GERM CELLS OF COELENTERATES 29 



specialized cells do actually become embryonic. But for the 

 present discussion the important point is the observation of the 

 varied potencies of the tissues of a differentiated adult organ 

 like the spleen. 



This brief account of some of the experimental investigations 

 upon cells and tissues of adult and embryonic animals is enough 

 to show the degree to which such tissues may change their 

 structure and function. It clearly demonstrates that body cells 

 are not so lunited in behavior and so predetermined in potency 

 as to render a change impossible. The difference between body 

 cells and germ cells is proved by such investigations not to be 

 so great as is usually held. 



5. Evidence fi'om cancer cells 



The studies which have been made upon cancers throw some 

 light upon the potencies of tissue cells. As is well known, it is 

 possible to transplant cancers from one animal to another 

 through many generations. Most of the cancers which have 

 been experimentally studied are tissue growths, not germ-cell 

 growths, and the ability of these cells to continue their growth 

 and proliferation for long periods of time is an indication of the 

 ability of tisssue cells to live and grow indefinitely. It is, of 

 course, perfectly clear that these cells do not produce other 

 cells of a widely different character, but they are more nearly 

 like embryonic cells, physiologically if not morphologically, than 

 the cells from which they originally came. This would probably 

 involve a sort of despecialization of the tissue cells with the 

 resumption of an embryonic potency. The germ-plasm theory 

 postulates a difference between the germ cells and the body 

 cells of such a sort that the former are conceived to have the 

 ability to live and develop indefinitely, while body cells have a 

 limited life. The behavior of the cells in cancerous growths may 

 do no more than show the ability of highly differentiated tissue 

 cells, under unknown or poorly known conditions, to regain this 

 power of repeated and indefinite growth; but this tends to break 

 down the distinction between germ cells and tissue cells in this 

 particular. 



