240 The Nature of Biological Diversity 



may contain informational cues as to direction of differentiation. 

 Chemical data on the nature of Wilde's material, particularly in 

 relation to Moscona's and to the conditioned medium of Niu (1956), 

 will he awaited with great interest. 



What do these facts add up to? Certainly not to definitive state- 

 ment about the mechanisms of induction, of aggregation, or of cyto- 

 differentiative control. They point, rather, to continued examination 

 of the couplings among these three phenomena, with emphasis on the 

 possibility that they share a common basis in microenvironmental 

 large-molecular materials. If it be assumed that these materials simul- 

 taneously are cell product and cell controls — both for the source cells 

 and their immediate neighbors — they would then provide the basis 

 for "external feedback" and a common matrix underlying the integral 

 character of many multicellular systems. How such materials operate, 

 if they do, in controlling intrinsic cellular activities is not at all clear. 

 In some fashion they must mesh with intracellular control mecha- 

 nisms radiating outward from the replicative genetic code. A number 

 of models are available from which to choose. They might operate 

 completely superficially, as "gatekeepers" on a permease model or 

 the modified molecular ecology model of Weiss. They might carry 

 "instructions" more deeply into the cell, to interact with the genetic 

 sites themselves, with messenger materials, or with cytoplasmic syn- 

 thetic sites. Or, an idea which attracts me particularly at the moment, 

 they might operate at or close to the cell boundary, modifying the 

 many subtle factors which influence polymerization and complexing 

 of monomers synthesized deeper in the cell. Whatever is the answer, 

 and most likely there will be more than one, when we have it we shall 

 be one step closer to rationalizing the mutual operation of replicative 

 and regulative factors in the control system of the cell. 



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