The method by which the turning back of the layers takes place was not, it appears, clearly described by Nussbaum in his first paper, for his account seems to imply, in certain passages, that the ectoderm may slide over the endoderm during the process, rather than that both layers always turn together. Ischikawa, who studied the problem later, gave a clearer account of the method of turning back. Nussbaum has stated in a later paper that he had described essentially the same process. In conclusion, it can be definitely stated that a transformation of ectoderm into endoderm cannot take place in hydra. Ischikawa also tried removing the endoderm from a piece by spreading it out and then killing the inner layer by weak acid applied with a brush, but pieces of this sort failed to regenerate a new endoderm. Tower has recently stated that if a living hydra is put into a strong light from an arc lamp of 52 volt 12 ampere capacity, that is focussed on the animal (after passing through an alum cell), the ectoderm cells fly off, but if the animal is kept, it subsequently produces a new ectoderm. Whether all the ectoderm is lost, or only the larger neuro-muscular cells, was not made out. One of the most unexpected discoveries of recent times in connection with the problem of regeneration is the renewal of the extirpated eye of triton and salamandra. Colucci first discovered in 1891 that if the eye is partially removed a new eye develops from the piece that remains and that _the new lens develops from the margin of the bulb_. Wolff, a few years later, not knowing of Colucci’s results, also found that after extirpation of the lens of triton, by making an incision in the cornea, a new lens develops from the edge of the old iris. Wolff pointed out the great theoretical importance of this result. The experiment has been repeated and confirmed by a number of more recent workers, so that there remains no question as to its accuracy. [Illustration: FIG. 60.--After Wolff. Regeneration of lens of eye of Triton. _A._ Edge of iris with beginning lens. _B, C, D._ Later stages of same. _E._ After Fischel. Whole eye with regenerating lens.] After the removal of the old lens the wound in the cornea quickly heals, and in the course of two or three weeks a thickening appears at one point at the edge of the iris (Fig. 60, _A_). The cells that produce this thickening are the ordinary deeply pigmented cells of the iris, where the outer layer of cells of the iris becomes continuous with the inner layer. The cells increase in number and produce a spheroidal ball that hangs down into the space formerly occupied by the lens (Fig. 60, _E_). The cells become clearer by absorbing their pigment and arrange themselves concentrically as in the normal lens. When fully formed the new lens separates from the iris and occupies the normal position.