[Illustration: FIG. 23.--_Planaria maculata._ _A._ Cross-piece, allowed to regenerate, then cut in two lengthwise, as indicated by line. _a-a⁵._ Regeneration of left half.] If a cross-piece is cut from a worm and kept until a small amount of new tissue appears over the anterior and posterior cut-surfaces, and if then the piece is split in two lengthwise, there will develop from each piece a new head out of the new material over the anterior surface. The result shows that the new material is at first totipotent, in the sense that it may still produce one or more heads according to the conditions. It is possible, of course, that the formation of the new head may have begun at the time of the experiment, but if it had, the development had not gone so far that a new arrangement was impossible. If, however, the piece is not cut lengthwise until just before the formation of a head (Fig. 23, _A_), then each half-piece produces at first a half-head, that completes itself later at the cut-side. Another experiment shows even more satisfactorily that the material over an anterior cut-edge may produce one or more new heads according to the conditions, and that the result is not connected with the region from which the new material is derived. If the anterior end of a planarian is cut off and then an oblong piece is removed from the middle of the worm, as shown in Fig. 24, _A_, it will be found, if the side parts are kept from fusing together in the middle line, that a new head develops at the anterior end of each part, as shown in Fig. 24, _c, c¹_. If, on the other hand, the two sides come together and fuse in the middle line, as shown in Fig. 24, _a, b_, the new material that appears over their anterior ends becomes continuous and produces a single head. In this case, although the middle part of the old tissue has been removed, a single head develops that is normal in all respects, and the eyes are not nearer together than when the middle part is present, as when regeneration takes place from an anterior cross-cut surface. [Illustration: FIG. 24.--_Planaria lugubris._ _A._ Showing where a piece, 4. was removed from middle of a worm. _a, b._ Regeneration of a single head. _c, c¹._ Regeneration of two heads. _D, E, F._ Regeneration of small piece, 4. that was cut out.]