Fig. 54.—A. After Harrison. Union of two tadpoles by posterior ends. Two days after operation. The line to the left of plane of union indicates where the two were cut apart. B. Tail of right-hand tadpole in A. Five days after cutting apart. C. Same. Nine days after cutting apart. D. Same. Ninety-five days after cutting apart. E. After Born. Combination of Rana esculenta (anterior) and Rana arvalis (posterior). Thirteen days after the operation.
species, even when they belonged to different genera. It is found, however, that some of these combinations can be more easily made than others, but it is not clear whether the difference depends upon differences in the sizes of the pieces, or the rate of growth of the ectoderm over the cut-surfaces, or to a deeper-lying lack of affinity between the tissues. A combination of Rana esculenta (anterior) with Bombinator igneus (posterior) was made. The combination lived for ten days, and then showing pathological changes, it was killed. Another combination is shown in [Fig. 54], E, in which the anterior part of Rana esculenta was united to the posterior part of Rana arvalis.[88] The blood of the posterior component was driven through the vessels by the action of the heart of the anterior component. The animal lived for seventeen days.
In all these combinations between different species, each developing part retains its specific characters, and, although in several cases one part received its nourishment from the other through the common circulation, yet no influence of one component on the other could be observed.
Harrison has succeeded in keeping an individual made up of two species, Rana virescens and Rana palustris, for a much longer time,—until, in fact, the transformation of a tadpole into a frog had taken place. Each half retained the characteristic features of the species to which it belongs.
The absence of regeneration after the union of the pieces may be attributed, in several cases, to the absence of this power in the region through which the cut has been made; but in other experiments this cannot be the explanation, since the power to regenerate can be shown to exist in the part. This is the case in an experiment carried out by Harrison and repeated later by myself. If the tips of the tail of two tadpoles are cut off and interchanged ([Fig. 55], A, B), a perfect union takes place between the two parts, and a single tail develops. Each of the cut-surfaces has the power to regenerate, but the union of the parts has suppressed the regeneration. If, however, like parts are not brought in contact, regeneration may take place in the region of union ([Fig. 55], D).
Both Harrison and I have made a number of experiments, in which the end of the tail of a tadpole of one species was interchanged with a similar part of another species. It is found that as the new tail grows larger the ectoderm of the grafted piece is carried out to the tip of the new tail, as shown in [Fig. 55], C, and does not cover all the inner tissues that belong to the same piece, the rest of the tail being covered by the ectoderm of the major component. If the tip of the tail is now cut off, as indicated by the line b-b in [Fig. 55], C, there are left at the exposed edge two kinds of ectoderm, and from the cut-edge a new tail regenerates, covered in part by each of the two kinds of ectoderm. I made this experiment in order to see if the new ectoderm would show any influence of its dual origin, especially along the line where the two kinds are in contact, but no influence could be detected. In another series of experiments the grafted tail was cut off, as shown in [Fig. 55], A, or in [Fig. 55], B, or in [Fig. 55], C, a-a. In these cases there is left exposed, at the cut-edge, the internal tissues of the two species. The new tail that regenerates is composed in part of material derived from one species and in part from that of the other, but each tissue remains true to its kind, and there is found no evidence of an influence of one on the other ([Fig. 55], E). These experiments show that even when the two kinds of tissue regenerate side by side, and unite to form a single morphological organ, there is no influence of a specific kind of one tissue on the other.
