178 PAUL WEISS 



ward on an intensified scale will go far in rectifying the current rather 

 abstract and often patently unrealistic concepts of "differential growth." 

 But even the existing knowledge, of which we have presented some 

 samples, is sufficiently compelling to discredit the growing fashion of 

 reducing, on paper at least, problems of differential growth to terms of 

 simple chemical reaction rates, with "growth stimulators" and "growth 

 inhibitors" called upon to do the "regulating." 



Elaboration of Growth Patterns 



According to the preceding sections, differential growth is simply 

 a corollary of differentiation, and differences in growth rates result 

 from a great variety of causes. As such, we have singled out the unequal 

 depletion of reproductive protoplasm in the building of specialized dif- 

 ferentiation products in different cell types, the various inequalities aris- 

 ing from shifts, aggregations and dispersals of cell groups in accordance 

 with properties acquired during differentiations, the resultant differ- 

 ences of geometric configuration, and hence, exposure of different parts 

 of a tissue, with the ensuing confinement of proliferative activity to 

 circumscribed zones of varying sizes, the different fates of differentia- 

 tion products, which are retained in some tissues but extruded in others, 

 and other related disparities contingent upon the divergent courses of 

 differentiation. Such inequalities in the growth pattern can, in turn, 

 secondarily influence the setting for subsequent steps of differentiation, 

 and the resulting changes will further modify the growth pattern. The 

 actual elaboration of the mature body thus appears as a complex se- 

 quence of transforming patterns in which the contributions of differen- 

 tiation and differential growth are intimately interlocked. Basically, 

 however, differential growth is not an agent but merely a product and 

 index of differentiation. 



This realization is of the utmost importance when it comes to inter- 

 preting the manifold general non-localized influences which modify the 

 growth process during later developmental stages, and which are im- 

 pressively illustrated by the diversity of phenotypic variants that can be 

 obtained from genetically identical germs: giants, dwarfs, duplications, 

 defects, disproportions, excesses, and suppressions. Since these can 

 often be directly related to aberrations of nutrition, hormones, oxygen 

 supply, physical factors (radiations, pressure), and even climatic 

 changes, all of these agents have at one time or other been claimed as the 

 "dominating" factors in the growth process. We realize now that any 

 factor that has an effect on the physico-chemical setting in which growth 



