Cellular Differentiation 79 



velopmental biology where cytology and cell physiology both enter. 



2. Account for the function of the differentiated cell. As stated in 

 the definition, the new cell type is meaningful because it contributes in 

 some specific manner to the construction of the multicellular organism 

 (i.e., without the appearance of the new cell type, the normal develop- 

 ment of the multicellular structure or the normal functioning of the adult 

 derived from it would be impossible). We must show the specific role 

 played by the new cell type and how its altered metabolism and structure 

 control or trigger later developmental processes in the rest of the organism. 



3. Account for the number of each type of cell. We must be able to 

 examine a very small sequence in the development of the multicellular 

 organism, pinpoint precisely the time at which a particular new cell type 

 appears, count the number of them in the cell population, and account 

 for the number that we see by studying the rate at which they appear and 

 the rate, if any, at which they disappear. 



4. Determine the manner in which the new cell type arose from its 

 parent. Here we return to the question broached in an earlier section. We 

 must determine whether, in a specific act of cellular differentiation, the 

 new cell type possesses genetic instructions different from those of its 

 parent or whether, faced with a new environment, it does different things 

 from what its parent did but uses the same old set of instructions. Cell 

 genetics in recent years has provided ways of distinguishing between 

 these alternatives and also has provided methods to determine in each 

 case some of the specific mechanisms involved. 



As you see, the description of even one step of cellular differentia- 

 tion is a tall order. At present no such step has been completely elucidated 

 in any organism and, in fact, even the most precise descriptions so far are 

 very primitive and not very quantitative. Consequently, the elucidation of 

 processes of cellular differentiation is one of the most exciting challenges 

 in modern biology and presents the hope that, by studying them, we can 

 uncover new mechanisms of cell variation. 



