I do not see how the opponents of the germ-plasm theory can explain these facts at all, for the appeal to external influences is here entirely futile, and that to internal liberating stimuli does not suffice, since these must be different after a part has been cut off from what they were when the limb developed normally, and also different from those which prevailed at the normal origin of the limb in ancestral forms. The four-jointed tarsus of the ancestors of our cockroaches did not arise as a result of amputation. We cannot therefore avoid referring the processes of regeneration to particular 'regeneration-determinants,' which are contained in the germ-plasm and are handed on in ontogeny with the other determinants from cell-division to cell-division, till ultimately they reach the cells which are to respond, or may have to respond, to the stimulus of injury by some expression of their regenerative capacity. As these determinants, as has been shown, can often only be very slightly subject to the influence of selection processes, they will, in many respects, lag behind in the phyletic development, and will tend to belong to an ancestral type of the relevant part. They will often remain for a long time at this ancestral level, and they will always adapt themselves to new requirements more slowly than the parts which arise in the normal way, and the determinants representing these in the germ. But the regeneration-determinants _are_ variable, and, indeed, are so hereditarily, and independently of the structure of the normal parts. They thus follow their own path of phyletic development, and this one fact is enough to secure a preference for the germ-plasm theory above others that have hitherto been suggested. None of these has even attempted an explanation of this fact; the tendency has rather been to call it in question. This, however, can be done at most only in regard to the explanation of the regenerations as atavistic, certainly not in regard to the progressive variations of the regenerated part, such as have been established by Leydig and Fraisse in regard to the lizard's tail. It may be doubted whether the most primitive insects had only four tarsal joints, but there is no disputing the kainogenetic deviation of the lizard's-tail.