The writer tried whether or not by tying a ligature around the middle of a piece of an Actinian this polarity could be suppressed; but the experi­ments did not succeed, inasmuch as the cells compressed by the ligature died, and were liquefied through bacterial action so that the pieces in front and behind the ligature fell apart. It is therefore impossible to decide whether or not a current or a flow of substances in a certain direc­tion through these elements is responsible for this polarity, though this may be possible. The writer found, however, that one condi­tion is necessary for the growth and regenera­tion of tentacles which also plays a rôle in the corresponding phenomena in plants, namely turgidity. The tentacles of Cerianthus are hollow cylinders closed at the tip, and by liquid being pressed into them they can be stretched and appear turgid. If, however, an incision is made in the body, the tentacles above the incision can no longer be stretched out. In one experi­ment the oral disk of a Cerianthus was cut off; very soon new tentacles began to grow at the top, and after having reached a certain size, an incision was made in the animal. The tentacles above the incision collapsed in consequence and ceased to grow, while growth of the others continued. On the lower edge of the incision new tentacles began to grow.

Fig. 27    Fig. 28   Fig. 29

It seems also possible that Morgan’s well-known experi­ment on regenera­tion in Planaria can be explained by a flow of substances. He[159] found that if a piece a c d b be cut out of a fresh-water Planarian at right angles to the longitudinal axis (Fig. 27), at the front end a new normal head, at the back end a new tail, will be regenerated (Fig. 28); but that if a piece a c d b be cut from a Planarian obliquely (Fig. 29) instead of at right angles to the longitudinal axis a tiny head is formed at the foremost corner of the piece a and a tiny tail at the hindmost corner b (Fig. [30]). Why is it that in the oblique piece the head is formed in the corner and not all along the cut surface as is the case when the cut is made at right angles to the longitudinal axis? The writer is inclined to believe that the right answer to this ques­tion has been given by Bardeen.[160] This author has pointed out the apparent rôle that the circulatory (or so-called digestive) canals in Planarians play in the localiza­tion of the phenomena of regenera­tion, inasmuch as the new head always forms symmetrically at the opening of the circulatory vessel or branch which is situated as much as possible at the foremost end of the regenerating piece of worm. He assumes that through muscular action the liquids of the body are forced to stream toward this end, and that this fact has some connec­tion with the forma­tion of a new head. There can be no doubt that the facts here mentioned agree with Bardeen’s sugges­tion. The oblique pieces in Morgan’s experi­ments which at first have the heads and tails outside the line of symmetry of the middle piece, gradually assume a normal posi­tion (Figs. 31, 32). The writer is inclined to believe that this is due to mechanical condi­tions. The head a e c of such an oblique piece is asymmetrical, the one side a e being less stretched than the other e c. The higher tension of the piece e c will have the effect of bringing e nearer c, since we know that acid forma­tion and hence energy produc­tion increases in propor­tion to surface, i. e., it must be the greater the more it is stretched. The reverse is true for the tail d f b, and the effect here will be that f will be pulled nearer d. In this way purely mechanical condi­tions are responsible for the fact that the soft tissues of the animal are gradually restored to their true orienta­tion.

Fig. 30   Fig. 31   Fig. 32

As a final possible example of the influence of internal secre­tion or substances contained in the blood may be mentioned the following curious observa­tion of Przibram.[161] In a crustacean, Alpheus, the two chelæ (pincers) are not equal in size and form, one being very much larger than the other. Przibram found that when he cut off the larger pincer in such crustaceans the remaining pincer assumes in the next moulting the size and shape of the removed large pincer; while in place of the removed pincer one of the small type is produced. Hence a reversal of the two pincers is thus brought about. If later on the large pincer is again cut off the process is repeated and the original dissymmetry is restored. Przibram was able to show that the nervous system has no connec­tion with this phenomenon.

The elements which have entered into the discussion thus far are, first, the flow of substances in preformed channels; second, the existence of general or specific substances required for the growing or regenerating organ. A third element is to be added; namely the “suc­tion” effect upon these substances of a developing organ. Thus we see that if one or a few of the notches in a leaf of Bryophyllum grow out the other notches of the leaf are inhibited from growing. There is enough material present in the leaf for all the notches to grow into shoots as is proved by the fact that all will grow out if they are isolated from each other. This was explained on the assump­tion that the notches of a whole which happen to develop first, create a flow of these substances from the rest of the leaf to themselves and thus prevent any getting to the other notches. We stated that this is supported by the fact that the few notches growing out in an undivided leaf grow more rapidly than the many shoots growing from each notch of a divided leaf. But why should a growing shoot or a growing point in general produce such a suction? I think this may be possible on the assump­tion that the consump­tion of these substances by the growing organs causes a low osmotic pressure of these substances in the growing region and this fall of osmotic potential will act as a cause for the further flow. This brings about the apparent “suc­tion” effect of the growing elements upon the flow of substances.

5. We mentioned that when a piece is cut from a Planaria between pharynx and head a new mouth is formed in the middle. It should also be mentioned that according to Child the piece after regenera­tion is smaller than it was before.[162] This indicates that material in the old cells has been digested or has undergone hydrolysis in order to furnish the nutritive material for the new head and tail, since the piece cannot take up any food from the outside before a mouth is formed. These phenomena of autodiges­tion—the process itself will be discussed in the last chapter—seem to occur in many (if not all) phenomena of regenera­tion. It may be that the collecting of red cells at the end in a Tubularian where regenera­tion is about to begin has to do with the furnishing of material by self-diges­tion, since these cells are partly at least destroyed in the process. It is of interest to look for more examples of autodiges­tion accompanying phenomena of regenera­tion.

Fig. 33   Fig. 34

Fig. 35