2l6 C. M. CHILD AND L. H. HYMAN. 



Whatever the terms in which we may finally define the axial 

 gradient, its existence and significance cannot be doubted. In 

 hydra, as in other forms examined (see references, Child, Hyman), 

 it is the earliest indication of the body axes, it is certainly asso- 

 ciated with quantitative differences in metabolic and general 

 physiological condition along its course, and physiological dom- 

 inance and subordination, as well as local specialization and dif- 

 ferentiation exist or arise in definite relation to it. In the light 

 of all the known facts, the only possible conclusion is that the 

 gradient is the simplest physiological expression of the axial re- 

 lation in organisms. In other words, this axial relation in its 

 simplest terms is a quantitative gradation in the condition of 

 living active protoplasm. 



The modifications of the gradient in hydra by contractile and 

 digestive activity are significant in that they constitute further 

 evidence for the value of the susceptibility method as a means of 

 investigating metabolic condition or "physiological state" in 

 simple organisms. Since the contractile mechanism in hydra is 

 merely a part of the body cell, excitation and muscular contrac- 

 tion affect the whole cell to some extent, and the altered con- 

 dition of the cell appears in its altered susceptibility. Similarly 

 the spreading or irradiation of excitation with both local muscular 

 contraction and local digestive activity also appears in the sus- 

 ceptibility relations of different regions. The simplicity and 

 diffuse character of the organization of hydra make it of particular 

 interest in this respect, because each excitation produces a more 

 or less diffuse effect. 



The experimental data indicate further that a greater degree 

 of excitation and change in physiological state is associated with 

 the muscular activity which brings about contraction of tentacles, 

 body or stalk, than with that concerned in their extension. Ex- 

 tension is doubtless due, at least in large part, to the activity of 

 transverse or circular muscle fibers, but the process is much less 

 rapid and less vigorous than that of contraction and occurs in 

 the absence of strong external excitation, while contraction is in- 

 duced by external excitation. Apparently the longitudinal mus- 

 cles are more numerous, or more powerful, or their contractile 

 activity is associated with more intense excitation, and it is this 



