ATTEMPT TO DEDUCE VERTEBRATE EYES FROM THE SKIN. 353 
retina bulged further and further inwards, the nerve-strand 
connecting it with the brain shifted down towards the base of 
the cup. It is quite indifferent whether we speak of the nerve- 
strand cutting down the sides of the ever-deepening retinal 
cup, or of the sides of the cup growing up on each side of the 
nerve. The vascular and connective-tissue elements which 
accompanied the nerve (see Diagram II) would now protrude 
into the cavity of the eye. Inthe completely invaginated eye 
(shown in Diagram III) it is obvious that the choroidal fissure 
could originally only have extended from the edge of the long 
palisade or lens cells to the entrance of the optic nerve. 
In Diagram IV I have indicated some of the adaptations 
which would be necessary to complete the Vertebrate eye. The 
lens had to be isolated, and its long cells rolled under it from 
all sides. We need not assume that the earliest isolated lens 
had its fibres arranged in the highly specialised manner 
characteristic of the higher Vertebrates; any folding of the 
long palisade cells under the more hardened central portion 
would be sufficient. The freedom of the lens and folding 
under of its long cells could, it seems to me, be brought about 
if the lens itself, under the action of contractile fibres in- 
truding with the optic nerve into the cavity of the eye, was so 
far moveable that its axis deviated through small angles from 
the optic axis of the eye. 
With regard to this method of accounting for the isolation 
of the lens, the following considerations are worth notice. 
The eye itself would have been immoveable through all the 
earliest stages of its evolution, but the Vertebrate eye is, as 
we know, a highly moveable structure. We are justified in 
concluding that its present powers of movement are but more 
perfect ways of attaining ends which, in its earliest stages, it 
must have striven to attain. It is not unlikely, therefore, that 
the connective-tissue strands which entered the eye in its 
earliest stages became attached to the edges of the primitive 
surface lens, and effected some simple movements. As the 
retina bulged inwards, these simple lens movements might be 
supplemented by slight movements of the whole eye,—these 
