A more striking illustration of the possible influence of tension of the parts is shown by an experiment with planarians. If the head of a planarian is cut off and the posterior piece is split partially in two along the middle line, as shown in Fig. 31, _A_, and then one of the halves is cut off just anterior to the end of the longitudinal cut, the result is as follows: A new head develops at the anterior end of the long half (Fig. 31, _B_), but no head develops on the posterior cut-surface, provided this part has reunited along the middle line with the long half, and a line of new tissue connects the anterior cut-surface of the long half and the more posterior cut-surface of the shorter half. At least this happens if the piece is not split too far posteriorly, _i.e._ through the region of the pharynx. If this is done, a new head may develop from the posterior cut-surface. In another way the development of the more posterior head can be brought about. If the shorter side-piece is kept from fusing with the longer side-piece in the middle line, it will invariably produce a new head (Fig. 31, _C_). The lack of development of the posterior head, when the two cross-cut surfaces are united by a connecting part of new material, can, it seems to me, be best explained by the influence of the developing anterior head, or of the new side on the posterior new tissue, and this influence can, I think, be better appreciated if we suppose some sort of tension to be the influence at work. Another example may be cited that shows even more clearly that the internal factor regulating the growth in the new part is probably some sort of tension. I refer to the development of the tail of fundulus from an oblique cut, or of the bilobed tail of stenopus from a cross cut. The assumption of the typical form that leads to the holding in check of the growth in certain regions, as compared with others, can be best understood, I think, as due to some sort of tension established in the different parts, that regulates the growth in those regions.