Driesch suggested that the red-pigment in the circulating fluid determines quantitatively by its presence how much of a hydranth is formed, or the size of the hydranth in relation to the rest of the piece. There seems to be no evidence in favor of this view and much against it. Loeb has not stated specifically whether he means that it is the pigment in the circulating fluid or that in the walls which acts as a formative stuff; the presumption is that he meant the latter. An examination of the piece during regeneration gives no evidence in favor of the view that the pigment moves into the region of the new hydranth. On the contrary, it remains constant in amount at all points except where the new hydranth is developing, and there is in this region unquestionably a large development of new pigment. The evidence for and against the idea that the red-pigment of tubularia is a formative stuff, or even building material, has been considered at some length, because it is the only case in which the hypothetical formative stuff has been definitely located in a specific, recognizable substance that can be followed during the process of regeneration. It is well, I think, to give the question full consideration, especially as the hypothesis often appears to give an easy solution of some of the problems of regeneration. In a later chapter the subject will be more fully treated. [Illustration: FIG. 29.--_Tubularia mesembryanthemum._ _A._ Short piece with hydranth at each end. _B._ Double piece with one circle of proximal tentacles. _C._ Double piece with only two proximal tentacles. _D._ Double proboscis with two sets of reproductive organs. _E-E³._ Double proboscis.] Since the red-pigment hypothesis does not explain the phenomenon of the formation of the partial structures in tubularia, we must look for another explanation. As the matter stands at present we can only assume that there is a _predisposition_ of a very small piece to form a larger partial structure than a smaller whole one. This problem of the method of development of small pieces of the stem of tubularia is further complicated by the development in many cases of double hydranths, or double parts of hydranths, as shown in Fig. 29, _A-E_. The first form (Fig. 29, _A_) shows two hydranths turned in opposite directions, that are united at their bases. Another form has only a single circle of proximal tentacles between the two proboscides (Fig. 29, _B-C_). In other forms there are only two proboscides, each with its reproductive organs (Fig. 29, _D_), and often there are simply two proboscides united at the base (Fig. 29, _E-E³_). It is the rule, even in longer pieces, that a hydranth appears at each end of the piece, if the piece is suspended or even lies on the bottom of the water; but