Regeneration

by Thomas Hunt Morgan

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.

The most surprising fact in connection with the development of the new lens is that it arises from a part of the body from which the lens of the eye never develops in the embryo of this form or of any other vertebrate. In the embryo the lens develops from the ectoderm at the side of the head and only secondarily unites with the optic cup, that has come from an evagination of the anterior wall of the fore brain. In the regeneration of the adult lens, however, the ectoderm covering the eye takes no part in the formation of the new lens,—in fact, it is separated from the eye by the thick inner, mesodermal layer of the cornea. The lens develops, as has been stated, from the already differentiated layers of the iris. It is a point of further interest to notice that the cells that form the transparent lens come from the iris cells that are in part at least filled with black pigment. If this pigment remained in the cells the new lens, while it might be structurally perfect, would be physiologically useless. The pigment disappears, however, as the lens develops. In this case we find a highly specialized organ, the lens, developing out of tissue also specialized in another direction. It does not simplify the problem to point out that the lens and the iris are both parts of the eye, since they have arisen from different parts of the body and have only secondarily come into apposition with each other. Colucci was contented to point out that both the embryonic lens and the regenerated one come from ectoderm and that the result can be brought into harmony with the “germ layer” hypothesis.

Wolff has called attention to the fact that the new lens arises from the upper edge of the iris, and that this is obviously the most advantageous position in which it could develop from the iris, since by its own weight it falls into place as it develops. If the lens had developed from any other point of the margin, its position would be less advantageous, as it might not be brought into its proper position.

Fischel, who has more recently studied the regeneration of the lens in the larvæ of Salamandra maculata, finds that after the removal of the lens the iris is thrown into wrinkles or folds and may stick at first to the cut-edge of the cornea. After the cornea has healed, the iris returns to its normal position. He finds that the first changes are more or less alike around the entire rim of the iris and involve a partial absorption of the pigment, a separation of the inner and outer layers at the edge, and a swelling of the margin. These changes go only a little way in those parts that do not produce a lens, but at the upper edge of the iris they go farther and lead to the formation of a lens in that region. He finds also that a new lens develops in animals kept in the dark as well as in those kept in the light, and in the same way.

Fischel also tried the effect of removing a part of the upper edge of the iris at the time when the lens was extirpated, in order to see if, in the absence of this part, the lens would develop from other parts of the uninjured margin of the iris. He found that the new lens still comes from the upper edge of the iris from the part left after the operation and not from the intact edge in other parts. This seemed to show that an injury to the iris is in itself a stimulus that starts the formation of a lens. This conclusion is made probable by the results of other experiments in which the iris was stuck at several points, when new lenses began to develop at several of these regions of injury. In some cases Fischel found that two or more lenses began to develop when the iris had not been intentionally injured; but it is not improbable that some sort of injury may have been effected when the lens was removed. Fischel, as has been said, removed extensive portions of the upper part of the iris and found that a new lens could be formed at the cut-edge, even in the region of the pars ciliaris; and, even after the removal of the entire upper part of the iris, lens-like structures may appear in the inner or retinal layer of the remaining region.

If instead of removing the lens it is displaced by pressing on the cornea until the lens leaves its normal position and comes to lie in the vitreous humor, a new lens develops from the edge of the iris, as though the old lens had been entirely removed from the eye, but in the experiments in which this was done the new lens was not well developed. The result shows that it is not necessary that the old lens be removed from the eye in order to induce the regeneration of a new one, but only that the lens lose its normal position in the eye.

In regard to the stimulus that determines the development of the lens, Fischel agrees with Wolff that gravity has a share in producing the result. The absence of the old lens from its normal position, as well as the wrinkling of the cornea, may also enter in as factors. Fischel takes issue with Wolff as to the interpretation of the result as an adaptation, and states that “the organism always responds to a change of relation in only one way, whose direction is already determined by internal structural relations, without regard to whether the result is adaptive or not. The response follows each stimulus in a way determined by the limited possibilities of the cells. With such a uniformity in the reaction, the idea of a fundamental adaptability cannot be connected, since the reaction that appears to us to be adaptive in a series of complicated changes may be non-adaptive in another series.”

Whether Fischel has here really met Wolff’s argument is, I think, open to question. It does not alter the result to show that factors already existing enter into the process, so long as the organism is so constructed that just those factors are present that bring about a useful response. That the response may be sometimes imperfect does not affect seriously the argument—in fact, it makes the case all the more remarkable if these imperfect attempts are in the direction of useful responses. Fischel sums up his conclusions as follows: “It is not necessary, and it is irreconcilable with the facts, to describe the formation of the lens in a teleological sense, and to bring this case forward as a proof of the universal application of a teleological principle. As has been already stated, the facts in regard to this case show much more clearly that the organism reacts to each change always in a manner that corresponds to its limited possibilities without regard to a teleological principle. A planarian, for instance, responds to a stimulus and makes a new head, even when it possesses one or more already; a tubularian produces a hydranth at its basal end, if this end is freely surrounded by water; an actinian forms a new mouth on the side of its body, etc.; so also do the cells of the pars ciliaris, and the pars iridica retinæ differentiate into lens fibres. Working blindly, without respect to the consequences as far as they concern the whole, the one thing only is produced for which the conditions are present that bring about its formation in the cells.”