By inserting small pieces of hypostome tissue into a host hydra whose cells were labeled with India ink (colloidal carbon), Broun and Bode found that the hypostome induced a new body axis and that almost all of the resulting head tissue came from host tissue, not from the differentiation of donor tissue (Figure 1A). In contrast, when tissues from other regions (such as the subhypostomal region) were grafted onto a host trunk, a head and apical trunk of a new hydra formed from the grafted donor tissue (Figure 1B). In other words, only the hypostome region could alter the fates of the trunk cells and cause them to become head cells. Broun and Bode also found that the signal did not have to emanate from a permanent graft. Even transient contact with the hypostome region was sufficient to induce a new axis from a host hydra. In these cases, all the tissue of the new axis came from the host.
Broun, M. and H. R. Bode. 2002. Characterization of the head organizer in hydra. Development 129: 875–884.