Hargitt has also made a number of grafting experiments on other hydroids. His most interesting results are those in which parts of two medusæ were united by holding their cut-surfaces together by means of bristles passing through the individuals. Hargitt also finds that while in certain hydroids it is possible to bring about a union of oral with oral end, or aboral with aboral, or oral with aboral end of the same species,[81] yet a permanent union between different species cannot be brought about. These results are in agreement with those of a number of writers who have recorded the difficulty or impossibility of uniting parts of different species of hydra. In a few instances it has been possible to unite temporarily a piece of a brown hydra with a piece of a green one,--as I have also seen accomplished,--yet the pieces subsequently separate. Wetzel succeeded in obtaining better results with two species of brown hydras, _Hydra fusca_ and _Hydra grisea_. In one experiment the head of _Hydra grisea_ was grafted on the body (from which the head had been cut off) of _Hydra fusca_. After five hours the pieces seemed to have united. Later a constriction appeared at the place of union, and the head-piece produced a foot near the line of union, and the posterior piece produced a circle of tentacles at its anterior end. Eight days later, when the animal was being killed, it fell apart into two pieces. It was observed that during the period of union a stimulus to one piece was not carried over to the other. Wetzel’s results seem to show that pieces of these two species of hydra unite at first, when brought together, as perfectly as do pieces of the same species, but the union never becomes permanent, a constriction appearing later at the line of union, and the pieces separating in this region. These results indicate, it seems to me, that the factors that bring about the first union are different from those that make the grafted pieces one organic whole. Other results indicate that the union of oral to oral end, or aboral to aboral end, while at first as perfect as between unlike surfaces, nevertheless is less permanent than when unlike surfaces are united; at least, subsequent regeneration is more likely to occur in the former than in the latter, and after this occurs the separation of the individuals often takes place. It seems, moreover, not improbable that a more permanent union results when similar regions are united by unlike surfaces, than when the union is at different levels. If, for instance, the anterior half of one hydra is united to the posterior half of another individual, the union is generally permanent; but if one or both of the pieces are longer than half the length, so that a “long animal” results, new tentacles are more often formed at the oral end of one component, and the parts subsequently separate. It may be that, at present, the data are insufficient to establish this general rule, and