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. [154] Loeb, J., Untersuchungen zur physiologischen Morphologie. I. Heteromorphose. 1891. II. Organbildung und Wachsthum. Würzburg, 1892. [Illustration: FIG. 22] [Illustration: 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. [155] Bickford, E. E., _Jour. Morphol._, 1894, ix., 417. [156] Driesch, H., _Science and Philosophy of the Organism_, i., 127. 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.