GRAFTING AND REGENERATION 169 



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, 1 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 constric- 

 tion 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 car- 

 ried 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 per- 

 fectly 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, sub- 

 sequent 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 per- 

 manent union results when similar regions are united by unlike sur- 

 faces, 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 no doubt other modifying influences must be also 

 taken into account ; but it is important that attention should be 

 drawn to this side of the subject. 



1 Pieces from male and female colonies of the same species also unite. 



