THE LIFE CYCLE 41 



in the course of the reactions which constitute metabolism, and, 

 secondly, of their physico-chemical properties. The substratum 

 once formed in the course of chemical reactions affords a basis for 

 the continuation of the reactions and for the further addition of 

 colloids. So far as the metabolic reactions are enzyme reactions, 

 the structural substratum of the organism must consist of the sub- 

 stances which for one reason or another are less susceptible to 

 enzyme action than other substances which are transformed without 

 forming a part of the structure. 



According to this view the colloid substratum and the morpho- 

 logical structure of the organism represent, so to speak, the sedi- 

 ment from the metabolic process. They are, in short, by-products 

 of the reactions which do not readily escape from the cell unless 

 they undergo decomposition and which are relatively stable. 

 Therefore they must constitute the more permanent constituents 

 of the cell and appear as a visible substratum or more or less perma- 

 nent structure of some sort. The constitution of the structural 

 substratum developed in different organisms differs because the 

 metabolic processes and the substratum already existing at the 

 beginning of development differ. The visible organism is then 

 the sediment left behind by the metabolic current: it consists of 

 t;he substances which the current is unable to carry farther. It does 

 not represent life any more than the sand-bar represents the river; 

 it is simply a product of past activity which may influence future 

 activity. Sixty years ago Huxley said concerning the cells: "They 

 are no more the producers of the vital phenomena than the shells 

 scattered along the sea-beach are the instruments by which the 

 gravitative force of the moon acts upon the ocean. Like these, 

 the cells mark only where the vital tides have been and how they 

 have acted" (Huxley, '53). And yet since Huxley's words were 

 written how many attempts have been made either to show that 

 this or that structural element of the organism represents some- 

 thing fundamental to life or to translate the phenomena of life 

 into terms of an invisible hypothetical structure! 



The visible structural features of the organic substratum possess 

 very different degrees of stability: some are evanescent, while 

 others persist throughout the life of the cell in which they arise. 



