WHAT ARE LIVING UNITS? 83 



chondria, chloroplasts, and other particulate inclusions, may also 

 undergo changes which are sometimes heritably transmitted. In 

 nature, mutations generally appear in a random, haphazard man- 

 ner, giving what nurserymen term "sports." The orderly and 

 regular development of individual organisms from seeds or fertil- 

 ized eggs indicates a more positive and dependable mechanism 

 for the differentiation of the original germ cell (zygote) into var- 

 ious types of cells, tissues and organs. In fact, differentiated cells 

 (heart, leucocytes, or cancer) in tissue culture may continue to 

 reproduce themselves true to their differentiated type, though de- 

 differentiation or reversion to some other type (e.g., "embryonic" 

 type) may occur. 



The essential characteristic of true life units is their ability to 

 increase the number of their kind by direction of chemical 

 changes, whereby non-living and generally simpler atoms, mole- 

 cules and masses are transformed into the catalytic and reproduc- 

 tive structure of the living entities. Incidentally, besides the 

 catalyzed molecules incorporated into the living unit or into its 

 autocatalyzed "descendants," many other molecules may be dis- 

 charged into the surrounding medium or milieu,* and this mole- 

 cular waste is useful and often essential to other living things. 

 Thus many thousands of tons of urea are being excreted daily into 

 the world's chemical economy, besides large amounts of many 

 other substances. Non-living entities, such as molecules of ben- 

 zene or sulfuric acid, do not increase in number through the 

 direct catalytic action of pre-existing or "parent" molecules of 

 benzene or sulfuric acid, which are typical non-living entities. 



Functional Life: Living vs. Dead 



In a more restricted sense of the term, we may designate as 

 "living" any cell, tissue, or organ from a living unit while it is 

 carrying on the processes it ordinarily did during the functional 

 life of that unit. Thus a frog's heart may be kept "alive" and 

 beating after removal, though the frog is functionally dead. The 

 death of the organism as a whole (somatic death) is more slowly 

 followed by cellular death, as the blood ceases to circulate and the 

 cells perish in the products of their own metabolism. By quick 

 and careful work, living tissues from dead persons have been 

 successfully transplanted onto the living. Utilizing and improv- 



* We have no English equivalent for this French term, and it has been adopted 

 into English by scientists. 



