Regeneration of the Lens, Iris, and Neural Retina in a Vertebrate Eye **

In all classes of vertebrates the lens arises in the embryo from the surface ectoderm, which normally comes into contact with the early optic vesicle as it grows out from the wall of the brain on either side. Spemann8 in his Silliman Lectures reviewed his experiments and those of others on the embryonic eyes of amphibians which showed that for the lens this contact has particular significance. If the contact fails to take place, no lens develops. If the normal presumptive lens-forming ectoderm is replaced by ectoderm taken from other regions of the body of the same or even another species of embryo, it will also be induced by the underlying eye tissue to form a lens rather than the skin it would have formed had it remained on the donor. A study of experimentally produced cyclopian monsters in amphibians" also showed very well the influence of the early developing eye upon the origin of lens tissue. The degree of fusion of the eyes determines the degree of fusion of the induced lenses. When the two eyes are completely fused into a large median-placed one, a single correspondingly large lens is induced if contact is made with the surface ectoderm. If there is no contact, a lens fails to develop.' When a lens is fully differentiated, it is suspended by zonular fibers extending from the ciliary processes. But if the original lens is detached from these fibers and entirely removed from the eye, leaving no remnant of lens tissue, what are the chances that it would be replaced by some form of regeneration? It would appear at first that replacement of the lost lens is not possible. Twenty years ago' we found that fragments of the lenses of various amphibians and fishes, implanted into the eye, have the ability to organize and form new lens fibers if they included a normally high proliferating region of the epithelium along the equator of the lens. The organization and growth were best in the eyes of younger animals. In some adult salamanders * Bronson Professor of Comparative Anatomy.