CHAPTER II 



THE CELL 



1. GENERAL CONSIDERATIONS 



A. THE CELL AS AN ELEMENTARY ORGANISM 



THE remarkable substance whose activity is the basis of all vital phenom-. 

 ena in both animals and plants, and which we call therefore the living sub- 

 stance, occurs not in a homogeneous aggregation but in the form either of 

 discrete masses called cells or of small bodies which represent transformed 

 cells (Schleiden, 1838; Schwann, 1839). Every living being and every sepa- 

 rate cell arises from a preexisting cell (omnis cellula e cellula, Virchow, 1855). 

 Many animals and plants throughout their lives consist of but one cell. In 

 others, from the original cell new ones arise; these in their turn multiply, 

 are transformed into a variety of shapes, and thus become adapted for special 

 purposes. In this way the independence which characterizes the single-celled 

 creatures is reduced to a considerable extent, so that cells detached from the 

 parent organism are as a rule unable to maintain themselves. 



The cell is therefore the beginning and the source of the entire body, and 

 the formed elements of which the body is constructed are each and all nothing 

 else than cells or cell derivatives. Correspondingly we may say that the 

 powers of the body as a whole represent the sum of the powers resident in 

 the individual cells and the cell descendants. 



In the study of vital processes we have thus in the first place to consider 

 the activity of cells. The discoveries, however, which have been made by direct 

 observation upon cells (especially those that are free-living) are not by any 

 means sufficient to serve as the basis for a complete presentation of the general 

 vital phenomena. Besides, there take place in the many-celled organisms, owing 

 to the differentiations occurring in them, many kinds of phenomena which do 

 not take place in the elementary organism, or which at least with our present 

 means cannot be demonstrated, and at all events can be investigated much more 

 thoroughly in the organs of the many-celled animals. In any general discussion 

 of vital phenomena, therefore, one should give the results obtained in the different 

 provinces of general physiology each its proper share of attention. Since, how- 

 ever, this text-book has for its special subject the physiology of man, I must 

 limit myself in the present chapter to the boldest outlines of general physiology. 

 Elsewhere the general vital phenomena will be discussed from time to time in 

 connection with the facts of special physiology. 



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