738 PATTERNS AND PROBLEMS OF DEVELOPMENT 



with proper care differential susceptibility may be a valuable, though a rough and 

 imperfect, means of indicating quantitative differences in physiological condition in 

 naked, aquatic organisms. As will appear, particularly in chapter iv, there is a general 

 parallehsm not only between cyanide susceptibihty and respiration but, as far as data 

 are available, between susceptibihty and quantitative differences indicated by other 

 methods, the indophenol blue reaction, differential dye reduction in low oxygen, etc. 

 With further investigation the question of the physiological significance of cyanide 

 susceptibility has become only one aspect of the problem of differential susceptibility— 

 at least as far as physiological gradients are concerned— for it has been found that 

 axial differential susceptibility to many other agents, both physical and chemical, in 

 certain ranges of concentration or intensity show the same relation to the axis con- 

 cerned as does cyanide susceptibility. Moreover, this relation appears not only in 

 differential lethal action but in differential effect on development. In the light of 

 these facts the problem of the physiological basis of differential susceptibility has be- 

 come a much more general problem. It appears now as the problem of the nature of the 

 factors in living protoplasms that are concerned in determining that differences in 

 susceptibility along physiological axes in early developmental stages and in many of 

 the simpler organisms throughout Hfe are so largely nonspecific for so many different 

 agents which act on protoplasms in different ways. Some consideration of this prob- 

 lem is undertaken in chapter iii. 



