Size as Structure and Function 5 



It is the purpose of these chapters to attempt to 

 identify some of the means which an individual or- 

 ganism, such as one trypanosome, has of measuring 

 itself so that growth shall proceed or stop, or fission 

 shall intervene, in the way that it naturally does. 



The cell theory. Unicellular organisms are charac- 

 terized by their universally microscopic size. They 

 are often designated as micro-organisms, though there 

 are some multicellular organisms which fall within 

 their range of sizes. The fact that living structures 

 cannot attain to the largest sizes without organization 

 into more than one cell, has usually been taken to mean 

 that some barrier of size limits the efficiency of single 

 cells. 



The cell theory, the very principle which gave a basis 

 for generalization with respect both to structure and 

 to function among organisms, has always recognized 

 that cells are minute, hence limited in size. But the 

 meaning of this simple fact is still obscure, even though 

 much discussion has flowed concerning surface-mass 

 ratios, surface tension-viscosity ratios, and many other 

 possibly important quantities. Actually, within the 

 general range of sizes among cells, the diversities of 

 bulk are so enormous that none but specific popula- 

 tions can be profitably discussed. 



How unicellular individuals are to be compared with 

 multicellular tissues, has never received a general an- 

 swer. In some respects and in some instances the anal- 

 ogy of the protistan individual to a single metazoan 

 cell is fairly complete. In other instances, as in multi- 

 nucleate types, protista are better regarded as non- 

 cellular in structure. Certainly cytoplasmic material 

 separated from all nuclear material can carry on 

 neither growth nor reproduction. Size is therefore 

 typically regulated only in living units consisting of 

 both nucleus and cytoplasm. 



