[Illustration: FIG. 46.--_A._ Two posterior pieces of hydra united by their oral ends. _B._ Two anterior pieces of hydra united by their aboral ends. _C._ A “long hydra” made by uniting three pieces; the middle piece reversed. _D._ After Peebles. Two posterior pieces of brown hydra united by oral ends, and one cut off near union. A new anterior end developed from the cut, aboral surface. _F._ After Peebles. Union of a nutritive and a protective polyps of hydractinia. Subsequently former cut off at line, 1-1. _E._ Union of two posterior pieces of hydra by oral ends. Subsequently one piece cut off at line, 2-2. _E¹._ New head regenerated in region of union, and a foot from aboral cut-end. _E², E³._ Fusion of two parts with a single hydra.] In other experiments of this same sort a foot generally developed where the two aboral surfaces came together, and the head-end separated from the rest of the piece. In another case a mouth and tentacles appeared at the place at which the oral ends had united. In a different kind of experiment, the anterior ends of two hydras were cut off and united by their aboral surfaces; then one of the components was cut in two, just back of the circle of tentacles. After five days two short, hook-like processes appeared at the cut, oral end. They produced a foot, by means of which the animal fixed itself. In this case it will be seen that a foot developed from an oral end. The result might not in itself be considered sufficient to show whether the development of a foot at the oral end of a piece is due to the influence of the other component, or is simply a case of heteromorphosis having no connection with the presence of the other component. Since heteromorphosis has never been observed in isolated pieces of hydra, the probability is that the result is in some way connected with the presence of the other component. Peebles has made a number of experiments, in which special attention was paid to this point. Fifteen anterior pieces were united in pairs by their aboral cut-surfaces, and then one component was cut in half, leaving an exposed oral end. Out of this number five pieces formed a new head at the cut-surface, and the pieces became attached by a foot, that developed at the region of union. Two others did not regenerate at the cut-surface, but became fixed as before, and neither regenerated nor became fixed at the cut-end. Three became attached at the cut, oral surface, but none of these developed a characteristic foot. The result shows, nevertheless, that some influence was present that inhibited the development of a mouth and tentacles at the oral cut-end, since these always develop in isolated pieces. In another series of experiments posterior ends were united by their oral surfaces, and then one of the two pieces was cut in two (Fig. 46, _E_). A new hypostome and tentacles developed at the region of union, and a foot at the aboral cut-surface, as shown in Fig. 46, _E¹_. An organism,