DIFFERENTIAL GROWTH I5I 



ing molecules (54). As complex macromolecules will mostly have dif- 

 ferent configurations at opposite ends, successive layers will consist of 

 different molecular species. Here we have, therefore, a process that, 

 once it has started from a given surface, could lead to a progressive 

 segregation with stratification of molecular species which were originally 

 intermingled. In this process molecular orientation is of paramount 

 importance, since it exposes the maximum number of specific receptive 

 ends to the interior. This consideration gains added significance in the 

 case of those polar molecules that have enzymatic properties, as parallel 

 orientation and packing might increase their effectiveness by turning all 

 active groups toward the substrate. Then selective surface adsorption 

 would become a prime factor not only in the segregation of existing 

 molecular species, but also in the creation of new ones. Moreover, the 

 molecular fringe which settles along the surface acquires master control 

 over substance transfer across the cell boundary in that it can selectively 

 facilitate or prevent the exchange of materials between the areas it 

 divides. 



The extent to which a surface layer grows inward by apposition, will 

 vary with the circumstances. For example, in some cells the outermost 

 layers form a thick and distinctive coat (myelin sheath of nerve fiber, 

 envelope of red blood cell, vitelline membrane of egg), while in others 

 they remain an inconspicuous "plasma membrane." The subjacent layers 

 (crust of egg, ectoplasms) likewise vary in thickness as well as in 

 sharpness of demarcation. It must be remembered, however, that strati- 

 fied molecular segregation need not be so massive as to become micro- 

 scopically discernible. 



The picture of surface organization here presented is incomplete 

 and greatly oversimplified. For instance the specific key molecules, which 

 were given such prominence, obviously lie interspersed with many more, 

 smaller, simpler and trivial molecules of less specific behavior, a mosaic 

 feature which we have not duly taken into account. It has been suggested 

 on the basis of studies on drug action, immunological reactions, and 

 radiation effects, that target points of specific response are present in 

 isolated patches occupying only a small fraction of the cell surface. A 

 similar discontinuity characterizes the structure of the chromosomes. 

 Accordingly, many of the specific effects assigned above to surfaces in 

 general will actually be confined to certain portions of the total surface 

 only. An extension of this thought leads to the realization that complex 

 molecular aggregates (particulates) may be subject to specific adsorp- 

 tion and fixation, with one of their constituent key patches effecting the 



