Feet with supernumerary digits can also be produced by artificial wounds. If the first and second and then the fourth and fifth toes are cut off, as indicated by the lines in [Fig. 43], E, so that a part of the tarsus and a part of the tibia and fibula are cut away (the third finger being left attached to the remaining middle portion), more toes grow out from the wounded surface than were removed, as shown in Fig. 43, F. A similar result may be obtained in another way. If the first and second toes are cut off by an oblique cut ([Fig. 43], G), and then after the wound has healed the third, fourth, and fifth toes are also cut off by another oblique cut (a part of the tarsus being removed each time), more toes are regenerated than were cut off[64] ([Fig. 43], H).
Tornier suggests that the double feet that are sometimes formed in embryos—even in the mammalia—have resulted from a fold of the amnion constricting the middle of the beginning of the young leg, in the same way as is brought about artificially by tying a string over the growing end of the regenerating leg of triton.
In many of these cases, in which the double structure is the result of splitting the part in the middle line, the completion of the new part is exactly the same as though the parts had been entirely separated. The only special problem that we meet with in these instances is that this doubling is possible while the piece remains a part of the rest of the organism. This shows that there is a great deal of independence in the different parts of the body in regard to their regenerative power, and that local conditions may often determine the formation of double structures.
It has been shown during the last decade that double embryos may be produced artificially by incomplete separation of the first two blastomeres. Driesch, Loeb, and others have demonstrated that if the first two cells of the egg of the sea-urchin be incompletely separated, each may produce a single embryo and the two remain sticking together. Wilson has shown in amphioxus that the same result occurs if the first two cells are partially separated by shaking. Schultze has shown in the frog that if at the two-cell stage the egg is held in an inverted position, i.e. with the white hemisphere turned upwards, each blastomere gives rise to a whole embryo—the two embryos being united, sometimes in one way, sometimes in another, as shown in [Fig. 63]. In this case it appears that the results are due to a rotation of the contents of each blastomere, so that like parts of the two blastomeres become separated. In the egg of the sea-urchin, and of amphioxus, gravity does not have a similar action on the egg, but the results seem to be due to a mechanical separation of the blastomeres. These cases of double structures, produced by the segmenting egg, appear to belong to the same category as those described above for adult forms—especially in those cases where pieces regenerate by morphallaxis.
Fig. 44.—A. Planaria lugubris, cut in two as far forward as region between eyes, regenerating half-heads. B. Same cut in two at one side of middle line. Smaller piece produced a new head. C. Planaria maculata, split in two. It produced two heads in angle. D. Another, that produced a single head in angle.
In connection with the production of double structures there should be mentioned a peculiar method of formation of new heads, first discovered by Van Duyne in a planarian. He found that if the animal is cut in two in the middle line, the halves being left united only at the head-end, as shown in [Fig. 44], D, C, there may appear one or two new heads in the angle between the halves. I have repeated this experiment with the same result, and have found that it may also occur when only a piece is partially split from the side of the body, as shown in [Fig. 44], B. In Van Duyne’s experiment the two new heads do not appear unless the cut extends far forward, but if the division extends into the region between the two eyes there may be formed, as I have found, a single eye on each side that makes a pair with the old eye of that side ([Fig. 44], A). It is evident in this case that each head has completed itself on the cut-side, the completion including the eye and the side of the head also with its “ear-lobe.” The result, in this case, is the same as though the pieces had been completely cut in two. If the cut does not extend quite so far forward there are usually formed one or two heads near the angle, each with a pair of eyes and a pair of ear-lobes ([Fig. 44], C). Sometimes a single head develops in the angle itself ([Fig. 44], D), and it is difficult to tell whether it belongs to one or to the other side, or whether it is common to both sides. Van Duyne spoke of the double and single head of the latter kind which he obtained as heteromorphic structures in Loeb’s use of the term. According to this definition, heteromorphosis is the replacement of an organ by one that is morphologically and physiologically unlike the original one, but this statement has been made to cover a number of different phenomena. The examples of heteromorphosis that Loeb gives by way of illustration of the phenomenon are: the production of a hydranth on the aboral end of tubularia, and the formation of roots in place of a stem in antennularia, etc. The formation of the heads in the angle in planarians does not appear to me to belong in this category. It seems rather that the phenomenon is of the same sort as the formation of a new head at the side of a longitudinal piece, and if so the new heads in the angle are, therefore, in their proper structural position for new heads belonging to the posterior halves. Even if it should prove true that a single head may develop exactly in the angle itself, and belong to both sides, it can be interpreted by an extension of the same principle.[65] The position of this median head turned backward suggests an obvious comparison with the production of the heteromorphic head in Planaria lugubris, but a closer examination will show, I think, that the two cases are different. The heteromorphic head is produced only when the head is cut off close behind the eyes. If cut off slightly behind this region, a posterior end is generally formed. But in the worms split lengthwise the head in the angle may be formed at a level much farther posteriorly than the eyes. If the split extends into the head, then the eyes that develop are the supplements of those of the old part. Our analysis leads, therefore, to the conclusion that the heads, or parts of heads, in the split worms are not heteromorphic structures but supplementary heads.
CHAPTER VIII
SELF-DIVISION AND REGENERATION. BUDDING AND REGENERATION. AUTOTOMY. THEORY OF AUTOTOMY
Self-division, as a means of propagation, is of widespread occurrence in the animal kingdom. In some cases the animal simply breaks into pieces and subsequently regeneration takes place in the same way as when the animal is cut into pieces by artificial means. In other cases the parts are gradually separated, and during this time new parts are formed by a process resembling that of regeneration after separation. A few zoologists have tried to show how the process of regeneration before separation has been derived from regeneration following self-division. It is our purpose to examine here the evidence in favor of this hypothesis.