METHODS OF PHYSIOLOGICAL ANALYSIS 71 



chiefly or perhaps entirely, so far as we can determine, on a certain organ; 

 another on another organ. In case of some agents, action is more or less 

 specitic in a certain range of concentration or dosage ; but above this range 

 the action becomes more general, and in sufficiently high concentration or 

 intensity such agents may affect the whole organism. For pharmacologi- 

 cal purposes agents which produce physiological effects have sometimes 

 been separated into those with specific and those with general action, but 

 this distinction cannot usually be sharply drawn. 



Another aspect of protoplasmic susceptibility appears in the relation 

 between the susceptibility of a particular species-protoplasm, individual, 

 or organ to a particular agent or to many agents and its physiological con- 

 dition. For example, the physiologically young individual is usually more 

 susceptible than the old to various agents within a certain range of con- 

 centration or intensity. Increase in motor activity increases the suscepti- 

 bility of the planarian to cyanide in a certain range of concentration 

 (Child, 1913a), but increased motor activity may decrease the suscepti- 

 bility of many animals to alcohol and certain other agents within certain 

 limits of concentration. Susceptibility of an organ system may change 

 greatly during development. Susceptibility of the nervous system to anes- 

 thetics or to strychnine changes relatively to that of other organs from the 

 medullary plate stage to the adult in vertebrates; but susceptibility of the 

 hydroid nerve net to these agents undergoes no such relative change, and 

 strychnine, like anesthetics, depresses nervous activity in the adult. 



From the viewpoint of developmental physiology the questions of chief 

 interest as regards susceptibility concern axial and regional differences in 

 susceptibility, susceptibihty patterns at the beginning and in early stages 

 of development, and the changes which occur as development progresses. 

 So far as data are available, axial or other regional differences in suscepti- 

 bility which are specific for particular agents are usually less evident in 

 earlier, than in later, developmental stages or absent, though regions of 

 high yolk content in eggs may be highly susceptible to fat-soluble agents, 

 and certain other differences which have been regarded as specific appear 

 in some forms. Graded differences in susceptibility (susceptibility gradi- 

 ents) appear as characteristic features of early developmental pattern. 

 As development progresses, these gradients may undergo alteration or 

 even obliteration, and new gradients may appear; but in many of the sim- 

 pler animals gradients of earlier stages persist throughout life. These 

 gradients constitute what has been called "differential susceptibility." 

 They are, in general, nonspecific to a high degree; that is, the gradient is 



