DYNAMICS OF MORPHOGENESIS 155 



isms, which have not heretofore been open to investigation. With 

 this method we are able to determine in a general way where 

 differences of rate exist and it is often possible also, with properly 

 devised experiments, to determine what part these differences in 

 rate plsiy in phenomena of development (Child '13). 



Since this method is essential for the most important results to 

 be considered in later papers, it is necessary, before going farther 

 in the analysis of experimental reproduction in Planaria, to de- 

 scribe the method and its application in detail. The first part 

 of this paper is therefore devoted to a description of the method, 

 then follows a consideration of the evidence which constitutes the 

 foundation of the method, and finally the question as to the 

 nature of the action of the agents used is briefly discussed and 

 some interesting lines of investigation are pointed out. 



II. THE PHYSIOLOGICAL RESISTANCE METHOD OF COMPARING 

 RATES OF METABOLIC REACTION 



1. General outline of the method 



The method is concerned with the length of life, i.e., the physio- 

 logical resistance of the animals or pieces in certain reagents 

 which decrease metabolism or in sufficient concentration kill. 

 As will appear below, a relation exists between the length of life 

 of an animal or piece in a solution of such a reagent of given con- 

 centration under standard external conditions and the rate of 

 metabolic reaction in the animal or piece. This being the case, 

 it becomes possible by standardizing the concentration of the 

 reagent used and the external conditions, to compare the rates of 

 reaction in different individuals, regions or pieces. The present 

 and following papers will show that this method is capable of 

 wide application and that it gives us a new means of attack on 

 various problems and opens up certain fields which have hereto- 

 fore been inaccessible. 



Assuming for the moment the correctness of the method, it is 

 evident that we compare by means of it, not the rates of single 

 simple chemical reactions but rather the total amounts of the 

 reactions or processes concerned which occur in a given length of 



