150 PAUL WEISS 



the fact that the molecular population of one system begins to react 

 differentially to the conditions introduced by the new contact system 

 (electric charges, surface tensions, combinations and reactions between 

 border molecules). The resulting interactions lead to (i) the segrega- 

 tion of a selected fraction of the molecular population into a surface 

 layer, and (2) the immobilization, hence, solidification, of this portion. 

 The mechanisms involved will vary, but for our present purpose they 

 can simply be lumped under a common term as ''selective adsorption." 

 Their common denominator is the fact that a given interface of given 

 constitution will intercept and retain certain member species of a mixed 

 molecular population more easily and more strongly than others. Conse- 

 quently, the favored species will become concentrated at the surface and 

 the less favored ones correspondingly diluted. A change in the outer 

 conditions may force the replacement of the former border population 

 by new and more appropriate species from the interior. A complex sys- 

 tem of this kind will have a labile phase, during which the molecular 

 surface grouping responds to changes in the physical state and chemical 

 composition of the adjacent outer system, and a secondary stable phase, 

 which follows the consolidation of the surface due to chemical bonding, 

 gelation, precipitation, etc. If the outer conditions are not uniform over 

 the entire surface, but vary locally, the adsorbed molecular populations 

 will be correspondingly variegated. The same cell can thus develop dif- 

 ferent types of surface zones on different sides exposed to different 

 environments. Moreover, each of the manifold types of interfaces in- 

 side the cell will also acquire its own appropriate type of covering. The 

 segregative power of specific surfaces can thus be readily understood. 

 In becoming adsorbed to an interface, polar molecules assume a com- 

 mon orientation. Their exposed ends thus constitute a new free surface 

 to which further molecular species of fitting configuration may be built 

 on in a second layer. This, in turn, may serve for the deposition of a 

 third group, and so layer upon layer may be stacked up in a specific 

 sequence, the molecules of each interlocking with those of the next.* 

 Although factual information is scarce, it might be surmised that the 

 apposition of new layers in living systems is governed by highly specific 

 relations among the combining molecules, perhaps due to the steric 

 fitting, key-lock fashion, of the specifically shaped ends of the conjugat- 



* Such stacking up of molecular films has actually been demonstrated by Langmnir 

 and Blodgett (23), and although their observations were made on rather simple models, 

 evidence of a similar molecular lamination at the surface has been found in at least 

 certain types of cells (10, 34). 



