56 FRANCIS O. SCHMITT 



In the case of fibers constructed of globular units it is easy to picture 

 possible intracellular processes. Given a sufficient concentration of the 

 appropriate globular protein molecules within the cell, their aggregation 

 or disaggregation would depend on the local environment with respect 

 to salts or activating enzymes. The actin-myosin cycle seems particularly 

 attractive as a prototype. In this case the aggregation or disaggregation 

 would depend on the presence of free adenosinetriphosphate (ATP) or 

 similarly acting substance. The concentration of the latter depends, in 

 turn, on the presence of ATP-ase and on the coupled reactions of inter- 

 mediary metabolism. Thus would be exemplified the close relationship 

 between the energy yielding reactions and structure formation in cells. 



Little vision is required to suggest such possible mechanisms, but 

 much experimentation is needed to isolate the proteins of cell structure 

 and to determine their physical and chemical properties. Especially 

 significant for the problem of growth is the fibrous system developed 

 during the mitotic cycle. Existing theories of the formation of the 

 spindle and astral fibers are well-nigh useless. It is necessary to tackle 

 this problem with the techniques of modern protein and enzyme chemis- 

 try. 



Little can be said about the mechanism of the formation of fibers of 

 the collagen class, the components of which consist essentially of one- 

 dimensional arrays. The fibers are presumably formed extracellularly. 

 Do the cells produce a precursor, a procollagen, which is converted into 

 collagen under the influence of the intercellular environment? Or do the 

 fibroblasts produce the one-dimensional arrays of collagen which then 

 pass into the extracellular space and there form fibers in a manner 

 analogous to the reconstitution of fibers from an acetic acid solution of 

 collagen by the addition of salt or elevation of pH ? 



To these questions there are as yet no satisfactory answers. Collagen 

 is a good case to investigate, for the fibrils can be identified in the elec- 

 tron microscope. 



Comparative Macromolecular Morphology 



Reference has been made to a classification of the fibrous proteins 

 according to two types, the collagen group and the group showing the 

 characteristic alpha x-ray pattern, including keratin, myosin, fibrin, and 

 paramyosin. Astbury has stressed the comparative aspect and has sug- 

 gested that nature employs only a few general patterns of structure with 

 major and minor variations on the main theme. 



