Fig. 20.—A, A¹. After Driesch. A. Piece of stem of tubularia cut off obliquely, showing oblique position of tentacles. A¹. Same, later stage. B. After Barfurth. Tail of tadpole regenerating from oblique surface. C. Tail of fundulus regenerating from oblique surface. D. After Hescheler. Anterior end of allolobophora regenerating from oblique surface. E. Piece of planaria, cut off by two oblique cuts, regenerating new head and tail. F, F¹, F². Three stages in the development of a new head (of a piece of bipalium) at anterior end of oblique surface.
in earthworms ([Fig. 20], D), both for the anterior and posterior ends. I have shown that it also takes place in the tail of a teleostian fish, fundulus ([Fig. 20], C), and have offered the following explanation of the phenomenon. The new material that is first laid down is, to a certain extent, indifferent as regards its axes. A symmetrical structure is then formed, with the old edge as a basis. The median point of the cut-edge connected with the median point of the outer surface of the new edge, gives the axis of symmetry of the new tail. The other regions assume corresponding positions. In the tail of the tadpole the position of the new notochord is determined by the cut-end of the old notochord and the median, outer point of the new material, and since the new material is at first equally developed along the cut-edge, or at least symmetrically developed, the new tail must stand at right angles to the cut-edge. This explanation will cover, I think, all cases of regeneration from an oblique surface. It assumes a law of symmetry in the new material that is in accordance with the observed position in which the new structure appears. The hypothesis makes no pretence to explain why the new structures should assume a symmetrical position, but given that they do, the observed result follows.
Fig. 21.—Planaria lugubris. Upper row. A. Part of head cut off obliquely; a-a⁴. Regeneration of new head. Lower row. B. More of head cut off obliquely; b-b⁴. Regeneration of same.
There are certain peculiarities connected with the regeneration from an oblique surface in planarians that may be considered in this connection. If the worm is cut in two by means of an oblique cut, as shown by the oblique line in [Fig. 21], B, the new head that appears on the anterior cut-surface of the posterior piece appears at one side and not in the middle of the oblique surface ([Fig. 21], B, b). The new head stands at right angles to the cut-surface. The anterior piece of the worm produces a new tail at the side of the posterior cut-surface, in the same way that the tail is formed in [Fig. 20], E. The tail also stands at right angles to the cut-surface. The new pharynx that develops in a piece of this kind appears in the middle of the posterior cut-surface, between the old and the new parts. It may extend somewhat obliquely in the new part, and point toward the new tail.
