70 PATTERNS AND PROBLEMS OF DEVELOPMENT 



axial dye-reduction gradient — must always, and of necessity, coincide 

 with the respiratory gradient of the same axis or that a respiratory gradi- 

 ent must always be distinguishable in an axis which shows a dye-reduction 

 gradient. Dye reduction is regarded as chiefly dependent on intermediate 

 oxidation-reduction reactions rather than on total oxygen use. On the 

 other hand, when respiratory gradient and dye-reduction gradient do co- 

 incide, each aids in confirming the significance of the other." 



DIFFERENTIAL SUSCEPTIBILITY 



Living protoplasms may be stimulated or depressed, accelerated or re- 

 tarded, in activity or specifically altered in condition by external physical 

 and chemical agents; and any of these effects, beyond a certain degree or 

 threshold, becomes injurious or lethal. Agents acting within the organism 

 may also affect a particular cell or cell group, or an organ or organ system, 

 in all these ways. These effects are indicative of the sensitiveness or sus- 

 ceptibility of Hving protoplasm to factors in its environment. If life is a 

 continuous dynamic equilibration, protoplasmic susceptibility signifies 

 that this equilibration process can be altered in various ways or arrested 

 by external factors. We do not doubt that, as far as the organism is con- 

 cerned, differences in susceptibiHty of regions and organs, whether specific 

 for certain agents or not, depend upon differences in physicochemical con- 

 stitution and physiological condition or activity of the protoplasms, re- 

 gions, or organs concerned. It is evident, however, that differences in sus- 

 ceptibiHty to a particular concentration or intensity of a particular agent 

 tell us little or nothing more than that differences of some sort in constitu- 

 tion or condition exist. In order to obtain all the information possible con- 

 cerning susceptibility to any agent, it is usually necessary to determine 

 the susceptibilities of individuals, in different physiological condition and 

 of different developmental stages, to a wide range of concentrations or in- 

 tensities of the agent and often of other agents as well, to compare the 

 susceptibilities of different species, and to check the data on susceptibility 

 with those obtained by other methods. Conclusions concerning the nature 

 of regional differences in protoplasmic constitution and condition have 

 often been drawn from results obtained with a single agent in a single con- 

 centration or intensity. Such conclusions are often highly uncertain. 

 Many external agents act more or less specifically on certain organs or or- 

 gan systems of adult individuals, particularly of forms in which a high de- 

 gree of histological differentiation is evident. A certain agent may act 

 " For further details concerning use of the method and for references, see Appendix II. 



