17 • GROWTH AND 

 REDUPLICATION 



u. 



p to this time we have been concerned at some length with the prob- 

 lem of differentiation. We have seen how cells change from an undifferen- 

 tiated state to a very highly differentiated state. And now we ask how the cells 

 required for this process of differentiation are obtained. 



The egg starts as one cell with a single nucleus, and from this single cell 

 thousands of billions of cells are derived through the process of growth and 

 cell division. During the process of growth all parts of the cell undergo dupli- 

 cation with the result that when the cell divides the two daughter cells resem- 

 ble the parent. In order for all parts of the cell to duplicate, chemical com- 

 pounds must be synthesized from raw materials. Nucleic acid and genes must 

 be duplicated in the nucleus, and in the cytoplasm a whole host of enzymes 

 dealing with respiration and general metabolism must be built up prior to 

 division. In addition, during growth and cell division there must be a dupli- 

 cation of those compounds which arise during the process of differentiation. 

 For we have seen that once a structure or a group of differentiated cells is 

 induced we can remove this group of cells from the inducing agent and have 

 them self-differentiate. During this self-differentiation, cell division results 

 in more cells, each of which differentiates in the absence of the original in- 

 ductor. After the inductor has acted, therefore, the cells become self-dupli- 

 cating and reproduce their own kind. There must be chemical substances re- 



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