64 INDIVIDUALITY IN ORGANISMS 



parts with a higher respiratory rate would appear by the 

 usual methods as electro-negative to regions of lower 

 rate. 



Some twelve years ago Mathews' observed a differ-^ 

 ence in electrical potential along the main axis of certain 

 simple animals, the hydroids, the parts nearer the apical 

 end being electro-negative to those nearer the basal end. 

 In these forms the susceptibiHty method indicates that 

 the metabolic rate decreases from the apical toward the 

 basal end; that is, in the same direction as the decrease 

 in electro-negativity. Probably a similar electrical 

 gradient exists in nerves, although in the nerves of the 

 higher animals the change is undoubtedly very slight 

 within the physiological limits of length. As regards 

 the plants also various data on the differences of electric 

 potential suggest the existence of metabolic gradients, 

 although the fact that the observations were made with 

 other objects in view often leaves the evidence incon- 

 clusive as regards the matter of gradients. 



In the early stages of development of the starfish I 

 have been able to make the axial metabolic gradient 

 directly visible to the eye by differential staining in the 

 living animal,'' the stain in this case consisting of a 

 colored precipitate formed within the cells by the oxida- 

 tion of certain substances added to the water. The rate 

 of formation of this precipitate in different cells differs 

 with the amount or activity of enzymes or other con- 

 ditions which influence the rate of oxidation. In those 



^ A. P. Mathews, "Electrical Polarity in the Hydroids," Am. 

 Jour, of Physiol., VIII, 1903. 



* Child, "Axial Gradients in the Early Development of the Starfish," 

 Am. Jour, of Physiol, XXXVII, 1915. 



