384 CELL HEREDITY 



to and from tht' allele whose equilihriuni is under eonsideratioii. This 

 process is called periodic selection or orthoselection. By means of it, 

 bacterial populations maintain a certain degree of uniformity. At the 

 same time they constantly improve themselves for the existing environ- 

 ment and contain a small proportion of mutants which might be pre- 

 adapted for some future enviroiuiient. The principle involves a con- 

 tinued selection of subclones within the population and may well operate 

 for any kind of variants among diploid somatic cells. 



The problem of selection is a large and complex one about which little 

 is known as yet. But it should be clear that population dynamics, while 

 certainly part of the subject of ecology, are at the same time a genetic 

 phenomenon. Population dynamics are presently being considered as a 

 means by which the formation of antibodies may be understood. It had 

 previously been supposed that these specific globulins in the blood- 

 forming tissues of the body were synthesized on the model of the in- 

 troduced antigen. Alternatively, it now seems possible that the plasma 

 cell population may be genetically heterogeneous with regard to the 

 antibodies it can produce. This heterogeneity may be achieved through 

 spontaneous mutation as the plasma cell tissue arises in development. 

 Selection is thought to operate in two ways. First, immature plasma cells 

 may be repressed by antigens reacting with the antibodies they produce; 

 hence, in the adult there are no cells capable of producing antibodies 

 against the animal's own antigens (tissues such as the lens of the eye, 

 which are out of contact with the circulatory system, being an excep- 

 tion). Second, in the adult, a foreign antigen may electively stimulate 

 the multiplication of a clone of cells already endowed with the capacity 

 to synthesize the corresponding antibody; the persistence of this clone 

 would account for continued immunity. 



Experiments have been performed to determine whether different 

 plasma cells can produce only one or another antibody. Single plasma 

 cells from animals immunized against two antigens (flagella of Salmonella 

 or bacteriophages) were found to produce only one or the other, or to 

 produce both antibodies, according to the condition of immunization. 

 Although exclusiveness is clearly not complete, it is still possible that 

 the plasma cells may have restricted potencies. 



In any event, it should be clear that there are quite a number of ways 

 in which hereditary differences may be established among somatic cells. 

 In all too few cases can we yet be sure which, if any, are operating 

 There is little doubt, however, that as studies on this subject continue, 

 especially when modeled on the approaches found fruitful with popula- 

 tions of microorganisms whose component cells are a soma, we may ex- 

 pect that there will be found facts and principles of some theoretical 



