16 • REGENERATION 



n the course of the last few chapters we have seen that the general ideas 

 and principles of early embryonic development apply also to the later 

 development of the various organ systems. Now we should like to ask 

 whether these principles still operate in the adult organism. In other words, 

 is an adult organism a fixed structure, incapable of change, or does it still 

 show some of the labile organization so characteristic of the tgg ? 



Dynamic state in the adult 



In the first place, the adult is deceptively stable; many changes take place 

 within the adult; many cells are destroyed and new ones take their places; 

 many growth processes occur which compensate for the loss of cells. The 

 magnitude of these growth processes is usually not appreciated. For example, 

 new red blood corpuscles develop in the human adult at the rate of about 

 80,000,000 per second. This high rate of growth is necessary to replace red 

 blood corpuscles which are destroyed. Various types of cells must also be 

 replaced, as for example, sperm cells. A single ejaculate of about 3 cc. of 

 semen contains about 300,000,000 sperm cells, and these must be replaced 

 by cell division of the spermatocytes. We could go on to list other types of 

 cells which need replacements, such as the skin cells and the cells in the 



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