INTRODUCTION 



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GROWTH is an interesting subject because it is a personal 

 subject. We have all grown. We may wonder at times 

 why we have grown no more than we have or why we grew the 

 way we did, but not many do more than wonder. Few of us at- 

 tempt to learn what growth is, how it occurs, and why, or to 

 answer any of the other fundamental questions we might ask 

 about this process which we all 

 experience. 



What is growth ? Most peo- 

 ple would probably answer 

 this question by saying that 

 when anything grows it gets 

 bigger. But that is evidently 

 not all that we include by the 

 term growth. A dog is not 

 merely an enlarged puppy 

 and a man is more than an 

 overgrown infant. The body 

 structure, proportions, and 

 functions change as an indi- 

 vidual grows. This phase of 

 growth is commonly called 



differentiation or is referred to by the term development, and 

 in all but the simplest living creatures it is intimately connected 

 with increase in size. 



We can illustrate what growth is by considering some ex- 

 amples. The amoeba consists of a soft bit of jelly, microscopic 

 in size (from 0.03 to 0.3 mm. in diameter). Microscopic ex- 

 amination shows that this jelly, which is called protoplasm, 

 consists of two distinct parts: a granular, almost transparent 

 outer part which forms the larger portion of the amoeba and 



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Figure I. Amoeba polypodia in successive 

 phases of division. The dark spot is the 

 nucleus; the light one, a contractile vacuole. 

 From Parker and Haswell's Zoology. By- 

 permission of the Macmillan Company. 



