V CONCLUSIONS CONVERGENCE OF CONCEPTS 533 



tion of the tissues a minimal and possibly ubiquitous concentration of the inducing 

 material would suffice to maintain the concentrating mechanism and consequently 

 enzyme induction. Possibilities for stabilization of such a system are indicated in 

 some of the articles and in the discussions included in the symposium referred to 

 above (Gaebler, 1956). 



Although in part speculative, the conceptual background of this scheme seems 

 to be of great significance for certain problems of differentiation since it suggests 

 that inducing substances may modify the PFS either with or without becoming 

 part of it, bringing about the formation of a new type of protein. What is important 

 in the parallelism between this scheme and embryonic induction is not the nature 

 of the inducing agent which in embryology can be expected to be an intermediary 

 metabolite, a nucleic acid, a protein, or even a surface rearrangement after cellular 

 contact. What is important is the suggestion of a mechanism for the modification of 

 the PFS itself which in microbes and embryonic cells may exhibit, in principle, simi- 

 lar properties irrespective of the possibly dissimilar nature of the modifying stimulus. 



Further analysis of the penicillinase system' has led to findings which seem of 

 importance for the elucidation of another aspect of diflferentiation, primarily be- 

 cause it has become apparent that bacterial strains without minimum quantities 

 of penicillinase do not respond to penicillin with enzyme formation. For example, 

 in the inducible strain Bacillus cereus ^6g the enzyme quantity increases from a 

 minimum level of 75 molecules of penicillinase per bacterial cell to 25,000 mole- 

 cules per cell after maximal induction and in this instance induction is, therefore, 

 not absolutely de novo formation of a protein. 



The relationship of these observations to the problem of diflferentiation is sug- 

 gested by the work of Sawyer ( 1 943a, b) ; Boell and Shen ( 1 944) ; Boell, Greenfield 

 and Shen (1955); Shen, Greenfield and Boell (1955), the results of which define 

 quantitatively the onset and progress of neural diflferentation by one specific bio- 

 chemical parameter. Since the diflferentiated nerve cell is distinguished by a high 

 cholinesterase activity, the rapid increase of this enzyme following induction can 

 be regarded as a quantitative index of the induction process. Therefore, one con- 

 sequence of induction of the central nervous system must be the activation of that 

 portion of the PFS which produces cholinesterase. From the data presented it is 

 likely that cholinesterase is present in very low concentration in most embryonic 

 tissues and also in preinduced ectoderm. If the enzymes found before and after 

 induction are identical it would have to be inferred, in the case of this particular 

 enzyme, that the induction process initiates merely a quantitative acceleration 

 of cholinesterase accumulation and that the intercellular loci for the formation of 

 this enzyme are already preformed at this time. A somewhat comparable situation 

 seems to prevail in the case of heart formation where heart myosin is found very 

 widely distributed in low concentrations before the formation of the heart actually 

 begins (Ebert, 1953, 1954a, Ebert et al., 1955). A biochemical site of heart myosin 

 production in the PFS of many primordial cells is thus present before bulk pro- 

 duction of the protein is initiated. Histogenesis of the heart muscle cell may merely 

 involve a relative acceleration of myosin accumulation in the heart primordia. 



^ M. R. Pollock, personal communications. 



Literature p. sjg 



