CELL DIFFERENTIATION 519 



activity of enzymes. Extremely difficult to measure until recently, it is 

 small wonder that most biologists and biochemists have avoided the subject 

 as much as possible and even dismissed it from existence with remarks 

 like, " Oh, we use COo to set the pH, that's all." If this were " all" in fact, 

 then the same pH could far more easily be obtained with one of the many 

 excellent buffers that are not volatile. Yet "setting the pH" with gaseous 

 COo is compulsory in tissue culture (J. H. Hanks [9]) : 



"The problem of pH control often appears baffling when cell culture 

 work is first undertaken. In view of the simplicity of many buffer systems 

 it seems almost a crime that a gas such as COo in equilibrium with HoCOg 

 and NaHCOg should be a major physiological mechanism of pH control, 

 that this system should be essential for respiration and growth, and that 

 it should, at the same time, afford such inefficient buffering action in the 

 working range of pH 7-8. Until such time as man or Maker may provide 

 a substitute, one must be prepared to fight the battle of CO.^." 



Returning now to cellular differentiation, let us picture nature trying 

 endless experiments at the protozoan level in an attempt to obtain more 

 than one kind of cell with which to build a metazoan animal. Stacking 

 identical protozoa together into larger and larger clusters would auto- 

 matically expose the central cells to higher and higher levels of pCO.,. 

 Since fatty cell membranes form no appreciable barrier to free CO2 

 molecules, the whole mass of respiring cells would form one large " field of 

 force" whose medullary pCOo would be far higher than the peripheral 

 cortex. Such a unifying "field of force " would represent a function of the 

 whole, for if it were cut in two, new gradients of pCOo would form within 

 each half just as they do when a pile of glowing coals is divided in two : 

 the centre of each aggregate soon is hotter than the newly exposed 

 periphery. 



Suppose a mutant should now arise among these protozoa, a mutant 

 whose DNA behaved differently under high and low levels of pCOo: i.e. 

 a protozoan with genetic material that could not be expressed pheno- 

 typically except when grown under conditions of high pCOo. Clearly such 

 a protistan would replicate until a critical mass was formed in whose 

 centre the pCO., reached the postulated threshold for the activation of 

 this additional set of mutant genes. Here then would be a mechanism by 

 which replication could lead to differentiation, and, with further elabora- 

 tion as during gastrulation, Aristotle's A could lead to B and C. 



Thus, it is known that slime-mould amoebae lay down walls of cellulose 

 only when they are buried deep inside the multicellular pseudoplasmodial 

 aggregate. Would it not be a great step forward if it were found that single 

 slime-mould amoebae form walls of cellulose when grown in isolation if 

 they are exposed to increased levels of pCOo ? 



Many years ago Rache\sky examined the mathematical relations that 



