Regeneration by means of morphallaxis takes place only in those forms in which the body is not made up of a series of separated parts. This kind of regeneration occurs in those organisms in which the normal growth consists only in the enlargement of a system of organs already present. A piece of an animal of this sort usually contains the elements of each kind of organ, and from these the new parts are produced, both by proliferation at the cut-ends and by the enlargement of the parts that are present in the piece. In forms with separate segments we find, in some cases, resemblances between normal growth and regeneration, as shown, for example, in the earthworm. There is present in the young worm a region in front of the last segment, or, rather, a part of this segment, from which new segments are formed. In the regeneration of the posterior end a knob of new tissue is formed, and out of this a few segments develop, the last one having a growing region similar to that in the young worm. The subsequent stages in the regeneration involve the formation of new segments from the last one, as in the young worm. There is no such growing zone at the anterior end of the young worm, and none is formed in the regeneration of an anterior end, so that only the segments that are first laid down in the new part are present in the new anterior end. An interesting comparison may be made between the phenomenon of growth and that of contraction and expansion of the protoplasm. The bending of heliotropic organisms toward or away from the light, and the similar bending of negatively stereotropic forms away from contact with a solid body, are supposed to be phenomena of growth, and resemble in many ways the phenomenon of contraction. In a plant that bends toward the light, it is found that the most obvious change involves the amount of water on the two sides of the stem, and this is most probably connected with a fundamental structural change in the protoplasm, that is too subtile for further analysis. In the regeneration of some forms it is found that they respond in the same way to light. While it cannot be demonstrated that these phenomena really depend on processes of contraction and of expansion, the results are nevertheless suggestive from this point of view. Furthermore, I think, one cannot study the regeneration of such forms as planarians, hydras, stentors, etc., without being struck by the apparent resemblance of the change in form that they undergo to a process of expansion. The idea of the expansion of a viscid body carries with it, of course, the idea of tension within the parts, and the return to the former condition is brought about by a release from the tension and a return to a more stable condition. If by the intercalation of new material the extended condition is fixed, a new state of equilibrium will be established.