THE AXIAL GRADIENTS IN HYDROZOA. 



edly associated with the fact that the muscles of hydra do not 

 constitute a special tissue, but are merely specially differentiated 

 portions of the body cells or of certain of them. This being the 

 case, there is every reason to believe, either that muscular ac- 

 tivity in hydra is merely one expression of a general excitation 

 of the cells concerned, or that excitation of the contractile por- 

 tion of the cell brings about changes in the physiological condi- 

 tion of the cell as a whole, or it may be that both of these possi- 

 bilities are realized under different conditions. 



Whatever the nature of this relation between muscular activity 

 and susceptibility it is necessary in analyzing the susceptibility 

 relations to take into account the condition and activity of the 

 muscular system. It is desirable to determine the susceptibility 

 relations in the body, not only before the muscular system has 

 developed, but also in fully developed animals under conditions 

 of quiescence, moderate and extreme muscular activity and with 

 experimental inhibition of muscular activity. Fortunately it is 

 possible to investigate and analyze more or less completely all 

 these different cases. Although the earlier embryonic stages are 

 not available for work of this kind, the earlier stages of bud-de- 

 velopment show little or no muscular differentiation and are to 

 be regarded as essentially embryonic stages. Animals which 

 have been attached by the foot and undisturbed for an hour or 

 more are usually quiescent, while detached animals show a much 

 greater degree of muscular activity. Moreover, different agents 

 and different concentrations of the same agent produce very dif- 

 ferent effects upon muscular activity before death. Cyanide, 

 for example, except in very high concentrations produces no ap- 

 preciable excitation at any time before death and, on the other 

 hand, inhibits muscular activity very gradually; it therefore af- 

 fords favorable conditions for the analysis of the differences in 

 susceptibility in non-motile buds and full grown motile animals 

 and between attached quiescent, and detached animals. 



All the other agents used produce more or less excitation accom- 

 panied by increased muscular activity in the earlier stages of their 

 action. This condition of excitation may continue for hours in 

 the lower concentrations of these agents, but is of course sooner 

 or later followed by depression, disintegration and death. With 



