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on each other, viz., the enzyme action which breaks up the food 

 supply into particles suitable for assimilation and the synthesis 

 of these particles to form living protoplasm. 



Considered apart from their association with living matter, 

 enzymes are catalytic agents and their action involves an inter- 

 mediate stage, during which they form an unstable combination 

 with the material (substrate) undergoing digestion ; then follows 

 dissociation of the digested substrate from the enzymes, leaving 

 the latter free to attack fresh substrate. But this is not neces- 

 sarily the whole story. Conditions may supervene which tend 

 to stabilise (instead of dissociating) the union between enzyme 

 and digested food material, the result being that enzyme action 

 ceases to be progressive and is replaced by a synthesis between 

 substances formerly acting as enzymes and material upon which 

 they acted. 



Conditions of this latter nature seem to be of particular 

 importance for cellular growth, in explanation of which it is 

 necessary to postulate a balance of two opposite factors — 

 (1) enzyme action, and (2) inhibition of this action; without the 

 aid of (1) the cell would die from starvation ; without the 

 restraining action of (2) it would die from " autolysis." Applying 

 this conception to the enzyme action of bacterial protoplasm, 

 it would appear that bacterial substances which act as catalytic 

 agents are also concerned with the synthesis of bacterial protein, 

 and that bacterial protoplasm may be regarded as a complex of 

 enzymes and the products of enzyme action, a complex which 

 involves the synthesis of these products {e.g., amino-acids) into 

 proteins. 



Starting with this idea that the cellular enzymes of bacteria 

 are not merely catalytic agents acting on a substrate, but are 

 also the nucleus upon which bacterial protoplasm is built up, 

 it would seem that, when a bacterium begins to grow, catalytic 

 action upon its environment is at first predominant; then it 

 passes through a critical phase which fixes or stabilises the 

 constituents of bacterial individuality, a phase which is marked 

 by the transition from catalytic action on the raw substrate 

 to synthetic action on the elaborated product. When full 

 development is followed by division, these processes are started 

 afresh. 



As bacteria breed true to species, it must be assumed that, 

 in the mechanism of growth and reproduction, there is a constant 

 repetition of the same processes, with identity of catalytic agents, 

 of products ready for assimilation, and of their synthetic combi- 

 nation. This precise uniformity becomes easier to understand 

 with the help of the idea that both catalytic and synthetic 

 action are attributes of the same protoplasmic substances. 



Then where is one to find opportunities for variation (within 

 the limits of species characteristics) ? Here the incidence of 

 what I have termed the " critical phase " in the transition from 

 catalysis to synthesis may be worth considering, as providing 



