When the hydranth protrudes from the stem it assumes an oblique position, as shown in Fig. 20, _A¹_. Driesch supposed the oblique position of the hydranth to be due to an oblique zone that develops behind the hydranth, but the result can best be explained, as certain other experiments that I have made seem to show, as due to the negative thigmotropism of the hydranth at the time it protrudes from the old perisarc. It turns away from the projecting side of the oblique end of the perisarc, as it does from any solid body with which it comes in contact. That this is the case is best shown by splitting the stem lengthwise into halves. In this case, although the two circles of tentacles may be laid down squarely (Fig. 25, _A_), the new hydranth protrudes at right angles to the old perisarc, as shown in Fig. 25, _B_. [Illustration: FIG. 25.--Piece of stem of _Tubularia mesembryanthemum_ split in two lengthwise. Formation of whole hydranth that turned away from contact with old perisarc.] _THE INFLUENCE OF INTERNAL ORGANS AT THE CUT-SURFACE ON THE NEW STRUCTURE_ In a few cases it has been discovered that the presence of certain organs at the exposed surface is necessary in order that regeneration may take place. The following experiment that I have recently carried out shows, for instance, the influence of the nerve-cord on the regenerating part. A few of the anterior segments of the earthworm are cut off, as shown in the left-hand figure in Fig. 26, and then a piece of the mid-ventral body wall of the worm is cut out, a part of the ventral nerve-cord being removed with the piece. The cut-edges meet along the mid-ventral line and fuse, closing the wound. As a result of the operation there is left exposed, at the anterior end of the worm, a cut-surface with all of the internal organs present except the nervous system. The anterior end heals over, but I have not observed the development of a new head at this level, although the exposed end is in a region at which, under ordinary circumstances, a new head readily regenerates. In several cases a new head developed at the point where the cut-end of the nervous system is situated, _i.e._ at the level _B_ in the figure. [Illustration: FIG. 26.--Left-hand figure X shows how, after cutting off the anterior end of _Allolobophora fœtida_, a piece of the ventral wall (including a part of the nerve-cord) is cut out. Right-hand figure _Y_ illustrates a more complicated operation, in which the piece of the ventral wall that is cut out is a little behind the anterior end.]