[Control of the morphogenetic steady state in the polyps of Hydractinia echinata : I. Biological-experimental...

1. At any axial level in the polyps the potencies to regenerate basal parts are suppressed, while apical regions can be regenerated. If regenerative formations occur at the basal end at all, they will become apical structures leading to bipolar forms with symmetrical axial pattern. 2. The probability of such heteropolar development is positively correlated to the distance between the apical pole and the basal cut. Moreover, the probability rises, at all distances, after removal of the hypostomes, which, therefore, are interpreted as being in a position of dominance. 3. The lag between amputation and the appearance of new tentacles increases from apex to base. This gradient, being less steep for heteropolar regeneration, ensures that the apical primordium normally will get a start to win the position of dominance. Accordingly the rate of heteropolarity rises after removing or destroying the apical primordium within 3-12 h after amputation of the hypostome. 4. The polar pattern of potencies changes after dissociation of tissue. Reaggregated cellassociations develop stolons, whereas the potency of stolon formation proves to be stably suppressed in all parts of intact polyps. This potency is released also after operative displacement of the polar axis by 90°. 5. Induction phenomena occur when two regions of different and distant axial level are brought into contact, either by transplantation or by parabiosis. The induced formations develop those axial regions that normally are located between the two combined regions. 6. In several experiments, including parabiosis combinations, implanting of marks and autoradiographic studies, the inductive system was shown to consist of two components: a) The apical system, the vector of which is directed from apex to base: regions above the stationary zone bring about the development of structures in more basal levels, that may occur spontaneously in regeneration and that belong to higher axial levels. b) The basal system, directed from base to apex: the hydrorhiza induces, in regions in which the tissue movement is orientated basally (regions below the stationary zone), structures the properties of which are characteristic of deeper axial levels and that can not be regenerated. The vectors of the inductive streams, therefore, are opposed to the vectors of tissue movement during steady state growth. After disconnecting the junction to the hydrorhiza, that is the source of the basal inductive stimulus, the basally directed stream of cellular material dies down. 7. Following ideas of gene physiology a model of the process regulating differentiation can be constructed, which will describe the development of polar patterns in regeneration, induction phenomena, budding and steady state growth, and which may attribute all morphogenetic processes to the same mechanisms of control. These mechanisms include activators as well as blocking factors subjected to the activating stimuli. The assumption of gradients established by diffusible substances is not implicitely necessary.