumed to have ultimately passed in a 
group through the palisade layer at the (? posterior) edge of 
the new specialised sensory area (see diagram). This ever- 
thickening and broadening nerve-strand appears to have carried 
through with it one or more loops of the vascular system of 
the cutis, together with strands of connective tissue. 
The specialisation of a portion of the palisade layer to form 
the retina of a purely epidermal eye such as that figured in 
the diagram would, in course of time, prevent that portion 
from yielding any mucus or horn cells towards the exterior, 
more especially as the palisade cells of the retina must be 
supposed to have early specialised into cuticular rods. The 
deficiency caused by this more or less effective barrier to the 
free passage of pigment granules through the retina towards 
the surface would have to be made up somehow, not only 
to form a dioptric apparatus, but also for the protection of 
the retina. It was along time before I could see how the 
palisade cells surrounding the retina could cover it over with 
a horny layer. The solution to the problem suggested in the 
diagram was arrived at by working backwards from the lens 
of the definitive eye (cf. Diagrams IV, III, II). The palisade 
cells round the periphery of the primitive retina appear to 
