GRAFTING AND REGENERATION l6l 



In a different. kind of experiment, the anterior ends of two hydras 

 were cut off and united by their aboral surfaces ; then one of the 

 components was cut in two, just back of the circle of tentacles. 

 After five days two short, hook-like processes appeared at the cut, 

 oral end. They produced a foot, by means of which the animal 

 fixed itself. In this case it will be seen that a foot developed from 

 an oral end. The result might not in itself be considered sufficient to 

 show whether the development of a foot at the oral end of a piece is 

 due to the influence of the other component, or is simply a case of 

 heteromorphosis having no connection with the presence of the other 

 component. Since heteromorphosis has never been observed in iso- 

 lated pieces of hydra, the probability is that the result is in some way 

 connected with the presence of the other component. Peebles has 

 made a number of experiments, in which special attention was paid 

 to this point. Fifteen anterior pieces were united in pairs by their 

 aboral cut-surfaces, and then one component was cut in half, leaving 

 an exposed oral end. Out of this number five pieces formed a new 

 head at the cut-surface, and the pieces became attached by a foot, 

 that developed at the region of union. Two others did not regener- 

 ate at the cut-surface, but became fixed as before, and neither regen- 

 erated nor became fixed at the cut-end. Three became attached at 

 the cut, oral surface, but none of these developed a characteristic 

 foot. The result shows, nevertheless, that some influence was present 

 that inhibited the development of a mouth and tentacles at the oral 

 cut-end, since these always develop in isolated pieces. In another 

 series of experiments posterior ends were united by their oral sur- 

 faces, and then one of the two pieces was cut in two (Fig. 46, E\ A 

 new hypostome and tentacles developed at the region of union, and 

 a foot at the aboral cut-surface, as shown in Fig. 46, E l . An organ- 

 ism, with one mouth and a circle of tentacles, and two bodies and 

 two feet, resulted. The bodies soon began to fuse together (Fig. 46, 

 F?} into a single one, and when the fusion had extended to the region 

 of the feet, they also fused into a single structure (Fig. 46, E 3 ), so 

 that a single hydra was produced. 



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. Pee- 

 bles 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. 



