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CHAPTER II 



THE STRUCTURAL BASIS OF THE BODY 



THE CELL 



LL the higher animals and plants a when submitted to microscopic examina- 

 tion, are seen to consist of structural units which are spoken of as cells. 

 In each organ we find a mass of these cells closely resembling one another 

 in all respects, and we may therefore regard the function of any organ as 

 the sum of the functions of its constituent cells. Indeed, any given reaction 

 of the whole body is the resultant of the reactions of the unlike cells of which 

 the body is composed. The cell is therefore the physiological as well as the 

 structural unit, and it is necessary to commence our study of the functions of 

 the animal body with some consideration of the functions and reactions which 

 are common to all the structural units. 



This composite structure is peculiar to the higher forms of life. Amongst 

 the lower forms, both animal and vegetable, an immense number of organisms 

 consist only of a single cell. In this cell are represented all the phenomena 

 of life, all the adapted reactions which we associate with the life of the higher 

 organisms. That the unicellular condition represents the more primitive 

 stage from which the higher ^organisms have been evolved in the course of 

 ages is indicated by the fact that every one of these higher organisms in 

 the course of its development passes through a unicellular stage, namely, 

 the fertilised ovurn. We may assume that the series of changes attending 

 e development of the higher organism from the egg is a repetition in sum- 

 ary of the changes which have determined the evolution of the species from 

 the primitive unicellular type.* 



The general characteristics of the cell present important similarities, 

 hether we are considering a cell which forms the whole of an organism or a 

 ell which is but an infinitesimal part of a highly developed animal. 



The name ' cell ' was first applied by botanists to the structural units 

 und by them in plant tissues, and involved therefore the idea of certain 

 ualities which do not enter into our present conception of the term. A 

 ction through the stem of a growing plant shows it to be made up of an 

 xegation of cells in the etymological sense of the word, i.e. small sacs 

 bounded by a wall of cellulose and containing cell sap. Immediately inside 

 e cellulose wall is a thin layer, the primordial utricle, which encloses at one 

 int a spherical or oval structure known as the nucleus. If the section be 



* This assumption is often spoken of as the * law of recapitulation,' 



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