DIFFERENTIAL GROWTH I 53 



(explantation or transplantation), though they may cease to produce 

 their customary products, never reacquire the faculty to give rise to any 

 but the products of their original line, when given another chance. 



Thus the conclusion is inescapable that the various cell types men- 

 tioned do not just temporarily exhibit different production processes, 

 but have themselves become chemically converted into different pro- 

 duction plants. That is, each cell type develops, in addition to the molecu- 

 lar species which it contains in common with all or several other cell 

 types, its own peculiar and distinctive array of molecular species. These 

 are the ones that account for the differential behavior of the various 

 protoplasms ; they are the true objects of differentiation. One is tempted 

 to identify them with specific proteins, but factual knowledge is still 

 far too inadequate for any generalization. We need much more precise 

 information on the time and mode of appearance of various cytochemical 

 constituents in the ontogeny of various cell strains. Such an inventory 

 is bound to contain the answers to numerous questions about which we 

 can as yet do no more than speculate. 



In contrast to differentiation, cell modulation implies no essential 

 change in the composition of the molecular population, but merely a 

 regrouping of the existing species. According to our previous discussion, 

 any change in the environment of the cell that alters the conditions in 

 the surface could cause a reshuffling of the content and replacement of 

 the surface population with marked changes in the behavior and mor- 

 phology of the cell, but without necessarily affecting its basic composi- 

 tion. However, whether such a modulation is fully reversible, i.e. 

 whether or not the reshuffled population can return once more to its 

 initial distribution when the original environmental conditions are re- 

 stored, depends on the degree of consolidation the surface has undergone 

 in the interim. 



These considerations suggest the possibility that all differentiations 

 start out as modulation, as mere molecular regrouping, in response to 

 changes in the cellular environment. Let us consider an embryonic cell 

 shifting from a site A to another site B, or, what amounts to the same, 

 a change in the environment of a stationary cell from a condition ^ to a 

 condition B. Exposure to the new contact conditions brings molecules 

 with affinities to B to the surface. Now, suppose this molecular coat 

 initiates a chain of chemical reactions, through which the composition 

 of the interior is materially changed. This implies the appearance of 

 novel molecular types as well as the disappearance of some of the exist- 

 ing species as they become converted into new ones. Such change cannot 



