DIFFERENTIAL GROWTH I49 



The concept of molecular ecology gives a more truly representative 

 picture of cell life than the older morphological concepts. It gives expres- 

 sion to the fact that what is "determined" in a given cell is only the 

 general statistical norm of the proportions, arrangement, and distribu- 

 tion of its constituent elements, but not an absolutely fixed and stereo- 

 typed pattern exactly repeated in each individual cell specimen. No two 

 cells are ever precisely congruous. It is in the very nature of the cell 

 that we can predict no more than the degree of probability of the occur- 

 rence of a particular process in a given place, and this only with reference 

 to what is happening in the rest of the cell. 



In animal ecology the sizes of the mobile units vary greatly. Some 

 animals move independently, others in flocks, and still others as hetero- 

 geneous groups. Similarly, in the molecular populations of the cell some 

 molecules move singly, others combine into larger bodies, submicro- 

 scopic or microscopic particulates, and still others form mixed ag- 

 gregates, with members of different species compounded in definite 

 proportions (e.g. lipoprotein complexes, coacervates, etc.). The forma- 

 tion of any such larger unit introduces a new element of complexity 

 (new specific surface, new conditions for adsorption, restraints of 

 mobility, etc.), and if one thinks the matter through to its logical con- 

 clusion, one realizes that cytodifferentiation effects its manifest order 

 through a series of steps of progressive complication, in which the basic 

 determinants are the properties and mutual affinities of the different 

 molecular species, and a frame of "conditions" prevailing in their com- 

 mon space, favoring the various species differentially. This frame or 

 ground plan, in relation to which the mobile elements will become or- 

 dered and sorted, must evidently possess a greater measure of physical 

 stability than the rest of the system. In a relatively fluid system, such as 

 a cell, the entities to answer this demand are the surfaces, in which 

 molecular mobility is restrained. Surfaces thus assume foremost rank as 

 ecological niches for the assembly and segregation of different segments 

 of the molecular population of the cell. Their mode of operation can be 

 briefly sketched as follows. 



Surface Organization 



Let us consider the interfaces which set off a cell from its surround- 

 ings, or a cytoplasmic inclusion from the matrix, or a chromosome from 

 the nuclear medium. Heterogeneous systems meeting along a common 

 border immediately affect each other across the border. This is due to 



