Regeneration . e it is the only case in which thehypothetical formative stuff has been definitely located in a specific,recognizable substance that can be followed during the process ofregeneration. It is well, I think, to give the question full considera-tion, especially as the hypothesis often appears to give an easy solu-tion of some of the problems of regeneration. In a later chapter thesubject will be more fully treated. INTERNAL FACTORS OF REGENERATION IN ANIMALS 6i Since the red-pigment hypothesis does not explain the phenomenonof the formation of the partial structures in tubularia, we
Regeneration . e it is the only case in which thehypothetical formative stuff has been definitely located in a specific,recognizable substance that can be followed during the process ofregeneration. It is well, I think, to give the question full considera-tion, especially as the hypothesis often appears to give an easy solu-tion of some of the problems of regeneration. In a later chapter thesubject will be more fully treated. INTERNAL FACTORS OF REGENERATION IN ANIMALS 6i Since the red-pigment hypothesis does not explain the phenomenonof the formation of the partial structures in tubularia, we must lookfor another explanation. As the matter stands at present we can onlyassume that there is a predisposition of a very small piece to form alarger partial structure than a smaller whole one. This problem of themethod of development of small pieces of the stem of tubularia is fur-ther complicated by the development in many cases of double hy-dranths, or double parts of hydranths, as shown in Fig. 29, D Fig. 29. — Tubularia mesembryanthemum. A. Short piece with hydranth at each end. B. Doublepiece with onecircle of proximal tentacles. C. Double piece vvilh only two proximal tentacles,D. Double proboscis with two sets of reproductive organs. E-E^. Double proboscis. The first form (Fig. 29, A) shows two hydranths turned in oppositedirections, that are united at their bases. Another form has only asingle circle of proximal tentacles between the two proboscides (, B-C\ In other forms there are only two proboscides, each with itsreproductive organs (Fig. 29, D\ and often there are simply two pro-boscides united at the base (Fig. 29, E-FJ). It is the rule, evenin longer pieces, that a hydranth appears at each end of the piece, ifthe piece is suspended or even lies on the bottom of the water; but 62 REGENERA TION in all these cases the basal hydranth develops about twenty-four hoursafter the apical one. In the short pieces, however, the two endsdevelop at the same t
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