The regeneration of the muscles of the limb of an adult animal, plethodon, has been recently worked out by Towle. The leg was cut off in the middle of the forearm. Extensive changes take place in all the muscles that extend across the level of the cut. The old fibres in the lower end of the muscle, _i.e._ those near the cut-end, disintegrate, and the number of nuclei greatly increases. The division of the nuclei seems to be direct, each retaining some of the old muscle substance about itself. From some of these cells the new muscle tissue is formed in the new part. Higher up in the forearm the muscle fibres break down to a smaller extent, and still higher up some of the old fibres may remain intact. New muscle fibres are also formed in the old muscle, especially in the region near the cut-end. The process of regeneration has not been so fully worked out in any other vertebrates as in those described in the preceding pages, although the regeneration of _single tissues_ or organs in the vertebrates has been extensively investigated. In all such cases it is found that like tissues give rise to like. In the planarians it has been found that during regeneration the ectoderm covers the exposed surface, and from it arises the new ectoderm; the digestive tract appears to come in part from the old tract and in part from the middle-layer cells; the nervous system appears also to develop out of the middle-layer cells that are found scattered through the body. These cells seem to form a sort of reserve supply that gives rise to the digestive tract, nervous system, and middle-layer cells in the new parts. From them also arise the new pharynx, and the lining of the pharynx chambers, as well as some other structures. It is impossible to say at present whether one and the same kind of cell may give rise to all these structures, or whether different kinds of cells are present in the middle layer, that cannot be distinguished from each other by the methods at present at our command. The changes taking place in the tissues of those animals that regenerate by morphallaxis have been only quite recently carefully investigated. Bickford stated that in tubularia the old differentiated tissue changes over directly into the tissue of the new part, and Driesch confirmed this statement. Stevens has studied by means of serial sections the different changes that take place. Division of both ectodermal and endodermal cells is found to occur, but especially the ectodermal. Whether all the ectodermal cells divide, or only some of them, is difficult or impossible to state, but whether this happens or not, all the old region goes over into the new hydranth.