The "head" consists of a conical hypostome region that contains the mouth and a ring of tentacles (which catch food) beneath it.
When hypostome tissue from one hydra is transplanted into the middle of another hydra, the transplanted tissue forms a new apical-basal axis, with the hypostome extending outward (FIGURE 22.4A).
These experiments have been interpreted to indicate the existence of a head activation gradient (highest at the hypostome) and a foot activation gradient (highest at the basal disc).
The head activation factor is concentrated in the hypostome and decreases linearly toward the basal disc.
THE HYPOSTOME AS ORGANIZER Ethel Browne (1909; also see Lenhoff 1991) noted that the hypostome acted as an "organizer" of the hydra.
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