24 THE CELL THEORY. 



The most simple organisms consist of a single cell. 



The most complex of countless numbers of cells so 

 united that they are thrown into groups (the tissues), 

 which are arranged into such structural relationships 

 that they form special mechanisms (the organs). The 

 whole has that mutual relationship of parts, both in 

 structure and function, that constitutes a complex yet 

 complete organism. 



An organism consisting of a single nucleated cell 

 possesses in a certain degree all the physiological 

 properties the power of accomplishing the physio- 

 logical processes, e. #., assimilation, disassimilation, 

 irritability, contractility, reproduction, etc., that any 

 organism possesses. 



An organism consisting of great numbers of cells 

 differs from the more simple one-celled form in that 

 in it development has proceeded to the point where 

 groups of cells have become more adept in performing 

 a certain physiological process (e. g., gland cells 

 secretion; muscle cells contraction), while other 

 groups accomplish better other processes ; thereby 

 there having been accomplished in the organism what 

 has been termed the "physiological division of labor." 



Every cell arises from a pre-existing cell by division 

 of the first. The countless multitudes of differently 

 formed cells of the most complex organism can in 

 each case be traced to a single cell, the ovum, all 

 having arisen from this one by growth and repeated 

 divisions. 



Hence all the properties of the cells produced by 

 the last division came by descent from the first. All 

 the forms of organs and their arrangement into the 



