EARLY DEVELOPMENT IN THE FROG. 315 



tensities of action, exist in relation to the physiological axes. 1 

 These differences in susceptibility may be demonstrated in several 

 ways: first, as differences in survival time of one region of the 

 egg or embryo as compared to other regions, under conditions 

 severe enough to kill without permitting acclimation to occur; 

 second, as differences in the degree of inhibition of growth and 

 development, or in certain cases as differences in the degree of 

 acceleration of these processes; third, as differences in the rate 

 or degree of acclimation to a certain range of less severe condi- 

 tions; fourth, as differences in the rate or degree of recovery 

 after temporary exposure to conditions that inhibit development. 



These differences in susceptibility determined in these dif- 

 ferent ways are all expressions of the fact that a "differential 

 susceptibility" to the action of external agents is a characteristic 

 feature of physiological axes in both plants and animals so far 

 as they have been examined with reference to this point. This 

 "differential susceptibility" appears according to experimental 

 conditions as differential disintegration associated with death, 

 and as differential inhibition, acclimation or recovery in develop- 

 ment. 



The high degree of uniformity in the susceptibility relations 

 in different organisms, both animals and plants, to a wide variety 

 of agents and conditions, viz., cyanides, anesthetics, acids, alka- 

 lies, various salts, certain alkaloids, physical conditions such as 

 extremes of temperature and certain conditions that may be 

 termed negative, such as lack of oxygen, indicate that in their 

 general features these susceptibility relations are independent of 

 specific qualitative differences in the protoplasm of different 

 forms. If this is the case they must depend upon quantitative 

 differences of some sort which are common at least to all forms 

 in which these differences in susceptibility have been shown to 

 exist. 



So far as present knowledge goes, the facts indicate that 

 susceptibility, in the sense in which the term is used here, is 



1 The extensive literature on this subject, previous to 1915, mostly the work 

 of Child, is treated in two of his recent books (19150, 19156) to which the reader is 

 referred for specific references. Other references are : Behre (1918); Child (1916^ 

 19166, igi6d; 19170, 19176, I9I7C, 19170*; 19190, 19196); Child and Hyman 

 (1919); Hyman (19160, 19166; 19170, 19176; 1919). 



