EARLY DEVELOPMENT IN THE FROG. 355 



reactions in protoplasm in a variety of ways depending upon the 

 ionization constant of the acid or alkali, concentration, reactivity 

 of the substances formed by their interaction with protoplasm, 

 etc. 



It is obviously impossible here to go into a detailed discussion 

 of the particular types of reaction set up between external 

 agents and living protoplasm. For a resume of this important 

 subject the reader is referred to Mathew's "Physiological Chem- 

 istry," especially chapter V., where references to the literature 

 may be found. 



In spite of the various factors concerned in the action of 

 external agents upon living protoplasm, the highly significant 

 fact remains that the susceptibility relations to external agents 

 whose action is severe enough to kill within a few hours or to be 

 distinctly toxic show not only a high degree of uniformity for 

 at least a large number of different agents, but also a very definite 

 correlation with metabolic conditions. 



As regards permeability of limiting membranes there can be 

 no doubt that it is an important factor in the movement of 

 substances into and out of the cell. But the passage of many 

 substances across the cell membrane in concentrations high 

 enough to kill the protoplasm follows only upon the destruction 

 of, or irreversible changes in the surface membrane and the con- 

 sequent more or less complete disappearance of its special 

 physiological characteristics. Furthermore, the cell membrane 

 is alive and therefore metabolically active to some extent, and 

 its peculiar properties as regards permeability are dependent 

 upon the fact that it is alive, since they disappear with its death. 

 It is evident then, first, that semi-permeability is more or less 

 closely associated with metabolic conditions and second, that 

 differential susceptibility to external agents cannot be inter- 

 preted in terms of permeability as distinct from metabolic 

 conditions. 



Experimental analysis has already shown' that many of the 

 so-called cases of specific action of external agents are not properly 

 speaking specific effects at all, but actually are differences in 

 degree rather than in kind. And the question may be raised 

 whether at least many other apparently specific effects will not 

 prove to be of the sarne sort. 



