VII. DIFFERENTIAL GROWTH 



BY PAUL WEISS^ 



THE appearance of different properties in initially equivalent cells 

 is called "differentiation." Growth being only one cellular attri- 

 bute among many, "differential growth" is to be subsumed under 

 the larger topic of "differentiation," and our discussion therefore will 

 have to center on the latter. Since the accent of this book lies more on 

 prospects than on retrospects, I intend to concentrate on the gaps in our 

 concepts and knowledge of differentiation and point to possible ways of 

 filling them rather than dwell on past achievements, which, though im- 

 pressive in themselves, are dwarfed by the task that remains to be ac- 

 complished. 



Biology has discovered the strength it can derive from a more rigorous 

 analysis of biological phenomena by the tools and standards of the 

 physical sciences, and if the study of development is to share the new 

 light with its more alert sister branches, such as physiology, genetics and 

 immunology, to name only a few, it will have to submit to the same 

 reorientation that has proved so beneficial with the others. This reorien- 

 tation is in the direction of greater precision, objectivity, and con- 

 sistency in the description and interpretation of phenomena, prefer- 

 ably in terms of physical and chemical order. 



It implies the adoption not only of the tools of the physical sciences, 

 but above all of their disciplined methodology. Analytical embryology 

 is still full of problems which it can only settle within its own right 

 and by its own techniques. But in doing this it must apply standards, 

 criteria, and methods of procedure as rigorous as those of the physical 

 sciences. It must strive to replace the abstract formalism and verbalism 

 of its groping infancy by a mature realism, in which a term stands 

 as symbol for a known or knowable entity or relation, rather than as 

 a soothing device of convenient elasticity to pretend knowledge where 

 there is none. We must not allow its analytical principles to be con- 

 taminated by relapses into animistic mythology, as we are doing when 

 we charge the unresolved residue of meticulous determinations of 

 physico-chemical properties to "organizers," "inductors," and similar 

 ill-descript agents. Labels must no longer be allowed to pass for con- 

 tent, nor generalities for explanations. What we need most in entering 



1 University of Chicago. Original work referred to in this article was aided by the 

 Dr. Wallace C. and Clara A. Abbott Memorial Fund of the University of Chicago. 



