76 PATTERNS AND PROBLEMS OF DEVELOPMENT 



occur, as in differential death and differential inhibition. On the other 

 hand, if the external action is slight in amount or intensity, the more rap- 

 idly equilibrating region will eliminate in some way the disturbing factor 

 or equilibrate to it more rapidly than a region which is equilibrating more 

 slowly. This is apparently what happens in differential tolerance, differ- 

 ential conditioning, and differential recovery. For susceptibilities which 

 are specific for particular agents, these relations do not hold; but they may 

 hold for different regions of a part or organ which, as a whole, is specifically 

 susceptible. Moreover, it is not necessary to assume that respiration or 

 oxidation is always the primary factor in the various expressions of differ- 

 ential susceptibility, but it appears impossible to account for the facts in- 

 dependently of metabolic activity; and in most organisms respiration or 

 oxidation-reduction appears to be a more or less trustworthy indicator of 

 such activity, or at least of some of its fundamental factors. The relation 

 between respiration or oxidation and susceptibility may not be the same in 

 all organisms or in all organs of adult individuals; when glycolysis is the 

 chief source of energy, it is obviously different from that in aerobic respira- 

 tion. All that is maintained here is that there appears to be a general 

 parallelism between the phenomena of differential susceptibility, particu- 

 larly in early developmental stages and in the simpler organisms, and the 

 basal metabolic activity of the protoplasmic system concerned. Differen- 

 tial susceptibility is at best merely one method of indicating differences in 

 physiological condition which appear to be quantitative and which, as will 

 appear in following chapters, are essential factors in development ; without 

 the aid of other methods it gives no direct information as to the nature of 

 the differences indicated.'^ 



THE QUESTION OF DIFFERENTIAL PERMEABILITY 



It was pointed out above that gradients indicated by external chemical 

 agents are not dependent merely on a permeability gradient. Neverthe- 

 less, the question whether a differential permeability is present along phys- 

 iological axes as one feature of the axial gradient is of interest. It might 

 perhaps be expected that rate of staining by vital dyes would throw some 

 fight on this question. MacArthur (1921) maintained that differential 

 permeability alone could not account for the gradients of staining and 

 susceptibility to the dyes. The writer's observations bear out his conclu- 



'3 For references to the literature on differential susceptibility and further discussion of the 

 question of its relation to metabolism see Appendix III. 



