SECTION VIII. 



THE REGULATION OF GROWTH AND FUNCTION. 



In all the members of a species the course of the chemical 

 changes in the various tissues and organs are fairly constant 

 and depart but little from a normal course. Upon these 

 changes depend not only the development and growth of 

 each tissue and organ and of the animal as a whole, what might 

 be called the static adaj^tatlon to surrounding conditions, but 

 also the various responses to changes in external conditions, the 

 functional adaptation. The development and the activities 

 of each organ are co-related and co-ordinated with those of all 

 the other organs, and in this co-relation three main factors 

 play a part. 



I. Heredity. 



This is primarily the result of the chemical changes 

 inherited from the parents. The principle of Inertia, that — 

 " Every particle of matter in the universe remains in a state 

 of rest or of uniform motion in a straight line, unless it is 

 acted upon by external force," is applicable to living as w.ell 

 as to dead matter. 



Generation after generation a similar piece of protoplasm 

 the ovum, undergoing the same molecular movements, is placed 

 in the same external conditions, and hence must undergo the 

 same course of development, under the influence of what may 

 be called Hereditary Inertia. A proof of it is afforded by the 

 development, both structural and functional, of embryonic 

 tissues removed from the body and kept in the plasma of the 

 animal blood. A fragment of the cell mass from which the 

 heart develops undergoes the change into the muscular fibres of 



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