DYNAMICS OF MORPHOGENESIS 159 



practically eliminated. From the three general rules stated 

 above concerning the relation between resistance, rate of reac- 

 tion and concentration, it is evident that care must be exercised 

 to use concentrations sufficiently low or sufficiently high, other- 

 wise wholly misleading results may be obtained. For example, 

 if the concentration is too low in a test by the direct method, the 

 animals or pieces with the higher rate of reaction may bec6me 

 acclimated to some extent and so may live longer than those with 

 the lower rate which do not become acclimated to any appreci- 

 able extent. In this case the observed relation between the resist- 

 ances would be the reverse of what it should be. 



On the other hand, if the concentration is too high in a test by 

 the indirect method, the animals with the higher rate of reaction 

 may be killed by the direct action of the reagent and so die earlier 

 than those with the lower rate of reaction, which become accli- 

 mated to some extent. Here again the results will be the reverse 

 of what they should be. 



These complications connected with the concentration on the 

 one hand, and on the other the fact that in my earlier experi- 

 ments only the indirect method, where further complications due 

 both to internal and external factors may arise, are responsible 

 for the long time and the large amount of work necessary for the 

 attainment of definite results. 



It is, however, a simple matter to determine the proper limits 

 of concentration for either the direct or the indirect method. 

 When the relation between the resistances of the animals or pieces 

 compared does not undergo inversion with further increase of 

 concentration, then the concentration is sufficiently high for use 

 by the direct method and the factor of acclimatization is not 

 involved. By decreasing the concentration from this point until 

 the factor of acclimatization does appear clearly we can deter- 

 mine a concentration for use by the indirect method. In prac- 

 tice of course concentrations sufficiently far above or below the 

 critical concentration are used so that there is no danger of con- 

 fusing the direct and indirect effects of the reagents. 



Of the two the direct method is the simpler and requires only 

 a few hours, where the indirect method may require days or weeks 



