12 



Heredity in Somatic Cells 



Somatic cells are those that make up the body or the soma of multi- 

 ceUular organisms. They are distinct from the gametes and their 

 immediate progenitors, which comprise the germ cells. The somatic cells 

 contained in the various tissues of the body are found to be structurally, 

 metabolically, and chemically different. Yet they all arise from the single 

 fertilized zygote which contains all of the genetic information used 

 for the differentiation that produces, during development, the dis- 

 tinguishable cell types with such a harmony in time and space. Each 

 type is usually represented by considerable numbers of cells which can 

 produce only their own kind or, when differentiation is incomplete, a 

 restricted number of kinds. Each differentiated somatic cell thus comes 

 to have a stable heredity characteristic of its type. This stability is 

 reflected even when somatic cells are removed from the body and cul- 

 tured. Although the environment in which it is found determines within 

 limits its condition at the moment, a somatic cell rarely escapes its 

 hereditary destiny and progresses in a different direction. But clearly it 

 is these rare escapes — these aberrant behaviors — which may provide a 

 key to the processes of hereditary variation in the somatic cell. Both 

 normal and abnormal development have their particular embryological 

 and pathological aspects, at the same time remaining basically problems 

 of cell heredity. 



NUCLEAR DIFFERENTIATION 



Do the nuclei change in the course of differentiation in accord with 

 the variety of cell types found? Clearly nuclei are critical for develop- 



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