[Illustration: FIG. 28.--_Tubularia mesembryanthemum._ Products of regeneration of short pieces. _A._ Piece that regenerated a hydranth in same way as do longer pieces, but with fewer tentacles. _B._ Pieces whose stem drew away from wall of old perisarc (cylinder in figures). _C._ Hydranth with almost no stalk. _D._ Hydranth without stalk. _E._ Distal part of hydranth with one long proximal tentacle. _E¹._ Similar, but more reduced. _E²._ Similar, with two tentacles at side. _F._ Proboscis with reproductive organs. _G._ Proboscis without reproductive organs.] endoderm, draws away from the chitinous perisarc, as shown in Fig. 28, _B_. A hydranth with a short stalk is then produced. In other cases, Fig. 28, _C_, almost all of the cœnosarc is used up to form the hydranth, and only a short, dome-shaped knob represents the stalk. In still other cases there may be no stalk at all (Fig. 27, _D_), but only the hydranth. Forms like the last two are more often produced from pieces of the distal end of the stalk. From very small pieces, forms like those shown in Figs. 28, _E-E²_, that represent only proboscides with a reduced number of tentacles, are sometimes formed. Reproductive organs may be present at the base of these pieces. A further reduction is shown in Figs. 28, _F_, _G_, that are proboscides with only the distal circle of tentacles; in one of these, reproductive organs are present around the base. Partial forms more reduced than these have not been found. If we examine the factors that determine the production of the partial structures, we find, in the first place, that the size of the piece is of the greatest importance. The reduced forms appear most often in pieces that are shorter than the average length of the hydranth-forming area. A second factor is connected with the region of the stem from which the piece is taken. Larger pieces from the distal end produce partial structures, especially hydranths with very short stalks (Fig. 28, _C_), or with none at all (Fig. 28, _D_). There are certain facts connected with this distal region, which lies just behind the hydranth, that should be mentioned in this connection. It was first discovered by Dalyell that a hydranth-head lives for only a limited time, and that when it dies a new head is regenerated from the region behind the old one. The stalk of the new hydranth continues to elongate for some time after the new hydranth has been formed. Whether this continuous growth in the distal end, or the normal formation of a new hydranth by it from time to time, can in any way be connected with the development of partial structures from this region cannot at present be stated. The distal part of the stem contains more of the red-pigment, that gives color to the stem and to the hydranth, than does any other part. Loeb first advanced the view that the red-pigment in the stem acts as a formative substance in Sachs’ sense, and determines the production of a