l8o PAUL WEISS 



differentially, depending on their respective metabolic rates, margins of 

 tolerance, reserves, substitutive capacities, and the like. Faced with de- 

 ficiencies, they compete for the available requisites in what Roux has 

 described as the "struggle among parts." One part's loss may either be 

 another part's gain, or may doom other parts, depending on the mode 

 and degree of their mutual dependence. The production of differential 

 growth defects by different vitamin deficiencies (59) may be pointed 

 out as a most instructive example. But the lessons from such deficiencies 

 are not reversible. While increasing the supply of growth essentials 

 from deficient to adequate levels will benefit growth correspondingly, a 

 further increase to levels of overabundance does not keep on enhancing 

 growth indefinitely. A limit will be reached when the production plant 

 works at full capacity, the maximum rate of production being deter- 

 mined by the intrinsic rates of the component processes, which, because 

 of the assembly-line character of growth, are limited by the slowest links 

 in the chains. A fuller discussion of these problems has been ably pre- 

 sented in other chapters of this book. 



Of course, since what we call the "normal" level of nutrients, vita- 

 mins, hormones, and other growth requisites still falls short of the at- 

 tainable optimum, the growth potentialities of the organism are "nor- 

 mally" not fully realized. Raising such growth factors to supranormal 

 dosages can therefore occasionally have the spectacular effect of bring- 

 ing out latent formations which under "normal" circumstances would 

 have remained unrealized. For example, overdosing a female opossum 

 with sex hormones causes the formation of prostate glands (26), a 

 capacity inherent in the urogenital tract but failing to come to expres- 

 sion at "normal" hormone levels. 



To the various humoral principles thus far recognized as secondary 

 regulators of growth will perhaps have to be added an even more gen- 

 eral category, namely, systems of the antigen-antibody type consisting 

 of organ-specific discharges and their molecular counterparts. The case 

 for this assumption has been presented on an earlier occasion (54), to- 

 gether with supporting experimental evidence. In the most condensed 

 version, it is as follows. The specific key molecules which arise progres- 

 sively in the course of differentiation (see above, page 153) act as tem- 

 plates ("positives") in the intracellular reproduction of more of their 

 kind. At the same time they cause the formation of molecular species of 

 complementary shape ("negatives"), which may or may not be an in- 

 tegral step in the reproductive process itself (double reversal). Com- 

 plementary pairs could inactivate each other by conjugation. The 



