ATTEMPT TO DEDUCE VERTEBRATE EYES FROM THE SKIN. 399 
the two inner pigmented layers of the iris, viz. that con- 
tinuous with the pigmented epithelium, becomes the lens 
epithelium on the distal face of the lens, while the inner 
yields the long lens-cells. This naturally raises the question 
whether the lens epithelium may not be the homologue of 
the pigmented epithelium, the lens fibres alone representing 
the palisade layer. It may be that this regenerative 
process is strictly recapitulatory, and that this is the right 
explanation of the facts. On the other hand, the lens epi- 
thelium passes so gradually into the lens fibres round the 
equator of the lens, that it is difficult to believe that the whole 
structure is not the result of the folding of a single con- 
tinuous epithelium. Again, the lens in the Vertebrate eye can 
hardly be such an entirely new structure, dating, as in the case 
of the lens of the pineal eye, simply from the time of the 
invagination, which this new suggestion would compel us to 
believe. It seems to me far more probable that it was an early 
modification of the external horny layers of the epidermis, 
which, as illustrated in the diagrams, became involved in an 
invagination. A comparative study of lenses, including those 
in degenerate eyes, might throw further light on this subject. 
The embryological development of the lens is, according to 
our view, so purely adaptive that it can hardly help us. 
It is obvious, further, that this method of deducing the 
Vertebrate eye from the skin has the advantage that this organ 
need no longer be any exception to the rule which obtains in 
the animal kingdom, that organs of sense develop directly and 
continuously out of the skin. Lastly, we must not forget the 
significance of the fact that the change of type which can be 
seen to have taken place in the Vertebrate eyes is easily refer- 
able to the change which’we know must have taken place in 
the character of the skin from the Invertebrate to the Vertebrate 
type. 
Having, then, shown that it is possible physiologically and 
morphologically to deduce the Vertebrate eyes from the skin, 
the pineal eye from the skin in its Invertebrate condition, the 
definitive eye from the skin in its Vertebrate condition, we 
