But the excision of the lens in these experiments was not by any means an unprecedented occurrence! It is true enough that newts in their pools are not liable to an operation for cataract, but it does not follow that the lens is never liable to injury, and could not therefore be adapted for regeneration. It can be bitten out along with the rest of the eye by water-beetles or other enemies, and as far back as the time of Bonnet and Blumenbach (1781) it was known that the eye of the newt would renew itself if it were cut out, given that a small portion of the bulb was left. But if this were removed the possibility of regeneration was at an end. Thus, before the first artificial excision of the lens, a regeneration-mechanism must have existed, by means of which the eye with its lens was reconstructed, and this depends on the characters of the cells of the eye itself--it is localized in the eye, and without the presence of a piece of eye-tissue no regeneration can take place. Is it then so especially remarkable that the lens should be renewed when it is artificially removed without the rest of the eye? The mechanism for its renewal is there, and is roused to activity whether the lens alone or other parts of the eye also be removed. We do not need, therefore, to assume the existence of a purposeful or adaptive force; it is more to the point to inquire where the regeneration-mechanism which suggests this inference is to be found.
A definite answer to this is given in a detailed experimental work recently published by Fischel. It corroborates what Wolff had already found, that the substance of the new lens develops from cells which cover the posterior surface of the iris, that is, from cells of the retinal layer of the eye. First, the margin of the pupil begins to react to the stimulus of the injury (extraction of the lens); its cells enlarge, become clear, while previously they were filled with dark pigment, and finally they proliferate. They thus form a cell-vesicle similar to the ectoderm-vesicle from which the lens arises in the embryo, and into this the already mentioned retina-cells from the posterior wall of the iris grow, elongate, and arrange themselves to form the so-called 'lens-fibres,' on whose form, arrangement, and transparency the function of the lens depends. This is marvellous enough, but not more marvellous than that a whole foot should grow on the cut stump of a newt's leg, or that a whole eye should arise from a residual fragment. Here, again, we do not know the processes which cause the arrangement of the cells and their often manifold locally-conditioned differentiations, in short, we do not know the _essential nature of regeneration_. But, in the meantime, we can endeavour to find out which cell-groups regeneration is bound up with in particular cases, so as to know where the vital particles, the 'determinants,' which condition regeneration, are placed by nature.
