In the regeneration of some of the lower animals, the transformation of a piece into a new animal of smaller size is brought about by a change in form of the piece itself, rather than through the production of new material at the cut-ends. If a ring is cut from the body of hydra, as shown in Fig. 5, _A_, the open ends of the ring are soon closed by the contraction of the sides of the piece, and in the course of a few hours the ring has become a hollow sphere; or, if the piece is longer, a closed cylinder. After a day or two, the piece begins to elongate, and four tentacles appear near one end (Fig. 5, _B_, _C_, _D_). The piece continues to elongate until it forms a small polyp, having the typical proportions of length to breadth (Fig. 5, _E_, _F_). It has changed into a new cylinder that is longer than the piece cut off, but correspondingly narrower. In this case there cannot be said to be a replacement of the missing parts, but rather, through the transformation of the old piece, the formation of a new whole. In planarians also the formation of a new worm from a piece involves a change in the form of the old part, as well as the addition of new material at the cut-end. If a cross-piece is cut out, as shown in Fig. 4, _D_, new material appears at the ends, but the old piece also becomes narrower and longer (Fig. 4, _D¹-D⁴_). If the old head is cut off, it produces new material at its posterior end (Fig. 4, _E_, _E¹_), and also becomes smaller as the new part grows larger (Fig. 4, _E²_, _E³_). In a land planarian, _Bipalium kewense_, a piece is transformed into a new worm, as shown in Fig. 6, _A_, _B_. In this case the old pigment stripes of the piece are carried directly over into the new worm, the piece elongating during the transformation. [Illustration: FIG. 7.--_Stentor cœruleus._ _A._ Normal, fully expanded individual. _A¹._ Same contracted. Line _a-a_ indicates where it was cut in two. _B, C._ Pieces after division. _B¹, B², B³._ Regeneration of three distal pieces (_B_) containing old peristome. _C¹, C²._ Regeneration of two proximal or foot pieces (_C_).]