THE LIFE CYCLE 49 



different in any visible way may show themselves by their behavior 

 to be physiologically different, so that the absence of visible differ- 

 entiation in the cell is not proof that the cell is completely unspecial- 

 ized. 



The substance of the undifferentiated cell is the general meta- 

 bohc substratum of the organism, and it is the chemical or physical 

 transformations of this substratum, or the addition of substances 

 to it, that constitutes morphological differentiation. Physiological 

 differentiation consists in the progressive development of certain 

 activities at the expense of others. 



While we know too Httle at present of the nature of the various 

 metaboHc processes and of the relation between metabohsm and 

 the cellular substratum to permit us to reach positive conclusions 

 concerning the nature of dift'erentiation, the facts at hand suggest 

 certain probabilities. In the first place the embryonic cell very 

 evidently has in general a higher metabolic rate, or capacity for 

 a higher rate, independent of external stimulation, than do differ- 

 entiated cells. Apparently the mere continuation of Hfe in the 

 cell without cell division brings about changes which decrease 

 the metabohc rate. Such changes may conceivably result from 

 gradual atomic rearrangements or from changes in aggregate con- 

 dition of the colloids. It is a well-known fact that emulsoid sols 

 outside the organism undergo slow changes in the direction of 

 coagulation, even when kept under as nearly as possible constant 

 conditions, and there is good reason to beheve that similar changes 

 occur in the colloids of the living organism. In the coagulation 

 of proteids by high temperatures time is a factor, i.e., the occurrence 

 of coagulation depends, not only upon the actual temperature, but 

 on the time of exposure to it: the lower the temperature, the longer 

 the time necessary to bring about perceptible coagulation. From 

 the character of this relation between time of exposure and tem- 

 perature it is inferred that, theoretically, coagulation must occur 

 at all temperatures above the freezing-point of the sol, its rate being 

 infinitely slow at low temperatures and increasing rapidly as the 

 temperature rises. The fact that coagulation changes do occur 

 slowly in colloid sols at ordinary room temperatures supports this 

 view. Lepeschkin ('12) has found that the relation between 



