These observa­tions show that the phenomena of correla­tion or of the influence of the whole over the parts is due to peculiarities of circula­tion or the flow of sap; and that the isola­tion prevents the sap from flowing away to other parts of the plant. There is no need for assuming the existence of a mysterious force which directs the piece to grow into a whole.

Fig. 21   Fig. 22

3. Phenomena of inhibi­tion or correla­tion such as we have described in Bryophyllum are not lacking in the regenera­tion of animals, as experi­ments on Tubularia show.[154] Tubularia mesembryanthemum (Fig. 21) is a hydroid consisting of a long stem terminating at one end in a stolon which attaches itself to solid bodies such as rocks, at the other end in a polyp. The writer found that if we cut a piece from a stolon and suspend it in an aquarium it forms as a rule a polyp at either end (Fig. 22), but the velocity with which the two polyps are formed is not the same, the polyp at the oral end of the piece being formed much more rapidly—a day or one or two weeks sooner—than the aboral polyp. The process of polyp regenera­tion at the aboral pole could, however, be accelerated and its velocity made equal to that of the regenera­tion of the oral polyp by suppressing the forma­tion of the latter. This was accomplished by depriving the oral pole of the oxygen necessary for regenera­tion, e. g., by merely putting the oral end of the piece of stem into the sand. It was, therefore, obvious that the forma­tion of the oral polyp retarded the forma­tion of the aboral polyp. This inhibi­tion might have been due to the fact that a specific organ-forming material needed for the forma­tion of a polyp existed in sufficient quantity in the stem for the forma­tion of one polyp only at a time. This idea, however, was found to be incorrect since when the stem was cut into two or more pieces each piece formed a polyp at once at its oral pole and regenerated the aboral polyps also, but again with the usual delay. It seemed more probable then that the cause of the difference in the rapidity of polyp forma­tion at both ends lay in the fact that certain material flowed first to the oral pole and induced polyp forma­tion here but that this flow was reversed as soon as the polyp at the oral pole was formed or as soon as the forma­tion of the oral polyp was inhibited by lack of oxygen. The partial or full comple­tion of the forma­tion of the oral polyp acted as an inhibi­tion to the further flow of material to this pole. This idea was supported by an observa­tion made independently by Godlewski and the writer that if a piece of stem be cut out of a Tubularia, and if the piece be ligatured somewhere between the two ends, the oral and the aboral polyps are formed simultaneously. This would be comprehensible on the assump­tion that the retarding effect which the forma­tion of the oral has on the aboral polyp was indeed of the nature of a flow of material towards the oral pole.

Fig. 23

Miss Bickford[155] found that the difference in time between the forma­tion of the two polyps disappears also when the piece cut from the stem becomes so small that it is of the order of magnitude of a single polyp. In that case two incomplete polyps are formed simultaneously at each end (Fig. 23). The new head in the regenera­tion of Tubularia arises, as Miss Bickford observed, from the tissue near the wound. At some distance from the wound in the old tissue two rows of tentacles arise, which are noticeable as rows of longitudinal lines inside the stem before the head is formed. Driesch noticed that the newly formed head is the smaller the smaller the whole piece. (This is true, however, only in rather small pieces.) There is, therefore, in small pieces a rough propor­tionality between size of head and size of regenerating piece. Driesch[156] uses this interesting fact to prove the existence of an entelechy, while we are inclined to see in it an analogue to the observa­tion of Leo Loeb, that the velocity of the process of healing in the case of a deficiency of the epithelium decreases when the size of the uncovered area diminishes. While we do not wish to offer any sugges­tion concerning the mechanism of these quantitative phenomena—they may be related in some way with the velocity of certain chemical reac­tions—we see no reason for assuming that they cannot be explained on a purely physico­chemical basis.

The writer noticed that certain pigmented cells from the entoderm of the organism always gather at that end where a new polyp is about to be formed. These red or yellowish cells always collect first at the oral end of a piece of stem. It may be that certain substances given off by the pigmented cells at the cut end are responsible for the polyp forma­tion, but this is only a surmise.

Another sugges­tion made by Child,[157] is that there exists an axial gradient in the stem whereby the cells regenerate the more quickly the nearer they are to the oral pole. If this were correct, and we cut a long piece from the stem of a Tubularia and bisect the piece, the oral pole of the anterior half should regenerate more quickly than the oral pole of the posterior half. According to the writer’s observa­tions on a Tubularian (T. crocea) growing in the estuaries near Oakland, California, both oral ends regenerate equally fast in such cases.

4. The phenomena of regenera­tion in Cerianthus membranaceus, a sea anemone, can be easily understood from the experi­ments on Tubularians, if we imagine the body wall of Cerianthus to consist of a series of longitudinal elements running parallel to the axis of symmetry of the animal from the tentacles to the foot. The number of these elements may be supposed to correspond to the number of tentacles in the outer row of the normal animal. Each such element behaves like a Tubularian, with this difference, however, that the elements in Cerianthus are more strongly polarized than in Tubularia, and that each one is able to form a tentacle at its oral pole only. This fact can be nicely illustrated in the following way: if a square or oblong piece (a b c d, Fig. 24) be cut from the body wall of a Cerianthus in such a way that one side, a c, of the oblong is parallel to the longitudinal axis of the animal, tentacles will grow on one of the four sides only; namely, on the side a b.[158] (Fig. 25.) The other three free edges are not able to produce tentacles. If an incision be made in the body wall of a Cerianthus, tentacles will grow on the lower edge of the incision (Fig. 26).

Fig. 24   Fig. 25   Fig. 26