THE AXIAL GRADIENTS IN HYDROZOA. 221 



dependent upon permeability and that permeability is inde- 

 pendent of rate of oxidation. The chief basis for this view is 

 the work of E. J. Lund on starvation, and it has been pointed 

 out that his conclusions depend upon an interpretation of his 

 data which in the light of the known facts is almost certainly 

 incorrect: if this is the case, the whole contention of these authors 

 fails. There are, moreover, certain data on susceptibility, such, 

 for example, as the results with the dyes in hydra and other 

 forms, which cannot be interpreted in terms of permeability in 

 any physical sense, and other data on susceptibility to lack of 

 oxygen, which show that other factors than permeability are 

 concerned in susceptibility, are soon to appear. As one of us 

 has stated elsewhere (Child, '19), the most important question 

 in this connection is the question of the nature of what current 

 terminology calls permeability. Extended discussion of this 

 point is postponed to another time, but it may be noted that no 

 purely physical conception of permeability accounts for all the 

 known facts, that according to all the evidence, the surface as 

 well as the interior of the cell is alive and the seat of metabolic 

 reaction, and that susceptibility, and even permeability itself, 

 are dependent, not merely upon the physical characteristics of 

 the surface layers as a membrane, but also, at least 'in part, upon 

 this living, chemically active condition. Apparently for the 

 Lunds permeability is some condition at the cell surface com- 

 pletely dissociable from the metabolic condition of the proto- 

 plasm, but their own evidence that such a condition exists is at 

 least far from conclusive as we have shown, and other adequate 

 evidence is difficult to find. 



SUMMARY. 



1. A study of the susceptibility of three species of hydra, H. 

 viridissima (viridis), H. vulgaris (grisea).and H. oligactis (fusca) 

 to various agents shows that a gradient in susceptibility exists 

 along the apico-basal axis of the body and in each tentacle. 



2. This susceptibility gradient is primarily a basipetal gradient, 

 i. e., the susceptibility decreases from the apical region basipet- 

 ally and after the tentacles arise, in each tentacle from the tip to 

 the base. This primary gradient appears in the earlier bud 



