In another experiment, twenty-two posterior ends were united in the same way, and then one of the two components was cut in two. In five cases a single head developed on the aboral end of the smaller piece ([Fig. 46], D). It is evident in this case that the union of the two pieces has been a factor in bringing about the development of an aboral head. Another of the grafts produced an aboral head, and also one in the region of union. The remaining sixteen grafts produced new heads, if they developed at all, only in the region of union. Peebles states that the regeneration of aboral heads takes place only when one component is cut off near the region of union of the two pieces.
In general, it may be stated in regard to these experiments in hydra that when pieces are united in the same direction, that is, by unlike surfaces, a single individual is formed and no regeneration takes place where the union has been made, but when like surfaces are brought together, although perfect union may result, a process of regeneration takes place later, at or near the line of union. Even the presence of cut-surfaces at one or both cut-ends of the united components does not generally affect the result, although, in a few cases, it may change it, in so far that heteromorphic regeneration may take place from one piece. This sometimes leads to a suppression of regeneration at the line of union. The experiments do not show, perhaps, conclusively whether the heteromorphosis of the smaller component is due to the polarity of the larger component effecting a change in the smaller one, or whether the closing of the oral end of the smaller component (by its union with the other) brings about the result. All things considered, it seems to me that the larger component has directly influenced the other.
King has found that if two posterior pieces of hydra are united by the oral cut-surfaces, and then after they have fused both pieces are cut off near the line of fusion, there develops from the small piece a single hydra, with a foot at one end and tentacles at the other. If only one of the pieces is cut off near the line of fusion, a new head develops from its oral surface, as Peebles had found. If two anterior ends are united by their aboral cut-surfaces, and then later both are cut off near the line of fusion, a single hydra develops from the small, double piece. If one of the components is cut off near the line of union, a foot develops from the oral cut-end. If in any of the cases the cut is made some distance from the line of union, then each cut-surface develops its typical structure. These experiments leave no doubt as to the influence of the larger piece on the smaller one. Especially interesting is the formation of one individual from two short pieces united in opposite directions. In this case we must suppose that one piece has the stronger influence on the combination (perhaps because it is a little larger), and determines the polarization of the other piece.
King finds that when two posterior pieces are united by their oral ends, regeneration of one or of two heads often takes place at the line of union ([Fig. 47], B, B¹, B²), as Wetzel had found. If a dark green individual is united to a light green one, it can be seen that in many cases the new heads are formed by both components, as shown in [Fig. 47], B¹. Later one of the posterior ends is absorbed, and the halves may then separate ([Fig. 47], B¹, B²). If a number of pieces are united, as indicated in [Fig. 47], E, a number of heads may be formed, and one or more of these may have a double origin. No evidences of separation of the pieces was observed in cases of this sort.
Fig. 47.—After King. A. Hydra split in two, hanging vertically downwards. Later the halves completely separated. B. Two posterior ends united by oral surfaces. B¹. Same; it regenerated two heads, each composed of parts of both pieces. B². Absorption of one piece leading to a later separation of halves. C. Two posterior ends united by oblique surfaces. Later one piece partially cut off, as indicated by line. C¹. Later still, two heads developed, one at N, the other at M. D. Similar experiment in which only one head developed, at M. E. Five pieces united as shown by arrows. Four heads regenerated, one being composed of parts of two pieces.
In one experiment two posterior pieces were united by oblique surfaces, as shown in [Fig. 47], C, and one of the two was afterwards cut across, as indicated by the cross-line. The subsequent regeneration that took place is shown in [Fig. 47], C¹. A head, composed of parts of both pieces, developed at the cut-surface M, and another in the region N in [Fig. 47], C, composed of material of one component. In another case, shown in [Fig. 47], D, a head developed only at the cut-edge, but it was made up of material from both components.
A series of grafting experiments of another sort has been made