Cellular Differentiation 75 



do so whenever the external supply of arginine is exhausted and it can 

 again synthesize the missing enzyme. 



c. The lymphoid tissue of an animal exposed to diphtheria toxin will 

 form antibodies ( proteins that specifically neutralize the toxin ) . Although 

 the genetic potential for making antibodies is ever present, only exposure 

 to the foreign toxin can trigger antibody synthesis. 



In brief, then, what a cell can do is determined by its genetic endow- 

 ment, i.e., the instructions provided by its genetic material. The geneticist 

 terms this endowment the genotype. The cell's actual activity is the result 

 of the interaction between the genetic potential of the cell and the environ- 

 ment in which the cell finds itself. What the cell does ( i.e., its structure and 

 metabolic capacities ) is termed the phenotype. 



Faced with the fact that during the development of a multicellular 

 organism, daughter cells can gain new capacities or lose old ones and so 

 become difiFerent both from their parents and from their sisters, we can 

 ascribe the change to one or both of two processes : 



1. The daughter cell receives from the parent its genetic endowment 

 intact and unchanged. However, the environment in which it must live is 

 different from that which the parent cell faced. Consequently, it synthe- 

 sizes cell constituents that its parent did not and vice versa. The altered 

 metabolism leads to morphological differences as well. 



2. The genetic endowment of the daughter cell is different from 

 that of the parent. Thus the act of cellular differentiation would involve 

 a change in the primary genetic material and the alteration would be 

 inherited by the progeny, if any, of the difiFerentiated cell. The genetic 

 macromolecules being altered, the enzymes constructed under their direc- 

 tion would be altered in quantity or kind. In this manner, the altered 

 genotype becomes amplified into an altered phenotype. 



This second process, though not given in chemical terms, was im- 

 plicitly contained in an early theory offered by A. Weissmann in 1900. 

 Weissmann proposed that, when a fertilized egg cleaved, daughter cells 

 did not receive identical sets of genetic determinants. Instead, these were 

 parceled out in a regular fashion depending on the location of the cell in 

 the embryo, so that different cells received different genetic endowments. 

 The difference in genetic content would account for cellular differentia- 

 tion. The parceling out according to location would insure normality of 

 form. Figure 38 is a summary of this scheme in which the original egg 

 cell contains all the genetic elements necessary to make a complete em- 

 bryo. After cleavage, its descendants not only contain different sets of 

 genetic elements, but the necessary cell types end up in the right places in 



