THE AXIAL GRADIENTS IN HYDROZOA. 419 



rate of chemical activity (with which are doubtless associated 

 other graded differences) constitute the basis of the phenomenon 

 of polarity. 



It appears further that there exist in these organisms per- 

 manent electric currents whose direction of flow bears a definite 

 relation to the respiratory gradient (for data on Tubularia, see 

 Hyman, '20, for other hydroids, Hyman and Bellamy, '22, 

 Lund, '22) .- This relation is the following: any part of the 

 organism is electronegative (in the external circuit) to any part 

 having a lower respiratory rate than itself. It is probable that 

 the gradation in rate of chemical activity is the chief cause of 

 the electrical gradient. Some biologists are of the opinion that 

 these electric currents constitute a tool, so to speak, which en- 

 ables one part of an organism to affect another part. 



There is some indication that in the hydroids the gradient is 

 steeper in the more apical levels and gradually flattens out basally. 

 This inference is drawn chiefly from the electrical data, the 

 potential difference being greatest in apical levels (Hyman and 

 Bellamy, '22, pp. 332-33, Lund, '22, p. 490); but in Corymorpha 

 the respiratory evidence is to the same effect (Hyman, '22). 

 Regardless of the slope of the gradient in distal levels, it appears 

 certain from electrical and other data that the gradient is slight 

 or absent or even reversed in the proximal levels of hydroid stems 

 (see references just given). It follows that at a certain distance 

 from the apical end, the primary gradation practically disappears 

 and new gradations running in the same or the reverse direction 

 may be initiated. The distance to which the primary gradation 

 extends coincides with the limits of the individual and beyond 

 this point buds, zooids, etc., may arise, if the constitution of the 

 protoplasm permits asexual reproduction; or if asexual repro- 

 duction is impossible, the basal or caudal parts of the organism 

 may be more or less independent physiologically or nervously of 



2 In a later paper ('25) Lund reversed his statement in the 1922 paper as to the 

 direction of the current in Obelia, without offering any explanation of the contra- 

 diction. It may be pointed out that Hyman and Bellamy ('22) tested the P. D. 

 along the main axis of colonies of a species of Obelia common at Friday Harbor and 

 identified by Professor Nutting as Obelia borealis and of Obelia genicidata at Woods 

 Hole; and in both species found distal levels electronegative to proximal in agree- 

 ment with the statement in Lund's 1922 paper but contrary to his statement in 

 1925. 



