42O L. H. HYMAN. 



anterior levels. The existence of an oral or apical end inhibits 

 the formation of any other oral or apical end within the distance 

 limit over which the control of the former extends (see Child, '15, 

 Chapter IV.). 



Differences in rate of regeneration (time between cutting and 

 completion of oral or apical structures) with respect to level 

 constitute further evidence of the existence of a metabolic 

 gradient in hydroids. The more apical the level in the whole 

 within the limits of the primary gradient from which the piece 

 is taken the more rapidly does it produce a new apical end. This 

 generalization has been shown to hold in a large number of lower 

 invertebrates, mostly ccelenterates, e.g., Eudendrium (Goldfarb, 

 '07), Tubularia (Driesch, Morgan, Child, etc., for references see 

 Hyman, '20), Corymorpha (Torrey, '10), Pennaria (Cast and 

 Godlewski, '03), Obelia (Billard, '04, Lund, '23), Cerianthus 

 (Child, '03), Planaria (Child, 'n), annelids (Hyman, '16). 

 Further proof of the correctness of this generalization with regard 

 to Tubularia is presented in this paper. It is also shown that 

 such axial differences in rate of regeneration are independent of 

 size or mass differences at different levels. It can scarcely be 

 doubted that the metabolic gradient is the direct or indirect 

 cause of the apico-basal gradation in rate of regeneration of oral 

 or apical structures. 



Another instance of the dependence of regeneration rate on 

 metabolic rate is the difference in these regards between organisms 

 of different ages (sizes). It is shown in this paper and previously 

 (Hyman, '20) that in Tiibularia the rate of regeneration is more 

 rapid the smaller the diameter of the piece; and further that the 

 rate of oxygen consumption is higher the smaller the diameter. 

 There is thus a correlation between rate of production of oral 

 hydranths and rate of respiratory metabolism; and it is scarcely 

 to be doubted that the latter is the direct cause of the former. 

 It is probable that in Tubularia the dimensions of the stem 

 (diameter, total length, size of hydranth) vary inversely with 

 age and that we are really dealing here with age and not size 

 differences. An inverse relation between respiratory rate and 

 age (size) appears to be universal throughout the animal kingdom. 

 In a previous paper (Hyman, '19) I reviewed this matter and 



