Problem of Polarity
The control of polarity in regenerating coelentrates like hydra has received much attention for many years. You are already aware that while a hydra is cut in half, the half with the basal disc will form a hew hypostome and the half consisting of the hypostome will generate a new basal disc. Furthermore, if a hydra is cut into various segments perpendicular to the body axis, then each middle segment will regenerate both of the a basal disc and a hypostome. So every region of the hydra can give rise to a new organism.
Yet hypostomes, as well foot does not make anywhere or at all levels along the longitudinal axis of the animal; they form just only at the distal end. This indicates that a series of gradients arise from the two poles in Hydra that is rigidly polarized along the distal proximal axis. Grafting experiments have given further evidence for the existence of gradients in the hydra. When the hypostome tissue was added to the middle region of other hydra it formed a new bud with hypostome extending outward. When the basal disc cells were likewise grafted the new bud extended to form a new pedal disc and foot. In addition if grafts of both basal disc and hypostome cells were grafted together to the gastric region no supernumerary parts made. This observation suggests which signals from opposite ends of the hydra tend to counteract each other, causing the hydra to lose its polarity. Other experiments have as well shown which normal regeneration of the hypostome can be inhibited when an intact hypostome is grafted into the body of the hydra.