The writer has observed more closely the transformation of an organ into more undifferentiated material in _Campanularia_ (Fig. 33), a hydroid.[163] This organism shows a remarkable stereotropism. Its stolons attach themselves to solid bodies, and the stems appear on the side of the stolon exactly opposite the point or area of contact with the solid body. The stems grow, moreover, exactly at right angles to the solid surface element to which the stolon is attached. If such a stem be cut and put into a watch glass with sea water, it can be observed that those polyps which do not fall off go through a series of changes which make it appear as if the differentiated material of the polyp were transformed into undifferentiated material. The tentacles are first put together like the hairs of a camel’s-hair brush (Fig. 34), and gradually the whole fuses to a more or less shapeless mass which flows back into the periderm (Fig. 35). It follows from this that in this process certain solid constituents of the polyp, _e. g._, the cell walls, must be liquefied. This undifferentiated material formed from the polyp may afterward flow out again, giving rise to a stolon or a polyp; to the former where it comes in contact with a solid body, to the latter where it is surrounded by sea water. These observations suggest the idea of reversibility of the process of differentiation of organs and tissues, in certain forms at least. We have to imagine that some of the cells or interstitial tissue is digested and that as a consequence the organ loses its characteristic shape. [163] Loeb, J., _Am. Jour. Physiol._, 1900, iv., 60. [Illustration: FIG. 34] [Illustration: FIG. 35] Giard and Caullery have found that a regressive metamorphosis occurs in Synascidians, and that the animals hibernate in this condition. The muscles of the gills of these animals are decomposed into their individual cells. The result is the formation of a parenchyma which consists of single cells and of cell aggregates resembling a morula.[164] [164] The writer quotes this after Driesch. Driesch,[165] experimenting on the regeneration of an Ascidian, found that when he cut off the gills and siphons of the animal the portion removed was able to regenerate a whole animal. The gill-piece excised contained no heart, no intestine, and no stolon, and all these organs were regenerated from the gills. In a number of cases the regeneration took place by bud formation at the edge of the wound, but in other cases the gills were transformed into an undifferentiated mass of tissue from which the missing parts of the animals arose by budding and new gills were formed. [165] Driesch, H., _Arch. f. Entwcklngsmech._, 1902, xiv., 247.