If this be really so it should be capable of proof, at least in so far that we should be able to establish that the power of completing or re-forming a damaged or lost part is a limited one, localized in certain parts and cell-layers. This can be actually proved, as may be seen from numerous cases in which the faculty of regeneration is associated with autotomy, that is, with the power of breaking off or dropping off a part of the body. Even in worms we find this power, as we mentioned before in speaking of the high regenerative capacity of _Lumbriculus_. This worm reproduces in summer by what is called 'schizogony,' that is, by breaking into two, three, or more pieces, and it does not seem to require a very strong stimulus, such as pressure of the end of the worm by the jaws of an insect larva, to start this rupture; it often follows from quite insignificant friction on the ground. Certainly the power of regeneration is so great in this animal that it is out of the question to talk of localizing the primary constituents of regeneration; almost every broken surface is capable of regeneration. But this localization is well illustrated in Insects and Crustaceans, which possess the power of self-amputation in their appendages, especially in their legs. As far back as 1826 MacCullock observed this remarkable power in crabs, and described the mechanism on which it depends. When the leg is irritated, for instance when it is pinched at the tip and held fast, it breaks off at a particular place. This line of breakage lies in the middle of the short second joint (Fig. 98, _A_ and _B_, _s_), just between the insertions of the muscles (_me_, _mf_, _m_) which extend from this line towards the extremity of the limb and in the opposite direction towards the body-wall. Between these muscle-attachments the external skeleton is thin and brittle, and forms a suture, _s_, which breaks through when the animal contracts the muscles of the leg convulsively, and thus presses the lower protuberance (_a_) against a projection (_b_) of the first upper joint. Crabs require to make a very considerable muscular exertion before they can throw off the limb, and therefore they can only do it when they are in full vigour. [Illustration: FIG. 98. The leg of a Crab, adapted for self-mutilation or autotomy. _A_, the first three joints of the limb, _I_, _II_, _III_. _s_, the suture, that is, a thin area on the second joint which is predisposed to breakage. _mf_, flexor muscle, _me_, extensor muscle, both inserted at the suture. _B_, the entire leg with its six joints and with the suture (_s_). Slightly enlarged. After MacCullock.]