166 The Nature of Biological Diversity 



trated attacks upon problems at the next higher levels of complexity. 

 Foremost among them are the problems of the nature, development, 

 and inheritance of structure at the higher supramolecular and micro- 

 scopically visible levels within the cell. 



The simplest working hypothesis to guide thought and investigation 

 on these problems, and one which seems to have wide current appeal, 

 is that higher levels of structure within the cell, above that of the 

 polypeptide product of the cistron or gene, are accounted for by three 

 factors: the physicochemical properties of reactants, their random 

 collisions, and the ionic and molecular constitution of the cell "soup" 

 in which the collisions occur. These factors are held to determine how 

 gene products are built up into multipolypeptide enzymes and struc- 

 tural proteins, how multienzyme systems come together in proper 

 sequential arrangement, how enzymes and substrates come together 

 and yield further products, how smaller ribosomes combine to form 

 larger ribosomes, and so on. The "self-assembly" hypothesis in its most 

 extreme form thus ultimately traces the building of all cellular struc- 

 ture to molecular contributions from milieu and genes and to random 

 collisions of previously unarranged reactants. While some molecular 

 biologists, at least in oral discussions, seem to have great confidence in 

 the full adecpiacy of the hypothesis, others adopt it tentatively with 

 the express purpose of seeing how far it can be carried, how much 

 structure can be accounted for without invoking additional factors. 

 That some degree of structure can already be explained in this way 

 seems evident. The question then is: Do we now know, or can we dis- 

 cover, whether the hypothesis is sufficient to account for all cell 

 structure? 



Doubts have long existed, especially among students of the ciliated 

 protozoa (Sonneborn, 1951, 1960; Wiesz, 1954; Ehret, 1960; Tar- 

 tar, 1941, 1961), as to the sufficiency of so simple a hypothesis in the 

 light of evidences for the essential role of a fourth factor, a specific 

 preformed organization of the cell or its parts. This organization 

 seems unable to arise de novo by gene action in any noncellular milieu 

 or unorganized cell "soup." Moreover, a nucleus from a cell of one 

 species often fails to make its own kind of cell when introduced into 

 an organized cell of a closely related species. Decisive further evidence 

 for an essential role of preformed cell structure in the inheritance of 

 cell structure will be presented in the first half of this chapter. In the 

 second half, some important literature will be considered in relation 

 to our work and an attempt will be made to search for the underlying 

 generalities and the mechanisms involved. 



Satisfactory experimental analysis of the genetic function of pre- 



