An analysis of the chemical control of polarized form in hydra

Development and maintenance of polarized form in Hydra is controlled chemically by a diffusible factor, concentrated in, and probably produced by nerves in the hypostomal region of the animal. This polarizing (or inductive) factor may be isolated from tissue homogenates of Hydra. It is recovered from the supernatant fraction after centrifugating the homogenate at 25,000 × G for 50 minutes. Concentrated amounts of the extracted substance induce “heteropolarity” in regenerating annuli of Hydra pirardi. Heteropolarity is expressed through the development of supernumerary tentacles and “head” regions. Over 40 tentacles may be induced in heteropolar individuals, compared to the normal number of eight. The polarizing effect of this material is not species specific; H. viridis, H. littoralis , and H. pirardi contain extractable amounts of the active factor. Heterogeneous invertebrate tissue such as Dugesia extract does not induce heteropolarity. Because the active material is both dialyzable and can be digested by trypsin, it is suggested that it may be a peptide. Although the mechanism of action of the polarizing component has not been fully determined, the active material probably is bound by the tissues of the assay animal during the incubation period. It appears to act by directing the differentiation of interstitial cells. Interstitial cells in the annulus tested probably bind excessive amounts of the inducer and differentiate into nerve cells. Secretion of additional inducer from these nerve centers initiates the heteropolar state. The fact that heteropolar animals possess more extractable material per animal than do normal unipolarized hydra substantiates this conclusion, and suggests that a single inductive substance, varying quantitatively apico‐basally, controls the direction of differentiation of neoblast cells in Hydra .