How gravity acts on antennularia has not as yet been determined. The only suggestion that we can offer at present is that it brings about a rearrangement of the lighter and heavier parts of the tissues. A rearrangement of this sort has been demonstrated when the egg of the frog is inverted, and in consequence certain changes are brought about in the development that will be described in another chapter.
EFFECT OF CONTACT
The contact of a newly forming part with a solid body has been shown by Loeb in a few cases, at least, to be a factor in regeneration. If a piece is cut from the stem of the tubularian hydroid Tubularia mesembryanthemum, and the piece held so that its basal end comes in contact with a solid body, a root develops at that end. If a piece is held in a similar position, but with its apical end in contact with a solid body, a root does not develop from this end. Evidently the development of a root in this form is also connected with an internal factor; but that there is in reality a reaction in this case, and not simply the development of a root at the basal end, is shown by the following experiment: If a piece is cut from the stem and suspended so that both ends are surrounded by water—it makes no difference whether the piece is vertical or horizontal—a hydranth develops first on the apical end, and then another on the basal end ([Fig. 15], B). When the apical end of a piece is stuck in the sand, leaving the basal end free, a hydranth develops on the latter, but not on the end in the sand.
Fig. 15.—After Loeb. A. A piece of the stem of margelis placed in a dish. Roots come off where stem touches dish, and polyps at other points. B. Piece of the stem of tubularia producing a hydranth at each end. C. Cerianthus membranaceus. Piece cut from side producing tentacles only on oral side of cut.
In another hydroid, Margelis carolinensis, studied by Loeb, the effect of contact is more easily demonstrated. If a branch of margelis is put into a dish of water and is kept from all motion, the parts that come in contact with the dish produce roots that attach themselves. Even the apical end of the stem may grow out as a root, as shown in [Fig. 15], A. Those parts of the branch that are not in contact with any solid object give rise to new hydranths. Another hydroid, Pennaria tiarella, also shows, according to Loeb, the same response to contact. In this connection it is interesting to find that a growing hydranth of pennaria, if brought in contact with a solid body, turns away from the region of contact and bends at right angles to the body which it touches. We find, once more, that a factor having an influence on the growth of the animal has also a similar influence on the regeneration.
Loeb has found that if pieces of the hydroid Campanularia are cut off and placed in a dish filled with sea water, all the hydranths that touch the bottom of the dish are absorbed and transformed into the substance of the stem. The cœnosarc may creep out of the stem wherever it comes in contact with the glass, and produce stolons that give rise to new polyps on their upper surfaces. Loeb shows that growth takes place at the end of the stolon that pushes out of the perisarc, and this growing region draws the rest of the cœnosarc after it. If a new hydranth appears along the old piece, the cœnosarc is drawn towards the hydranth.
EFFECT OF CHEMICAL CHANGES IN THE ENVIRONMENT
Temperature, light, gravity, and contact are the most familiar kinds of external physical agencies that have a direct influence upon the growth of organisms. Food, though coming from the outside, yet acts only after it has entered the body. Organisms that live in water may be affected by the quantity and the kinds of the salts contained in the water, and also by the dissolved gases. The only experiments that have been made to show the influence of this last class of agents on animals are those made by Loeb. He placed pieces of the stem of tubularia in sea water of different degrees of concentration. After eight days the pieces, that had meanwhile produced hydranths, were measured. It was found that the maximum growth in length takes place, not in normal sea water, but in a much diluted solution. Loeb interprets this result to mean that the cells of tubularia must have a certain amount of turgidity in order to grow, and this is possible so long as the concentration does not pass a certain limit. This limit is reached by the addition of 1.6 grams of sodium chloride to each 100 c.c. of sea water. With a decrease in the concentration, the cells become more turgid, the maximum point corresponding to the maximum amount of growth. Below this point the solution is supposed to act as a poison. The most important result of this experiment is to show that the maximum growth does not take place in sea water in which the animal is accustomed to live, but in a much more dilute solution. Normal sea water contains about 3.8 per cent of salts; the maximum growth takes place in a solution containing only 2.2 per cent. Not only is the length of the stem greater in the latter solution, but the thickness of the stem is also greater. The stem is smaller in a solution containing more salt than that contained in ordinary sea water.