Chapter 3 



CELLS AND THE CELL PRINCIPLE 



I. THE CELL PRINCIPLE AND ITS IMPORTANCE 



The cell principle states that all living animals and plants (or those 

 which were once alive) are made of cells and that all life phenomena 

 and abilities are fundamentally cellular in nature (Figs. 8 and 9). This 

 principle is important in biology because since its formulation it has 

 stimulated the study of a wide range of living phenomena under a com- 

 mon point of view. 



The cell principle was clearly and definitely formulated by the Ger- 

 man botanist Schleiden and the German zoologist Schwann in 1839, al- 

 though cells had been rather crudely and inaccurately studied previously. 

 An Englishman named Hooke had studied cells as early as 1665. 



The original formulators of the cell principle did not have accurate 

 and detailed accounts of cells, but much of that information has been 

 contributed by multitudes of scientists since. The principle has been 

 proved repeatedly by these later investigators, but its general purport 

 and content are much the same today as at the time of its adoption. 



A study of this principle shows that plants and animals, although ap- 

 parently different, are really organized and constructed along common 

 lines or units. It shows that the functions of a normal animal or plant, 

 as well as those of an abnormal, diseased organism, are but the expres- 

 sions of the activities of the individual cells. This principle also influ- 

 enced the study of physiology by showing that cells and their activities 

 are at the foundation of this phase of ^science. It also paved the way for 

 much of the unified scientific experimentation of hundreds of biologists, 

 thereby profitably influencing their progress and research. It also laid 

 the foundation for the modern specialized branch of biology known as 

 cytology, which deals with the study of the finer parts of cells. 



Early investigators used the term cell because they saw the cell wall 

 or container and practically ignored the extremely important substance 

 within. To them tissues looked like the cells of a honeycomb, or some- 

 thing in which other things might be placed. Felix Dujardin (1801- 

 ' 1860) studied and recognized the real importance of the cell contents, 

 especially among the lower animals. Hugo von Mohl in 1846 found 



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