no doubt other modifying influences must be also taken into account; but it is important that attention should be drawn to this side of the subject. Grafting experiments in planarians have so far been carried out in only the two cases which I have described. In one of these the anterior ends of two short pieces of _Bipalium kewense_ were united (Fig. 50, _A_). Neither piece produced a head at the region of union. Later the pieces were cut apart by an oblique cut that passed across the line of union (Fig. 50, _C_), so that each piece retained at its most anterior end (at one side) a piece of the other individual in a reversed position. A head developed at the anterior (and lateral) end of each piece, in such a way that a part at least of the small reversed piece was contained in the new head (Fig. 50, _D_). In the other case two pieces of bipalium were united by their posterior cut-surfaces. Each piece produced a new head at its free end, and the pieces greatly elongated, but remained sticking together (Fig. 51). [Illustration: FIG. 50.--_A._ Two pieces of _Bipalium kewense_ united by anterior ends. _B, C._ Later stages of same. Line in _C_ indicates how pieces were cut apart. _D._ Two worms produced by these pieces. All drawn to scale.] [Illustration: FIG. 51.--Two pieces of _Bipalium kewense_ united by posterior ends. Each regenerated a head at anterior end.] A large number of experiments have been made by Joest in grafting pieces of earthworms. The cut-surfaces were held in contact by means of two or three threads passing through the body wall of each piece and tied across, so that the pieces were drawn together and held firmly in that position. Joest found that pieces of the same or of different individuals could be united in various ways, and the union become permanent. If the anterior end of one worm is united to the posterior end of the same, or of another worm, a perfect union is formed, and no subsequent regeneration takes place (Fig. 52, _A_). Long worms can be made by uniting two pieces, each more than half the length of a worm, or by uniting three pieces, as shown in Fig. 52, _C_. Short worms can be formed by cutting a middle piece from a worm, and uniting the anterior and posterior pieces (Fig. 53, _D_). Joest found that when a short worm is made in this way, so that no reproductive region is present, the new worm does not produce new reproductive organs. It is conceivable that new reproductive organs might have been produced either in the old segments, or by the formation of a new reproductive region between the two united pieces, but neither process takes place. In the long worms two sets of reproductive organs, etc., are present. This sort of union is, however, less permanent, as the worms often pull apart.