SECTION III. 

 REPRODUCTION. 



CHAPTEE I. 



THE NATURE OF REPRODUCTION, AND THE 

 ORIGIN OF PLANTS AND ANIMALS. 



REPRODUCTION is the process by which the different kinds of organized 

 bodies are perpetuated in continuous series, notwithstanding the limited 

 term of existence allotted to each individual. It includes the phe- 

 nomena of the production, growth, and development of new germs, as 

 well as the whole history of the successive changes in the organs and 

 functions, and the consequent modifications of external bodily form pre- 

 sented at different periods of life. 



All organized bodies pass through certain successive stages of de- 

 velopment, in which their structure and functions undergo corresponding 

 alterations. The living animal or plant is mainly distinguished from 

 inanimate substances by the continuous changes of nutrition and growth 

 which take place in its tissues. These nutritive changes correspond in 

 activity with the other vital phenomena ; since the production of these 

 phenomena depends upon the regular and normal continuance of the 

 nutritive process. Thus the organs and tissues, which are the seat of 

 a double change of renovation and decay, retain nevertheless their 

 original constitution, and continue capable of exhibiting the vital phe- 

 nomena. 



These changes, however, are not the only ones which take place. 

 Although the structure of the body appears to be maintained in an 

 unaltered condition by the nutritive process from one moment to an- 

 other, or from day to day, yet a comparative examination at greater 

 intervals of time will show that this is not precisely the case; but that 

 the changes of nutrition are, in point of fact, progressive as well as mo- 

 mentary. The composition and properties of the skeleton are not the 

 same at the age of twenty-five years that they were at fifteen. At the 

 later period the bones contain more calcareous and less organic matter 

 than before ; and their solidity is increased, while their elasticity is di- 

 minished. Even the anatomy of the bones alters in an equally gradual 

 manner; the medullary cavities enlarging with the progress of growth, 



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