PHYSIOLOGICAL CHARACTERISTICS OF AXIATE PATTERNS 103 



imal cut ends of a piece are negative to the middle region, a cut end is 

 negative to the hydranth, and a distal cut end is negative to a proximal 

 but may become positive during reconstitution. In Pennaria pieces an 

 end undergoing reconstitution is usually externally negative to other re- 

 gions; in Eudendrium it is usually positive. It does not appear from 

 Earth's data whether the potential of a developing hydranth differs from 

 that of a developing stolon or whether reconstitution of the Tuhularia hy- 

 dranth by reorganization of cells already present and reconstitution by re- 

 generation, that is, by outgrowth of new tissue from the cut end before 

 hydranth development, may account for some of these species differences. 

 At present the data appear to indicate that, in general, the apical region 

 is externally electronegative in intact hydroids with fully developed hy- 

 dranths; in forms like Tuhularia and Corymorpha, in which hydranth re- 

 constitution occurs by redifferentiation of a part of the stem without out- 

 growth of new tissue, the reconstituting hydranth is usually externally 

 negative to other regions; in forms like Obelia, in which outgrowth of new 

 tissue resembling a stolon precedes hydranth development at its free end, 

 this growing region is externally positive. Moreover, it is possible that the 

 sign of potential difference between the outgrowth and other levels of the 

 piece may differ according as the outgrowth is stolonic in character or a 

 hydranth-stem axis, and the potential of outgrowing tissue may differ 

 from that of the differentiating or differentiated hydranth. Further inves- 

 tigation is necessary to determine whether these suggestions have any 

 value; but concerning the presence of electric-potential differences as 

 characteristics of axiate pattern in hydroids there can be no doubt. Lund 

 regards the potentials as associated with, and dependent on, oxidation- 

 reduction. He has shown that in Obelia the axial potential difference can 

 be reversibly decreased or reversed in direction by cyanide, ether, and 

 chloroform — in other words, the axial potential exhibits a differential 

 susceptibility to these agents. 



In several species of hydromedusae Hyman and Bellamy found the 

 distal end of the manubrium most strongly negative externally, the mar- 

 gin of the umbrella next, then the subumbrellar surface, and the exum- 

 brellar surface positive to other parts. 



The galvanotactic reaction of hydra and of various hydroids and medu- 

 sae shows definite relation to the gradient pattern of the body or part con- 

 cerned." 



The gradients indicated by differential susceptibility and differential 



" Hyman, 19326; Bancroft, 1904; Hyman and Bellamy, 1922. 



