SECTION III 

 THE CELL AS THE BASIS OF ORGANIC ACTIVITY 



CHAPTER 15 



CELLS AND THEIR ACTIVITIES 



Despite the tremendous variety of the organic world, there are a 

 few unifying principles which demonstrate the essential unity of all 

 living matter. One of these principles is the cell theory which states 

 that with but rare exceptions all living things are composed of cells or 

 cell products. 



Like most great principles, the formulation of this theory and its 

 implications was dependent upon the work of a great many different 

 investigators in seemingly unrelated fields. This work started some 

 300 years ago and is continuing at an increasing pace at present. The 

 gross anatomy of plants and animals was quite well known even at 

 the beginning of the sixteenth century, but until some method of mag- 

 nification was developed, the finer details of structure could not be 

 discerned. Lenses were known even in antiquity, and in Europe 

 they were manufactured from the thirteenth century onward. In that 

 same century, Roger Bacon suggested that these lenses be used as 

 spectacles and predicted the invention of the miscroscope and telescope. 

 Near the end of the sixteenth century, a Dutch father and son by the 

 name of Janssen combined some lens in a tube in such a way as to make 

 it possible to magnify small objects, but the actual invention of the 

 microscope should probably be credited to Galileo. 



The invention of the microscope stimulated the curiosity of a great 

 many individuals whose names are still familiar to many of us. Some 

 of these names have been immortalized by being applied to various 

 structures which these people first discovered. Obviously it is impos- 

 sible to give a full recital of all these men, but a few names should 

 be included because of outstanding contributions. 



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