THE LIFE CYCLE 53 



According to the theory of differentiation developed here, the 

 self-determined rate of metabolism of the cell must be to some 

 extent an index of its degree of differentiation. This is to be ex- 

 pected, since the metabolic rate must depend upon the condition 

 of the metabolic substratum. It is important to note that it is 

 the metabohc rate, as determined by conditions existing within 

 the cell independently of external stimulation, which is thus related 

 to the degree of differentiation. IMany highly differentiated cells 

 with a low, self-determined metabolic rate are capable temporarily 

 of a very high rate when stimulated from external sources. Such 

 increases in rate are evidently the result of changes in the cellular 

 substratum which are largely or wholly reversible. What their 

 nature is we do not know certainly, although various theories of 

 stimulation have been advanced. As differentiation proceeds 

 beyond a certain stage, even the metabohc rate following stimu- 

 lation decreases and the cell becomes less and less capable of per- 

 forming its special function as a differentiated cell. 



In general, a greater degree of differentiation of cells is one of 

 the features which distinguish the so-called higher organisms from 

 the lower. A comparison of the cells of higher and lower forms 

 and of their course of differentiation seems to indicate very clearly 

 that the physiological stabihty of the substratum must be greater 

 even in the embryonic cells of the higher than in those of the lower 

 forms in order to serv'^e as a basis for the more rapid and greater 

 differentiation which the higher forms show. Whether the rate of 

 metabolism per unit of weight and under similar conditions of tem- 

 perature, etc., is lower in the higher than in the lower forms is not 

 at present known, but there is some evidence that it is. If increase 

 in physiological stability of the cellular substratum has occurred 

 during the course of evolution, it must have been an essential 

 factor in determining the increase in structural complexity which 

 is so characteristic a feature of evolution, and structural evolution 

 must then be regarded as in some degree an equihbration process, 

 a change from a less stable to a more stable condition. 



The orderly sequence of the process of organic differentiation 

 and the constancy of the results in a given species must result from 

 certain definite characteristics of the organic individual. My 



