CELL DIFFERENTL\TION 52I 



Rachevsky's fourth factor is the level of pCOa in the general back- 

 ground. Human tissues, for example, have a background of 5 •3^0 atm. 

 for this exact level of pCO., is carefully maintained in the arterial blood 

 stream by the medullary centre of the brain. Most fresh-water and marine 

 animals in contrast are exposed to a background of about 0-03",', atm. [i i], 

 for this is the level of pCO., that exists in water that has been equilibrated 

 with normal air. 



Four separate factors then contribute to the pCO., existing at the 

 centre of a spherical cell where the chromosomes usuallv are found. Of 

 these, the single most important is probably the size of the cell or cell 

 aggregate itself, for here the parabolic cur\e rises as the square of the 

 radius. Is this the reason that a large cell like an amoeba liquefies its 

 central protoplasm ? Certainly no amoeba can build a steep Rache\ skv 

 gradient on a permanent basis, for its protoplasm is continuallv rolling 

 over and thus destroying the geometrical relations that caused it to arise 

 in the first place. 



Experimental study of amoebae in our laboratory suggests that their 

 reversible solation-gelation is pCOo dependent. Pantin, for example, has 

 used pH-dependent vital dyes to show that the cvtoplasm of newly 

 forming pseudopods is decidedly more acid than the older gelated 

 material [12]. \\'e ha\e taken acti\ely migrating amoebae that are extended 

 in thin strands like the horns of a deer and exposed them to 20",, CO.,. 

 Almost immediately, the ends of such staghorns begin to soften and 

 "melt" back into the body of the amoeba so that it soon assimies a uni- 

 formly round and spherical appearance. Clearly the normal inside-out 

 gradient of pCO.2 has been abolished by the artificial application of 20",, 

 CO., from the outside-in. If now this same animal is examined 20 min. 

 later, it will be found to be moving around in normal staghorn fashion 

 just as if it were not still under 20",, CO.2. What has happened in these 20 

 min. .- Clearly the answer is that continued respiration on the part of the 

 amoeba's large mass of cytoplasm has allowed a new inside-out gradient 

 to form even against an outside background of 20",, CO.2; the inside of 

 such an adjusted amoeba therefore is again softer than the outside and 

 once again it can begin to flow out from a central liquid pool in successive 

 " lar\a flows " that cool and solidify as they loose their excess CO.,. Experi- 

 mental evidence for this view is provided by the fact that this adjustment 

 to 20'j'o COo does not occur under anaerobic conditions. Further evidence 

 is provided by the fact that dinitrophenol does not inhibit normal amoeboid 

 movement even though it is known to uncouple specifically oxidation from 

 phosphorylation [13], and hence inhibits ATP production while allowing 

 CO.2 production to continue. Finally, it should be mentioned that a pC02 

 of 50",, atm. permanently liquefies such an amoeba and reduces it to a 

 round sphere that is unable to form pseudopodia. Bv this x'lew, amoeboid 



