CHAPTER XV 



EXPERIMENTAL STUDY OF GROWTH 



Introductory 



Normal Growth 



Although growth is recognized as one of the fundamental 

 properties of living things, comparatively little zoological work 

 has been done in this field. Botanists have paid more attention 

 to the phenomena of growth and with marked success. In al- 

 most every field of biological investigation the process of growth 

 is directly or indirectly involved in the changes that take place ; 

 yet the connection between these changes and growth is often 

 obscure, for as yet we know almost nothing in regard to what 

 takes place in the protoplasm during growth, and very little in 

 regard to the causes that incite growth or inhibit it. 



In this and in the following chapters I shall attempt to give 

 some of the most suggestive results that have been obtained 

 regarding the growth of animals, although the purely physio- 

 logical side of the question, where most, in fact, has been 

 accomphshed, will occupy a secondary place. Attention will be 

 directed more particularly to the gross phenomena of normal 

 growth and to the external factors that influence the growth of 

 animals. 



The process of growth may be said to begin with the egg and 

 to end with the adult. While in some animals the adult condi- 

 tion coincides in a very general way with the condition of sexual 

 maturity, in other animals growth may continue throughout the 

 length of fife of the animal, becoming smaller in. amount as the 

 size increases beyond what we may speak of as the adult condi- 

 tion. In a general way we may class these two kinds of growth 



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