Fig. 42.—Planaria lugubris. A. Two heads produced after operation similar to that in [Fig. 24]. Each head about half size. B. Worm split in half through level of pharynx. New half-worms larger than half of normal worm.
the head that regenerates from an undivided stem, yet the number of tentacles on each head is more than half the average number. This is connected apparently with the fact that the circumference of each half is greater than half the circumference of the original stem. Planarians with double tails, produced by partial splitting, have been described by Dugès and by Faraday, and it has also been shown that by partial splitting of the anterior end of the worm two heads can be produced. Van Duyne, Randolph, and Bardeen and I have obtained the same result. Each half completes itself on the cut-side and produces a symmetrical anterior end. If one of the heads is cut off, it will be again regenerated. If the heads are united very near to the trunk, as in Fig. 42, A, they may never grow to the full size of the original head, as I have found; but if the pieces have been split posteriorly, so that each head has a long anterior end, then each one may become nearly as large as the original head ([Fig. 42], B). We see in these cases the influence of the region of union on the growth of the new part. If the new part is near the region of attachment, the smaller size of the latter restrains the growth of the new head; but if the region of union is farther distant, the head may grow more nearly to its full size despite the influence of the region of union. King has found in the starfish that if the arm is split lengthwise, each half may complete itself laterally and a forked arm result. An additional entire arm may be formed by splitting the disk partially in two between two arms. If the cut-edges do not reunite a new arm will grow out from each cut-surface ([Fig. 38], E). In this case the development of the new arm cannot be accounted for on the assumption that the typical form completes itself, since a sixth arm cannot be supposed to be a typical structure in the starfish. The result must depend on other factors, such as the presence of an open surface in a region where the cells have the power of making new arms.
Barfurth has been able to produce a double tail in the tadpole by the following method: A hot needle is thrust into one side of the tail, so that the notochord and the nervous system are injured. The tail is then cut off just posterior to the region injured by the needle. A new tail grows out from the cut-end, and also in some cases another tail grows out at the side where the notochord was injured by the needle. The injury to the notochord and the removal of tissue immediately about it leads to a proliferation of cells, around which other tissues are added and the new tail produced.
Lizards with double tails have often been described,[63] and it now appears that all these cases are due to injuries to the normal tail. Tornier has succeeded, experimentally, in producing double and even triple tails. If the end of the tail is broken off, and the tail is then injured near the end, two tails may regenerate, one from the broken end and one from the region of injury ([Fig. 43]). Under natural conditions this might occur if the tail were partially bitten off and the end of the tail lost at the same time. A regenerated tail may produce another tail if it is wounded. A three-tailed lizard may be made by cutting off the tail and then making two injuries proximal to the broken end. Two of the new tails may be included in the same outer covering if they arise near together, as shown in [Fig. 43], B. Lizards with two or three tails may be produced in another way. If the tail is cut off very obliquely, so that two or three vertebræ are injured, there arises from each wounded vertebra a cartilaginous tube that forms the axis of a new tail. Tornier thinks that the regeneration is the result of overnourishment of the region where the injury has been made, but this does not seem in itself a sufficient explanation. Tornier has also been able to produce, experimentally, double limbs in Triton cristatus in the following way: The limb is cut off near the body, and, after the cut-end has formed new tissue, a thread is tied over the end in such a way that it is divided into two parts. As the new material begins to bulge outward it is separated into halves by the constricting thread, and each part produces a separate leg ([Fig. 43], D). The soles of the two feet in the individual represented in [Fig. 43], D, are turned toward each other. The femur is bifid at its outer end, and to each end the lower part of one leg is attached. The bones in this part are fused together at the knee, so that only the foot portions can be separately moved.
Fig. 43.—After Tornier. A. Lacerta agilis. Produced by partly breaking off old tail. New tail arises at place of breaking. Old tail also remains. B. Three-tailed form—two tails being united in a common covering. Old tail had been cut off (it regenerated the lower branch from cut-end) and two proximal vertebræ that had been injured. C. Additional limb of Triton cristatus produced by wounding femur. D. Double foot of triton cristatus produced by tying thread over regenerating stump. E. Foot of Triton cristatus. Dotted lines indicating how foot was cut off. F. Regeneration of same. G. Another way of cutting off foot. H. Result of last operation.
The same method used to produce double tails in the lizard can also be used to produce double legs. The femur is broken in the vicinity of the hip-joint, and the soft parts are cut into over the break. Then, or better somewhat later, the leg is amputated below the broken part. A new limb regenerates from the cut-end, and at the same time another limb grows out from the broken femur ([Fig. 43], C). The same result is reached if the femur has a slit cut into it in the region of the hip-joint, so that it is much injured. Later the leg is cut off below the place of injury. A double leg is the result.