DIFFERENTIAL GROWTH 1 37 



ones that impress us optically, and to appreciate the development of 

 these properties we must bear in mind that we are dealing with material 

 systems in space and time, in which whatever is present at any one stage 

 is the result of antecedent processes. Thus the conventional "charac- 

 ters" by which we distinguish differentiation are in reality only inci- 

 dental manufacturing samples of the cellular production plant. And 

 just as one kind of industrial plant differs from another not only by its 

 different end products but by the whole machinery and processing 

 method turning out those products, so a differentiated cell of one kind 

 differs from that of another kind not only in its final equipment (pig- 

 ment, fibrils, secretions, etc.) but in the basic mechanisms through 

 which that equipment has been produced. 



Tracing pigment cells, muscle cells, or blood cells back into earlier 

 phases of their development, we find, or postulate, that they contain 

 then, instead of the terminal products of melanin, myosin, or hemo- 

 globin, half-finished "precursors." But if we go back still further, 

 beyond the "precursors of precursors," the future products lose their 

 identity and we are faced with nothing but the plant capable of turning 

 them out, and perhaps some of the ingredients. Now, evidently a plant 

 designed to turn out chiefly melanin must be different from one de- 

 signed to make contractile myofibrils. We express this fact in the names 

 "melanoblast" and "myoblast" given to those early stages, and we 

 generally concede that they are intrinsically different, although the dif- 

 ferences are no longer demonstrable by ordinary microscopic or chem- 

 ical techniques. By further refinements of technique we can extend 

 the range of our discriminative powers ; structurally, by the ultramicro- 

 scope, polarization microscope, electron microscope and x-ray diffrac- 

 tion; chemically, by microtests (including cytological stains), spec- 

 troscopy, microincineration, etc. These techniques will undoubtedly 

 reveal criteria of differentiation much earlier in the embryonic history 

 than was possible with the cruder classical tools of observation. This 

 is a line of investigation worthy of vigorous prosecution. But we must 

 expect that it too will come to an end short of retracing the whole 

 ontogenetic course of a given cell strain, for we are still essentially 

 concerned with tracing "products" rather than the production processes 

 that lie behind. Once we can detect the "product," most of the story 

 of its production is over and we have certainly missed its essential be- 

 ginnings. It looks as if the analytical methods at hand are of no avail. 



Preformistic theory would deny the validity of our dilemma. It 

 would assert that all distinctions of later stages have existed, though 



