STRUCTURE OF A TYPICAL CELL 



133 



confirmed, and now stands as one of the fundamental generaliza- 

 tions of biological science, being known as the Cell Doctrine. A 

 complete statement of this unifying conception of Biology would be 

 that all living organisms are composed of cells, which are the units 

 of structure and function, and of cell products. In complex organ- 

 isms these units are not isolated; but the coordination, both struc- 

 tural and functional, between the different kinds of cells, is ex- 



FiG. 73.— j\I. Schleiden, 1804-18S1 (on the left); and Theodor Schwann, 



1810-1882 (on the right). 



(From Locy, "Biology and its Makers," copyright, 1908, by Henry Holt and Co., 

 reprinted by permission.) 



pressed in the well-known fact that the animal, as a whole, con- 

 stitutes a higher type of unit, the individual. This is sometimes 

 called the Organismal Theory. It is based upon the observed fact 

 that cells associated in a complex organism are coordinated, by 

 their own activities, in such a way that the animal is an individual. 



Structure of a Typical Cell 



A cell may be correctly defined, in a general way, as a mass of 

 protoplasm containing a nucleus. It is true, however, that certain 

 cells, notably the red blood cells of mammals, do not contain 

 nuclei after they reach the final stage in their development. If 



