A number of other experiments that I made, in which pieces of antennularia were fixed to a rotating wheel, gave negative results, since neither roots nor stems appeared on the pieces. The rubbing of the ends of the piece against the water as the wheel turned round, or else the agitation of the water, prevented, most probably, the regeneration from taking place. How gravity acts on antennularia has not as yet been determined. The only suggestion that we can offer at present is that it brings about a rearrangement of the lighter and heavier parts of the tissues. A rearrangement of this sort has been demonstrated when the egg of the frog is inverted, and in consequence certain changes are brought about in the development that will be described in another chapter. _EFFECT OF CONTACT_ The contact of a newly forming part with a solid body has been shown by Loeb in a few cases, at least, to be a factor in regeneration. If a piece is cut from the stem of the tubularian hydroid _Tubularia mesembryanthemum_, and the piece held so that its basal end comes in contact with a solid body, a root develops at that end. If a piece is held in a similar position, but with its apical end in contact with a solid body, a root does not develop from this end. Evidently the development of a root in this form is also connected with an internal factor; but that there is in reality a reaction in this case, and not simply the development of a root at the basal end, is shown by the following experiment: If a piece is cut from the stem and suspended so that both ends are surrounded by water--it makes no difference whether the piece is vertical or horizontal--a hydranth develops first on the apical end, and then another on the basal end (Fig. 15, _B_). When the apical end of a piece is stuck in the sand, leaving the basal end free, a hydranth develops on the latter, but not on the end in the sand. [Illustration: FIG. 15.--After Loeb. _A._ A piece of the stem of margelis placed in a dish. Roots come off where stem touches dish, and polyps at other points. _B._ Piece of the stem of tubularia producing a hydranth at each end. _C._ _Cerianthus membranaceus._ Piece cut from side producing tentacles only on oral side of cut.]