H, 



Growth 



and 



Form 



ow an organism grows and the processes, both in- 

 ternal and external, that control and limit 

 growth are fundamental biological prob- 

 lems. A wide variety of living organisms 

 has served as objects of study. Much has 

 been learned by examining the growth 

 of populations of microorganisms— bac- 

 teria, protozoa, algae, and fungi. In re- 

 cent years, biologists have been able to 

 remove tissues of higher animals and 

 grow them outside the body (tissue cul- 

 ture). In some cases, it has been possible 

 to disaggregate such tissues and to cul- 

 tivate the cells as dispersed individuals 

 exactly as if they were microorganisms. 

 Even the cells of human beings (tumor 

 cells, kidney cells, etc. ) can be so treated 

 and have been found to obey the same 

 rules of growth and nutrition as their 

 more primitive brethren. Intact higher 

 animals have been grown under con- 

 trolled conditions in which the natural 

 diet is replaced by mixtures of known 

 and chemical compounds delivered in meas- 



ured amounts. The rat is a favorite lab- 

 oratory animal for this purpose, but other 

 animals such as the fruit fly ( Drosophila ) 

 and the university undergraduate (vol- 

 untarily ) have been used. 



The first lesson learned was that 

 the term "growth" has several meanings 

 and that an organism may grow in one 

 sense but not in another. Growth, under 

 one interpretation, is an increase in the 

 number of cells, which is found by count- 

 ing the total cell population or a meas- 

 ured sample from it. Growth can also be 

 described as an increase in protoplasmic 

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