38 Introductory Biology 



plant tissues to be made of cells which in turn were composed of that 

 essential material which he named protoplasm. Max Schultze (1825- 

 1874) stated that all living cells are made of similar protoplasm or, in 

 fact, that this mass of organized protoplasm really is the cell and that 

 bone, chitin, and similar products are manufactured by the active, living 

 protoplasm. 



The cell is now considered as the unit of structure of animal and 

 plant tissues; that is, units of which they are constructed, as bricks are 

 the units of which brick walls are made. The cells are also units of 

 function or physiology because the functions and activities of any living 

 organism are the sum of the individual cell activities composing that 

 organism. Each cell works as a unit, performing its particular duty. 

 However, there must be, and is, a proper interdependence, interfunction- 

 ing, coordination, and subordination if the organism as a whole is to 

 function normally and efficiently. The cell is also a unit of develop- 

 meiit and growth because even a complex animal or plant with its many 

 cells has grown and developed through the division and increase in size 

 of its individual cells. Thus, the development of an organism is due to 

 the properties and activities of its various cells, each acting as a unit, 

 each contributing its part to the development of the organism as a whole. 

 Cells are also units of heredity because the embryo receives from each 

 parent a single sex cell which carries the characteristic determiners. The 

 embryo grows and its future cells are given these determiners through the 

 process of mitosis (indirect cell division). There is in this manner a 

 direct hereditary continuity between the parents of one generation, their 

 germ cells, and the newly formed offspring of the next generation. Cells 

 not only transmit hereditary determiners from one generation to the 

 next, but also through successive cell divisions they retain those hereditary 

 characteristics with which the young embryo is endowed. If it were not 

 for this efficiency of our cells, we might at certain periods in our life 

 fail to retain the characteristics given us by our parents. 



Since biology is one of our oldest sciences, one might wonder why the 

 cell principle was not formulated before 1839. Undoubtedly the follow- 

 ing will help us to answer such a question : ( 1 ) There was a lack of 

 scientific instruments with which to study cells effectively before that 

 time. (2) Experimental science as we know it today was not yet preva- 

 lent. (3) Gross or macroscopic anatomy demanded the attention of 

 biologists previous to that time so that a detailed knowledge of cells was 

 not extremely vital. Biology was studied more or less in a general way 

 and a greater emphasis was placed on nature study than on detailed 

 work. (4) A revival of interest in the embryology of organisms in the 



