THE CELL 47 



Tissues. In both the Protozoa and the Protophyta, all the 

 various activities involved in living are performed in and by a single 

 cell. In other words, a single-cell animal must move about and cap- 

 ture its own food, perform the processes that digest and convert it 

 into chemical form that the protoplasm may have energy and ma- 

 terials for maintenance, must escape from enemies or injurious con- 

 ditions around it, and must reproduce itself. But among the 

 Metazoa and Metaphyta the variouG activities are taken over and 

 carried on by different types of cells. For example, in the body of a 

 metazoon some types of cells exaggerate the function of contractil- 

 ity. In general, these are termed muscle cells. Others have devel- 

 oped the function of irritability very highly; these are the nerve 

 CELLS. This specialization of function or division of labor is accom- 

 panied by differences in structure and in metabolism; it is called 

 DIFFERENTIATION. A group of cells of the same sort, together with the 

 intercellular substances incorporated with them is known as a 

 TISSUE. Thus the muscle cells make up muscle tissue; bone cells 

 together with their deposits form bone tissue; nerve cells compose 

 nervous tissue. A tabular list of the tissues of the human body aids 

 in familiarizing one with the nature of tissues. (Fig. ii). 



Organs. Various types of tissues are combined into working 

 units called organs. For example, the human stomach is made up of 

 a variety of tissues, muscular, glandular, epithelial, and so on, and 

 functions as a unit in performing a special phase of the conversion 

 of foods into forms which the body may utilize. The term organ 

 refers to a functional unit and does not necessarily mean that all 

 organs are made up of multicellular tissues. Only metazoon organs 

 are composed of tissues. In every cell are structures that are func- 

 tional units and are properly termed cell organs. Similarly, in the 

 Protozoa there are many types of cell organs consisting of special- 

 ized protoplasm. One must realize that such organs are made up of 

 protoplasmic structures that are not cellular in character. The 

 diminutive term organelle is often applied to cell organs. 



