ABSTRACT
The factors controlling hypostome formation in hydra have been investigated using transplantation techniques. Two factors have been demonstrated, (a) inhibition of hypostome formation and (b) threshold for inhibition.
The presence of a hypostome inhibits the formation of another hypostome. This confirms the results of other workers.
Inhibition can act in either a disto-proximal or a proximo-distal direction.
The level of inhibition is lower in proximal than in distal regions, i.e. the level or effectiveness of inhibition declines as the distance from the hypostome increases. An axial gradient in level of inhibition is postulated.
The difference in behaviour of regions from different axial positions when transplanted to the basal disc, i.e. when subjected to identical inhibiting conditions, indicated that regions have different thresholds for inhibition. Distal regions have higher thresholds than proximal regions; an axial gradient in threshold is postulated. The idea of a threshold for inhibition is a new concept as far as regulation in hydroids is concerned.
The interaction between threshold and level of inhibition is believed to control hypostome formation. Polarized regulation of hydra into a distal hypostome and a proximal non-hypostomal region is discussed in terms of these concepts.
The results are discussed in relation to those obtained by other workers on other hydroids. Re-interpretation of these results suggests that inhibition and threshold are controlling factors in the regulation of other hydroids.
