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.” _THE PART PLAYED BY THE “GERM-LAYERS” IN REGENERATION_ Our examination of the origin of the tissues and organs in the new parts has shown that in most cases the old tissues give rise to the same kind of tissue in the new part; or in some other cases, as in the nervous system, the regenerating organs arise from the same “layer” as that from which they develop in the embryo. These facts have led many writers to state that the tissues and organs in the regenerated part arise from the same germ-layers as do the same parts in the embryo. It is supposed that ectoderm gives rise to ectoderm, and to those structures that arise from the ectoderm in the embryo, as, for instance, the nervous system, stomodæum, etc. The endoderm is supposed to give rise to endoderm, and to endodermal structures, and the mesoderm to mesoderm and its derivates. So fixed has this opinion become that it is not uncommon to find investigators proclaiming the triumphant success of their results, because they have been able to trace the organs in the regenerated part to the same germ-layers that give rise to these organs in the embryo. Before deciding as to the value of this point of view, let us examine briefly the foundations of the so-called germ-layer hypothesis.