198 C. M. CHILD 



There are also in certain cases regional factors which make the 

 application of the indirect method difficult. When, for example, 

 the rate of reaction in different parts of the body is very different 

 it is sometimes difficult to find a single concentration which will 

 give results by the indirect method for both. 



In animals like many of the coelentera and turbellaria the con- 

 tinued existence of the characteristic structural features after 

 they have once developed is more or less directly dependent on 

 the maintenance of a certain relatively high rate of reaction. 

 Decrease in the rate below a certain limit is followed either by 

 dedifferentiation or more usually by death. It is difficult to com- 

 pare rates of reaction by the indirect method in such cases for 

 the direct depressing effect of the reagent often kills or hastens 

 the death of certain relatively highly differentiated parts, even 

 when the concentration is so low that it has little or no effect on 

 other parts. The hydranth region of the hydroid Corymorpha 

 is a case in point. In all my tests by the indirect method the 

 hydranth region dies before the stem, although young hydranths 

 live longer than old ones in the same solution. These cases need 

 further investigation, but apparently we have here simply a 

 structure in which the range of acclimatization is narrowly lim- 

 ited by the high degree of differentiation which has resulted from 

 the relatively high rate of reaction. It will probably be found 

 true in general that structural differentiation, especially where 

 it occurs in consequence of a high rate of reaction, limits the 

 range of adaptation to depressing media. Moreover, the older 

 such a structure becomes the more narrowly is its range lim- 

 ited. In Tubularia, for example, the mere change from open 

 water to the laboratory usually brings about the death of the 

 hydranths, apparently in consequence of the decrease in rate of 

 reaction accompanjdng the change in conditions : use of a depress- 

 ing agent hastens the death of the hydranth still further because 

 it brings about a still further decrease in the rate of reaction. 



And finally there are the factors of time and labor to be consid- 

 ered in connection with the indirect method. New solutions must 

 be made up every day or two and the renewal of the solutions in 

 the flasks requires much time. The direct method requires only 



