If a piece is cut from the anterior part of a worm by two oblique and parallel cuts, the new head appears at one side of the anterior cut-surface, and the new tail at the other side of the posterior cut-surface. The new pharynx appears in the new material of the posterior part in the middle line. Thus the middle lines of the new head and tail and pharynx lie in different positions, yet these parts are subsequently brought into the same line. This is done by the head extending more forward and becoming broader, the tail growing backward and also becoming broader. The old piece becomes narrower at the same time. These three changes going on simultaneously produce a new symmetrical worm. In one form, _Planaria lugubris_, the symmetrical form is reached largely by the forward growth and the enlargement of the head, and the growth backward and the enlargement of the tail (Fig. 22, _B_). In _Planaria maculata_ the old part shifts, so that it forms a new median line connecting the median line of the new head and new tail. This is best shown when the piece includes the old pharynx (Fig. 22, _C_). The pharynx is also shifted, so that its anterior end points towards the side at which the new head lies, and its posterior end towards the new tail. The result is that a new symmetrical worm is formed, as shown by the series of figures in Fig. 22, _C_. In _Planaria maculata_ the changes take place largely in the old part, and the old material extends throughout the entire length of the new worm. In _Planaria lugubris_ the change takes place largely in the new parts (Fig. 22, _B_). The general method in the latter species by which the symmetry is attained can be best shown by cutting the worm in two by an oblique cut just in front of the genital pore (Fig. 22, _A_). The posterior piece produces a new head at the side, and a new pharynx appears along the border between the new and the old parts, as shown in these figures. Its posterior end touches the middle line of the old part, and from this point it extends obliquely across the new tissue towards the middle of the new head. As regeneration goes on the new head is carried farther forward, it becomes larger, and the main region of new growth is found to be, in the figure, to the left side of the new part. As a result of these changes the new head turns forward, and comes to lie nearer the middle line of the old part. The pharynx is also turned more forward, and finally, as the new parts enlarge, the symmetrical form is produced. The internal factors that are involved in the development of these oblique pieces are very difficult to analyze. The position of the new head and tail at one side of the cut-edge is the most difficult phenomenon of all to explain. We may, I think, safely regard the first new material that is proliferated along the cut-edge as totipotent, and our special problem resolves itself into discovering what factor or factors determine that the new head is to form at the