222 C. M. CHILD AND L. H. HYMAN. 



stages, in quiescent attached individuals of H. viridissima and 

 H. vulgaris at all stages and in young quiescent individuals of 

 H. oligactis. 



3. This primary gradient is modified in various ways by re- 

 gional differences in functional activity and particularly by the 

 special activities of the stalk, a more or less specialized organ 

 which develops at a relatively late stage from the basal body re- 

 gion. 



4. Regional differences in the contractile activity of the mus- 

 cular apparatus produce the most conspicuous modifications of 

 the susceptibility gradient. Such modifications can be produced 

 by detachment of the animals, which leads to special activity of 

 the stalk, by the general excitatory or special irritant action of 

 certain agents, such as the dyes used, and by certain combin- 

 ations of excitation and paralysis with anesthetics. 



5. Alterations of the primary susceptibility relations are pro- 

 duced, not only by regional differences in muscular activity, but 

 by regional differences in digestive activity. 



6. The very direct and marked effect upon susceptibility of 

 regional and local differences in muscular and other functional 

 activity results from the simple and diffuse organization of hydra. 

 The muscle, for example, is a part of the body cell and the whole 

 cell shares in the excitation connected with muscular contrac- 

 tion, and the facts indicate that most or all local excitations 

 spread or irradiate to a greater or less degree to adjoining regions. 



7. In stages after the development of motor activity suscep- 

 tibility decreases in general with advancing physiological age, 

 but the bud in stages preceding the development of motor ac- 

 tivity is less susceptible than later motile stages. 



8. The differences in the susceptibility relations of the three 

 species under various conditions suggest that of the three H. 

 oligactis possesses, as its .structural and functional characteristics 

 indicate, the highest degree of regional specialization. 



9. The facts confirm previous work in that they indicate the 

 existence of a relation of some sort between susceptibility and 

 metabolic rate. Within the ranges of concentration of the agents 

 used, susceptibility evidently varies in general directly with 

 metabolic rate, or with the rate of energy-liberating metabolism. 



