CHAPTER III. 



PROTOPLASM AND THE CELL. 



IT has been shown in the last chapter that life is inherent in 

 a peculiar substance, protoplasm, occurring in definite masses or 

 cells. In other words, protoplasm is the physical basis of life, 

 and the cell is the ultimate visible structural unit. Protoplasm 

 and the cell deserve therefore the most careful consideration; 

 but because of the technical difficulties involved in their study 

 only such characteristics as are either obvious or indispensable to 

 the beginner will here be dwelt upon. 



Historical Sketch. Organs and tissues are readily visible, but 

 in order to resolve tissues into cells something more than the 

 naked eye was necessary. The compound microscope came into 

 use about 1650, and in 1665 the English botanist Robert Hooke 

 announced that a familiar vegetal tissue, cork, is made up of 

 "little boxes or cells distinct from one another . ,' Many other 

 observers described similar cells in sections of wood and other 

 vegetal tissues, and the word soon came into general use. It 

 was not until 1838, however, and as a consequence of a most 

 important improvement in the compound microscope, viz., the 

 invention of the achromatic objective, that cellular structure 



came to be recognized as an invariable and fundamental charac- 



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teristic of living bodies. At this time the botanist Schleiden 



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brought forward proof that the higher plants do not simply con- 

 tain cells but are wholly made up of them or their products ; and 

 about a year later the zoologist Schwann demonstrated that the 

 same is true of animals. This great generalization, known as 

 the ' ' cell-theory ' ' of Schleiden and ScJmvann, laid the basis for 

 all subsequent biological study. The cell-theory was at first de- 

 veloped upon a purely morphological basis. Its application to 

 the phenomena of physiological action was for a time retarded 



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